What is a Weir?

Being able to control the level of water in a river is beneficial in quite a few ways. Historically, mills relied on water power to drive saws, grinding wheels, and other equipment. Raising the water level in a river can also allow boats and ships to navigate areas that would otherwise be inaccessible. Finally, having control of a river can help mitigate the damaging impacts of flooding. But, how we do get this type of control over the level in a body of water? Hey I’m Grady and this is Practical Engineering. On today’s episode, we’re talking about wiers.

A weir is a small dam built across a river to control the upstream water level. Weirs have been used for ages to control the flow of water in streams, rivers, and other water bodies. Unlike large dams which create reservoirs, the goal of building a weir across a river isn’t to create storage, but only to gain some control over the water level. Over time, the term weir has taken on a more general definition in engineering to apply to any hydraulic control structure that allows water to flow over its top, often called its crest. In fact, the spillways of many large dams use weirs as control structures. So how do they work?

If you watched my previous video on the basics of open channel hydraulics, you’ll remember that for subcritical flow, that is slow, tranquil flow seen in most rivers, the depth is controlled by downstream conditions. That means adding a weir across a river will increase the water level upstream. But by how much depends on the flow. This is the equation for flow over a weir. We’re not going to do any calculations here, but it’s important to know the factors that govern the performance of our hydraulic structure. This equation says that the amount of flow that passes over the weir depends on three factors: the length of the weir, the height of the water level above the crest of the weir, and this coefficient which changes depending on the geometry of the weir. The graph of a hydraulic structure’s flow versus water level is called its rating curve, and this is the rating curve for a typical weir.

In many cases, a weir is a passive structure, meaning once it’s installed there’s no way to change this rating curve. And that’s not always ideal. Streams and river are subject to tremendous variability in flow rate. A hydraulic structure may normally flow a small amount, but in flooding conditions be asked to pass incredible volumes of water. With a passive structure and fixed rating curve, that variability in flow means tremendous variability in the water level upstream. During a flood, a weir may back up the water badly enough to cause damage upstream. If you’re using a weir for the spillway on a dam, you might have to build your dam much higher just to handle the water level that occurs during very rare but extreme cases, increasing the overall costs of the structure. Ideally, hydraulic structures used to control water level would have a flat rating curve, meaning over a wide range of flows, you only get small changes in level. So how could we flatten this curve?

Going back to the weir equation, there are only two other parameters available to increase the flow for a given water surface. We could improve the geometry of the weir to increase its efficiency. Different shapes of weirs can pass flow more efficiently and thus have a higher discharge coefficient, but this has a practical limit. The most efficient shape for a weir is to match the curve that the water would take off of a sharp crest. This part of the flow is called the weir’s nappe, and the shape that matches it is called an ogee. With ogee-crested weirs, we can get discharge coefficients as high as around 4, but that’s pretty much the limit. The other parameter we can change is the length of the weir, but in many locations, the available footprint for the weir is a fixed size that can’t be increased. Even if the footprint isn’t fixed, increasing the length of the weir can add significant costs.

Of course, this challenge is easy to address if we allow for structures with moving parts. Many dams and spillways have large gates or valves to control flow. There are a wide variety of types of controlled outlets used on hydraulic structures, including crest gates that act like weirs that can be raised or lowered. The benefit is that the structure’s capacity can be increased while flows are high by opening gates, and then decreased when flows return to normal. Controlled structures provide more flexibility in how water gets released or held back, essentially turning a static rating curve into a family of curves which can be selected from to meet the operational goals.

Of course, controlled outlets come with a major disadvantage of increased complexity, and in many cases, requiring an actual person be available 24/7 to operate the gates and make releases based on inflows. So what if we could get the benefit of a controlled outlet without the disadvantages of increased complexity and operational obligation? Well, there’s one other trick that hydraulic engineers have up their sleeves.

Remember when before I said that you could only fit a certain length of weir within a fixed footprint. That’s not completely true. We can actually fold a weir to get more length within a given space. This is called a non-linear weir and it’s used in situations where you want greater discharge within a given footprint but without the need for actively controlled outlets. To show how this works, I’ve built this flume and some model weirs. This first weir just goes directly across the flume with no bends. I’ll mark the water level in the flume first using this straight weir. Now, with the same flow rate, I’ll replace the linear weir with the folded version. This has just about twice as much weir length in the same footprint. You can see that, even though the weir is passing the same amount of flow, the water level is lower, almost half the distance to the crest from the original level. We’ve flattened the rating curve, allowing for greater discharge at a lower water level. Non-linear weirs with folded cycles like this are call labyrinth weirs and they’re becoming more common as hydraulic control structures. There are also rectangular versions called piano key weirs.

It’s easy to see how beneficial weirs can be, from generating power to improving navigation, controlling floods, and even acting as the spillways for dams. With all those benefits, there are definitely some downsides as well. Impoundments across rivers affect the aquatic environment. Low head dams can also pose a serious danger to swimmers and boaters, a topic I’d like to discuss in the future. In fact, many old weirs that are no longer needed are being replaced or completely removed to restore the river to its natural state. But as long as we need to control the flow of water in our constructed environment, weirs will continue to be an important tool for a hydraulic engineer. Thank you for watching, and let me know what you think!

What is a Hydraulic Jump?

Controlling the flow of water is one of the fundamental objectives of modern infrastructure, from flooding rivers to irrigation canals, stormwater drainage facilities to aqueducts, and even the spillways of dams. So, engineers need to be able to predict how water will behave in order to design structures that manage or control it. And fluids don’t always behave the way you’d expect. Hey, I’m Grady, and this is Practical Engineering. On today’s episode, we’re talking about one of the most interesting phenomena in open-channel flow: the hydraulic jump.

Fluid dynamics might sound as complicated as rocket science, but unlike rockets, you probably already have some intuitions about how water flows. The study of how water with a free surface behaves, that is not confined within a pipe, is known as open channel hydraulics. This field is especially useful in civil engineering where structures can’t usually be tested at scale. We can’t build a dam, cause a flood to see how well the spillway works, and then rebuild it if the performance isn’t up to standards. Instead, engineers need to be able to predict how how well hydraulic structures will perform before they’re ever constructed. This is the definition of engineering: to take theoretical knowledge of science and physics (in this case fluid dynamics), and apply that information to make decisions about the real world.

One of the most important parameters in fluid dynamics is velocity, or how quickly the water flows. Sometimes velocity is a good thing, like when you’re trying to move a lot of water quickly, for example in a flood. Sometimes velocity is a bad thing, like if you’re trying to avoid erosion. Either way, it’s almost always a key criterion when designing hydraulic structures. But the velocity of flow isn’t the only velocity that’s important in fluid dynamics. We also care about the velocity of waves or how quickly pressure disturbances in a fluid can travel. If the flow velocity is exactly equal to the wave speed, we call the flow critical. But it’s more likely that these two velocities are different. Slow, tranquil flow conditions are called subcritical. In this case, the wave speed is faster than the flow velocity. You can see that the waves can travel against the flow direction. Because of this, the depth is controlled by downstream conditions. You can see that anything I do upstream isn’t changing the depth of this flow. Fast moving flow is called supercritical. In this case, the flow velocity is faster than the wave speed. You can see that waves aren’t able to propagate upstream. Supercritical flow is controlled on the upstream side, so nothing I do downstream affects the depth of the supercritical flow above.

A flow profile can naturally transition from subcritical to supercritical (that is from slow to fast), for example if a channel changes to a steeper slope or a cliff. Many types of flow measurement devices rely on forcing a flow to transition from sub- to supercritical because there will be a unique relationship between flow rate and depth for a given geometry. Maybe we’ll talk more about flow measurement in a future video. But, when flow transitions the other direction - when a fast-moving supercritical flow transitions to a more tranquil subcritical condition - something much more interesting happens: a hydraulic jump.

The classic demonstration of a hydraulic jump can be seen at the bottom of your sink. Open the faucet and watch how the flow behaves. You can see the fast moving water right as the flow hits the sink and the abrupt transition of the hydraulic jump to a slower moving flow. But the sink demo isn’t the best example because it happens due to surface tension, not gravity. Plus it’s kind of a boring. So I built this flume in my garage to give you a better look at the hydraulics. If I open the upstream gate by just a little bit, I can create supercritical flow in the flume. Now, if I obstruct the area downstream, I can force the flow to transition into subcritical. Right where the flow transitions, you can clearly see the hydraulic jump.

This phenomenon happens naturally in certain locations. Steep mountain streams often have supercritical flow crashing into rocks and changing slopes, creating whitewater and turbulence and the occasional hydraulic jump. Also, a tidal bore occurs when an incoming tide forms a wave that travels upstream against a river. These events only occur in a few places across the world, but it’s fascinating if you get to see it in person. In many cases, the bore travels as a moving hydraulic jump, similar to what you see here in my flume. But, jumps aren’t just natural phenomena. They’re important in hydraulic structures as well, especially for energy dissipation.

A major part of the job of a civil engineer working in the field of hydraulics is designing against erosion from the flow of water. When we try to control flow of water, it often leads to the potential of having fast moving, erosive conditions. For example, when we put water in a culvert rather than allowing to flow over a roadway, it can pick up speed in the pipe. When we line a ditch or creek with concrete, the smoothness speeds up the flow compared to natural conditions. And when we make releases from a reservoir behind a dam into a spillway, the water can come roaring down at extremely high velocities. This supercritical flow can cause erosion and eventually lead to failure of the structure. So, most hydraulic structures will be equipped with some form of energy dissipator on the downstream end to reduce the velocity of flow and protect against erosion.

There are all kinds of hydraulic energy dissipators, but for large structures like spillways, the most common types rely on the formation of a hydraulic jump. Because a hydraulic jump causes so much turbulence, it is able to effectively dissipate hydraulic energy as heat. So many energy dissipators, also called stilling basins, are designed to force a hydraulic jump to occur. There are many types of stilling basins, but most use different combinations of blocks, end sills, and overall geometry to control how the hydraulic jump forms. The turbulence stays within the stilling basin with the objective of having smooth, tranquil, subcritical flow leaving downstream, minimizing the potential for erosion which would otherwise threaten the integrity of the structure.

Hydraulic jumps don’t just serve utilitarian purposes. Recreational whitewater courses can be found across the world, and many of these courses make use of hydraulic jumps as artificial rapids. In fact, many kayak parks started out as obsolete dams in need of removal, a perfect opportunity for replacement with something more beneficial to the community and the environment. Freestyle kayaking, also known as playboating, involves performing tricks in a single spot. Playboaters use natural and artificial hydraulic jumps to stay in one spot. I’ve never tried this myself but it looks like a lot of fun. Next time you see water flowing in a open channel, try to identify if it’s sub- or supercritical, and keep your eye out for hydraulic jumps. Thank you for watching, and let me know what you think!

How Water Towers Work

Clean water is one of humanity’s most fundamental needs, and those of us who live in urban areas usually get our water from some kind of centralized public system. Operating a water system is a major responsibility that has implications for public health and safety. In dense urban areas, a clean and abundant supply of water is an absolute necessity, not just for drinking, but also for sanitation and firefighting. And it’s not just something we need every so often; water is a constant necessity both day and night, on weekends, holidays, and any time in between. So, the job of finding enough water, making it safe to use, and then reliably distributing it to the system customers with almost no downtime is a monumental task that requires a lot of infrastructure. And probably the most visible component of a public water system is the elevated storage tank, also known as a water tower. I’m Grady and this is Public Works, my video series on infrastructure and the humanmade world around us.

Let’s say you’re the owner of a public water system. You’ve found a source of water of sufficient quantity for your customers, you’ve found a way to clean that water so it’s safe for them to use, and now it’s time to send the water on its way. There are a few ways we can get water from one place to another. One of them is just to carry it there. Whether it’s on the back of an animal, on a truck, or a bottle in your backpack, we still physically carry water all the time. But it’s usually not the most efficient way. The first infrastructure dedicated to water conveyance was the open channel. Whether in a ditch, canal, or aqueduct, the water is carried by gravity, sometimes over very long distances. We still use open channels to carry water for irrigation and drainage, but they have some disadvantages as well. The water is exposed to pollution and contamination, channels bisect the land, making it difficult to get across, and the water can only flow to areas of lower elevation than where it started. And that last one is a big disadvantage, especially if you’re trying to deliver water to an area with hills or mountains. So most public water systems today rely on pipes for distribution.

Simply putting a top on an open channel allows us to take advantage of pressure to move fluids where we want them to go. Just like electrons in a wire flow from high to low voltage, a fluid in a pipe will flow from high to lower pressure. So, if you raise the pressure at one end of a pipe, you can send your clean water to anywhere you want it to go. And how do you raise the pressure of water? With a pump. A pump is a device that moves fluids. In some cases a pump literally lifts the fluid to a higher elevation, but in most cases a pump imparts energy to a fluid by raising its pressure. And pumps, especially the size of pumps that serve entire cities, are expensive. So if you’re tasked with choosing the size of the pump you need for your public water system, what do you do? Maybe you measure the amount of water that the city uses in a given day and select a pump that can match that flow rate. Let’s see how that would work.

It’s midnight in your city and most of your water customers are asleep. Besides the industrial customers that run 24/7, water demands are minimal, and your pump is having no trouble meeting them. But around 5 am, automated sprinkler systems start kicking in. Around 6 am, people start waking up, taking showers, brushing their teeth, cooking breakfast, all things that require water. It doesn’t take long before the water demand exceeds the capacity of your pump. Almost right off the bat, your new pump can’t meet your system demand, because it was only sized for the average. Water demand in large urban areas can vary significantly over the course of a normal day, with the peak hourly demand (usually in the morning or evening) sometimes being as much as 5 times the average daily demand. So, if you are trying to meet your customer’s water needs using just pumps, instead of just one, you might need as many as five pumps (or one huge pump that can do the work of 5). And not only that, you’ll have to be constantly cycling the pumps on and off to meet the variable demand, increasing the wear and tear on your equipment. And here is where storage comes in. Let’s add a water tower to the system and try this experiment again.

It’s midnight and demand is low, but your pump is running full wide open. Instead of water flowing customers, it’s flowing into your water tower, filling the tank slowly but surely. As morning comes and demand starts to increase, your pump continues running. It’s not able to meet the demand on its own, but the stored water in the tank is making up the difference. All your customers are getting the water they need. As people start their day, demand again drops below average. But, the pump keeps running and the extra flow goes into the tank. Demand again begins to spike as the residents of the city start cooking dinner, taking baths, and watering the plants. All this extra water use drains the tank again before most people go to bed and the cycle starts again.

It’s pretty easy to see how storage makes your water system more efficient. It smooths out the peaks and valleys of water demand not just on your pumps but all your upstream infrastructure, including your water treatment plant and raw water supply. Without storage, all those facilities would need to be sized for peak demand, increasing their cost. With enough storage, pumps and other infrastructure can be sized for average demands, saving not only cost, but also complexity, because you don’t have to predict changes in demand and respond accordingly. Sometimes those peaks and valleys are predictable, but sometimes they’re not. Some of the biggest water demands in urban areas are from fires. Without a firefighting force and enough water to supply them, fires can burn out of control in dense urban areas. In fact, many of the deadliest disasters in history were fires in cities before modern water systems. Now most municipalities and building codes have minimum requirements for the amount of water that must be available to firefighters. And having water stored and ready, like in a water tower, goes a long way to being able to respond to an emergency.

You may thinking, c’mon Grady. This is nothing new. Storage is the age-old solution to any situation where the supply doesn’t match the demand. And, yeah, it might not be anything remarkable to store water in a big tank. But water towers aren’t just big tanks, they’re big tanks elevated above the ground. And that’s because water towers aren’t just storing water; they’re also storing energy. Water distribution systems rely on pressure to get the water where it’s going. If you’ve ever taken a shower with low water pressure, you know how frustrating it can be, because you just can’t get enough water out of tap. Pressurizing a water system is also important for public health. Without enough pressure in the pipes, contaminants could make their way into the system through taps or small leaks. Most water systems get their pressure from pumps, and it takes a lot of energy to maintain this pressure. So, having the ability to store not only the water itself, but also the energy that has been imparted to it by the pumps is important. In some areas, where electricity costs vary based on demand, you can run the pumps at night when electricity is cheap to fill up your water tower. Then, leave the pumps off during the day when electricity is more expensive, allowing just the tower to pressurize the system and serve your customers. Storing energy this way is also carried out at a larger scale to help with electrical grid reliability, but that’s a topic for another video. Elevated storage is also beneficial during a power outage, by keeping the system pressurized even when pumps are out of service.

But how elevated do they need to be? You might know that the pressure within a body of water is related to the depth. The deeper you go, the greater the pressure. Just like in a pool or the ocean, a water distribution system has the same relationship between depth and pressure. It just happens to be confined within a series of pipes. So, you can imagine a water distribution system as a virtual ocean under which we all live, and the water surface in elevated storage tanks represents the surface of the virtual ocean. Imagining a water system this way makes it easy to see the challenge of delivering water to customers at the right pressure. If our cities were flat, this would be pretty simple. All the buildings would sit at the same depth in the virtual ocean. But most areas have at least some amount of topographic relief. Customers at low elevations are at the bottom of the virtual ocean, where pressures can be too high. You might think this is a good thing, but plumbing pipes and appliances are only rated to certain pressures, so exceeding those ratings can cause serious damage. Sometimes buildings at low elevations will be equipped with special valves to reduce the pressure. Customers at high elevations will be near the surface of the virtual ocean, having very low water pressure. As I mentioned, this can be not only frustrating, but also lead to contamination of the system. To solve this challenge, many large cities maintain separate distribution systems called pressure zones, each with their own water tower, to serve customers at different elevations within the city.

But, what happens if you need to serve customers at different elevations in the same location? Tall buildings, like skyscrapers, can have adequate water pressure on the lower floors, while the higher floors can go up near the surface or even above the virtual ocean in the water distribution system. So, instead of relying on city water pressure, most tall buildings use their own pumps to provide water to the upper floors. And some cities, like New York, even require that each building have its own elevated storage tank.

Not every city uses water towers. Some have their entire water supply at a higher elevation, minimizing the need to add pressure to the system. And, sometimes it just makes more sense to rely on pumps alone to keep the system up and running. After all, water towers aren’t cheap, they take up quite a bit of space, and they can allow water to stagnate if it isn’t circulated enough. But, with public water supplies, reliability is key. And, it’s been a long time since gravity was knocked offline from a thunderstorm, so elevated storage tanks (in some form or fashion) are definitely here to stay. Thank you for watching, and let me know what you think!

What is Prestressed Concrete?

Talk to any concrete professional and they’ll tell you the first rule of concrete is this: it’s pretty much guaranteed to crack. But not all cracking is considered equal, and there is a way to reinforce concrete to minimize its negative impacts. Hey I’m Grady and this is Practical Engineering. Today we’re talking about prestressed concrete.

Despite its excellent qualities as a structural material, concrete has some weaknesses, too. One that we’ve discussed in previous videos is that it has almost no strength against tension. Concrete can withstand a tremendous amount of compressive stress, but when you try to pull it apart, it gives up easily. Concrete’s other weakness is that it’s brittle. It doesn’t have any “give” or stretch or ductility. Combine these two weaknesses, and you get cracks. Concrete loves to crack. And if you’re designing or building something made of concrete, understanding how much and where it’s going to crack can be the difference between the success and failure of your structure.

To understand how engineer’s design reinforced concrete structures, we first have to understand design criteria - or the goals of the structure. The obvious goal that we all understand is that it shouldn’t fall down. When a car drives over a bridge and the bridge doesn’t collapse, the structure is achieving its design criterion of ultimate strength. But, in many cases in structural engineering, avoiding collapse actually isn’t the limiting design criteria. The other important goal is to avoid deflection, or movement under load. Most structural members deflect quite a bit before they actually fail, and this can be bad news. The first reason why is perception. People don’t feel safe on a structure that flexes and bends. We want our bridges and buildings to feel sturdy and immovable. The other reason is that things attached to the structure like plaster or glass might break if it deflects too much.

In the case of reinforced concrete, deflection has another impact: cracks. The reinforcement within concrete is usually made from steel, and steel is much more elastic than concrete. So, in order to mobilize the strength of the steel, first it has to stretch a little. But, unlike steel, concrete is brittle - it’s doesn’t stretch, it cracks. So that often means that concrete has to crack before the rebar can take up any of the tensile stress of the member. This demonstration is from a test in a previous video showing a conventionally reinforced concrete beam. Go back and check that video out if you haven't seen it yet. Notice how this beam is resisting the load on it, even though it is cracked at the bottom. It’s meeting design criterion number 1 - it’s holding the load (in this case 6 tons) without failing. But it’s not meeting design criterion number 2 (serviceability) - it’s deflecting too much and the concrete is cracked. Those cracks not only look bad, but in an actual structure, they could allow water and contaminants into contact with the reinforcement, eventually causing it to corrode, weaken, and even fail.

One solution to this problem of deflection in concrete members is pre-stressing, or putting compressive stress into the structural member before it’s put into service. This is normally accomplished by tensioning the reinforcement within the concrete. This gives the member a compressive stress that will balance the tensile stresses imposed in the member once it is put into service. A conventionally reinforced concrete member doesn’t have any compression to start with, so it will deflect too much well before it’s in any danger of not being strong enough to hold the load. So with conventional reinforcement, you don’t even get to take full advantage of the structural strength of the member. When you prestress the reinforcement within concrete, you don’t necessarily increase its strength, but you do reduce its deflection. This balances out the maximum load allowed under each of the structural design criteria, allowing you to take fuller advantage of the strength of each material.

There are two main ways to prestress reinforcement within concrete, and of course I built a couple of beams to demonstrate. The first method is pre-tensioning. And yes that terminology is a little confusing. It’s pre-stressed because the steel is stressed before the member is put into service, but pre-tensioned because the steel is stressed before the concrete cures. To make this work, I had to build a little frame to go around my concrete beam. This frame will hold the steel in tension while the concrete cures. I installed threaded rods through the mold and frame, and then tensioned these rods by tightening the nuts. I tried to use the pitch of the ringing to get them at around the same tension, and you can see how much my frame is flexing from the force in these steel rods. The other method for pre-stressing steel is post-tensioning. In post-tensioning, the steel is stressed after the concrete cures, but still before the member is put into service. In this beam I cast in smooth plastic sleeves in the mold. The steel rods can slide easily within the sleeves.

Once both molds were prepared, I filled them up with concrete. And I finally got a construction grade concrete vibrator as well. This machine helps get all the air bubbles out of fresh concrete before it cures, a process called consolidation. After the concrete’s has had some time to cure, it’s time to test the beams out. On the pretensioned beam, I can unscrew the nuts and take off this frame. Because the concrete hardened around the bolts, the steel rods are still under tension inside this beam. I put it under the hydraulic press for testing, and the results are easy to see. In a conventionally reinforced beam where the steel is simply cast into the concrete without any tension, cracks start forming at around 4 tons. In the pretensioned beam, the cracks didn’t appear until double that force at around 8 tons. The tension already in the steel is able to take up the force of the press without requiring the beam to flex.

For the post-tensioned beam, I inserted the steel reinforcement after the concrete had cured. Then I tightened the bolts on the rods to pre-stress the steel. Under the hydraulic press, the results are nearly identical. The tension in the steel held beam in compression for much longer than a conventionally reinforced member could. Of course, the cracks eventually appear, but it takes much more force before they do. That’s because, adding force to the beam is not creating tension, but just reducing the compression that’s already been introduced through the tension in the steel rods.

It’s important to point out that we didn’t necessarily make these beams stronger. Both the steel and concrete have the same strength as they would without prestressing the steel. But, we did increase the serviceability of member by reducing the amount of deflection under load. Of course, none of these examples actually failed because of the reinforcement, and that wasn’t the point of the demo. But, it’s still more fun to test everything to failure. Pre-stressed concrete is used in all kinds of structures from bridges to buildings to silos and tanks. It’s a great way to minimize cracking and take fuller advantage of the incredible strength of reinforced concrete. Thank you for watching, and let me know what you think!

Was Roman Concrete Better?

The largest unreinforced concrete dome in world is on the Pantheon. It’s not a modern marvel, but rather an ancient Roman temple built almost two thousand years ago. So, if concrete structures from the western Roman Empire can last for thousands of years, why does modern infrastructure look like this after only a couple of decades? Hey I’m Grady and this is Practical Engineering. In today’s episode, we’re taking a look at the factors that affect the design life of concrete.

If you haven’t seen the previous videos in this series about concrete, here’s a quick synopsis. We’ve talked about how concrete’s made, why it often needs reinforcement, and how that reinforcement can sometimes lead to deterioration. Concrete reinforced with steel bars is the foundation of our modern society. The reinforcement is required to give the concrete strength against tensile stress. We use steel as reinforcement because of its strength, its similar thermal behavior, its availability, and low cost. But steel has an important weakness: it rusts. Not only does this corrosion reduce the strength of the reinforcement itself, but its by-product, iron oxide, expands. This expansion creates stresses in the concrete that lead to cracking, spalling, and eventually the complete loss of serviceability - i.e. failure. In fact, corrosion of embedded steel reinforcement is the most common form of concrete deterioration. But it hasn’t always been that way.

The Romans got around this problem in a very clever way: they didn’t put steel in their concrete. Simple enough, right? They harnessed the power of a few clever structural engineering tricks like the arch and the dome to make sure sure that their concrete was always resisting compression and never tension, minimizing the need for reinforcement. One of those clever tricks was just making their structures massive, and I mean that literally, because the simplest way to keep concrete in compression is to put heavy stuff on top of it, for example, more concrete. We use this trick in the modern age as well. Most large concrete dams are gravity or arch structures that rely on their own weight and geometry for stability. In both gravity and arch dams, the shape of the structures are carefully designed to withstand the water pressure using their own weight. You can see how they get larger, the deeper you go. So, even with the tremendous pressure of the water behind the structure, there are no tensile stresses in the concrete, and thus no need for reinforcement.

But lack of steel reinforcement isn’t the potential only reason Roman concrete structures have lasted for so long. One of the other commonly-cited suggestions for the supremacy of Roman concrete is its chemistry. Maybe they just had a better recipe for their concrete that somehow got lost over time, and now those of us in the modern era are fated to live with substandard infrastructure. In fact, in 2017, scientists found that indeed the combination of seawater and volcanic ash used in ancient roman concrete structures can create extremely durable minerals that aren’t normally found in modern concrete. But that’s not to say that we can’t make resilient concrete in this modern age. In fact, the science of concrete recipes, also known as mix design, has advanced to levels a Roman engineer could only dream of.

One of most basic, but also most important factors in concrete’s chemistry is the ratio of water to cement. I did an experiment in a previous video that showed how concrete’s strength goes down as you add more water. Extra water dilutes the cement paste in the mix and weakens the concrete as it cures. The Romans knew about the importance of this water to cement ratio. In historical manuscripts, Roman architects described their process of mixing concrete to have as little water as possible, then pounding it into place using special tamping tools. Interestingly enough, we have a modern process that closely mimics that of the ancient Romans. Roller Compacted Concrete uses similar ingredients to conventional concrete, but with much less water, creating a very dry mix. Rather than flowing into place like a liquid, RCC is handled using earth moving equipment, then compacted into place using vibratory rollers like pavement. RCC mixes also usually include ash, another similarity to Roman concrete. It’s a very common construction material for large gravity and arch dams because of its high strength and low cost. Again, these are usually unreinforced structures that rely on their weight and geometry for strength.

But, not everything can be so massive that it doesn’t experience any tensile stress. Modern structures like highway overpasses and skyscrapers would be impossible without reinforced concrete. So, generally we like our concrete to be more viscous or soupy. It’s easier to work with. It flows through pumps and into the complex formwork and around the reinforcement so much more easily. So, one way we get around this water content problem in the modern age is through chemical admixtures, special substances that can be added to a concrete mix to affect its properties. Water reducing admixtures, sometimes called superplasticizers, decrease the viscosity of the concrete mix. This allows concrete to remain workable with much lower water content, avoiding dilution of the cement so that the concrete can cure much stronger. I mixed up three batches of concrete to demonstrate how this works.

In this first one, I’m using the recommended amount of water for a standard mix. Notice how the concrete flows nicely into the mold without the need for much agitation or compaction. After a week of curing, I put the sample under the hydraulic press to see how much pressure it can withstand before breaking. This is a fairly standard test for concrete strength, but I’m not running a testing lab in my garage so take these numbers with a grain of salt. The sample breaks at around 2000 psi or 14 MPa, a relatively average compressive strength for 7-day-old concrete. For the next batch, I added a lot less water. You can see that this mix is much less workable. It doesn’t flow at all. It takes a lot of work to compact it into the mold. However, after a week of curing, the sample is much stronger than the first mix. It didn’t break until I had almost maxed out my press at 3000 psi or 21 MPa. For this final batch, I used the exact same amount of water as the previous mix. You can see that it doesn’t flow at all. It would be impossible to use this in any complicated formwork or around reinforcement. But watch what happens when I add the superplasticizer. Just a tiny amount of this powder is all it takes, and all of a sudden, the concrete flows easily in my hand. In many cases, you can get a workable concrete mix with 25% less water using chemical admixtures. But most importantly, under the press, this sample held just as much force as batch 2 despite being just as viscous as batch 1.

The miracle of modern chemistry has given us a wide variety of admixtures like superplasticizers to improve the characteristics of concrete beyond a Roman engineer’s wildest dreams. So why does it seem that our concrete doesn’t last nearly as long as it should. It’s a complicated question, but one answer is economics. There’s a famous quote that says “Anyone can design a bridge that stands. It takes an engineer to build one that barely stands.” Just like the sculptors job is to chip away all the parts of the marble that don’t look like the subject, a structural engineer’s job is to take away all the extraneous parts of a structure that aren’t necessary to meet the design requirements. And, lifespan is just one of the many criteria engineers must consider when designing concrete structures. Most infrastructure is paid for by taxes, and the cost of building to Roman standards is rarely impossible, but often beyond what the public would consider reasonable. But, as we discussed, the technology of concrete continues to advance. Maybe today’s concrete will outlast that of the Romans. We’ll have to wait 2000 years before we know for sure. Thank you for watching, and let me know what you think!

Does Rebar Rust?

Concrete reinforced with steel is the literal foundation of our modern society. Reinforcement within concrete creates a composite material, with the concrete providing strength against compressive stress while the reinforcement provides strength against tensile stress. But, while steel reinforcement solves one of concrete’s greatest limitations, it creates an entirely new problem: Corrosion of embedded steel rebar is the most common form of concrete deterioration. So what are we doing about it? Hey I’m Grady, and this is Practical Engineering. On today’s episode, we’re testing out some innovations in concrete reinforcement.

Although unprotected steel is naturally prone to corrosion, or rusting, when it gets embedded into concrete, certain factors usually work to protect it. First is the obvious protection of simply being shielded from the outside environment by a relatively impermeable and durable material. Water and contaminants usually can’t make their way through the concrete to the steel. The second form of protection is the alkaline environment. The high pH of normal concrete creates a thin oxide layer on the steel that provides protection from corrosion. But, in some cases, this protection isn’t enough. One of the main sources of corrosion to rebar is salt. Whether through exposure to saltwater near a marine environment or application of deicing salts to make roadways safer during the winter, these chloride ions can make their way through the concrete, corroding the steel reinforcement. And when steel corrodes, it creates iron oxide that expands inside the concrete. This expansion generates stress, sometimes called oxide jacking, and is the one of the primary causes of concrete deterioration. So, how do we prevent these chloride ions and other contaminants from reaching the steel and causing corrosion? The first line of defense is cover.

Cover is the minimum distance between the outside surface of the concrete and the reinforcing steel. And, depending on exposure and application, certain codes specify different amounts of concrete cover, generally between 25 and 75 millimeters or 1 to 3 inches. Cover is one of the reasons good concrete work takes so much effort before the concrete ever shows up on the job site. Installing strong formwork and lots and lots of wire tying all the reinforcement together help to make absolutely sure that, through all the jostling and walking over and general chaos that comes when it’s time to actually place concrete, the rebar stays where it was designed to be embedded within the final product. Neglecting these steps can cause rebar to sink to the bottom of a slab or come too close to an outside surface before the concrete cures, eventually leading to premature corrosion of the reinforcement due to lack of cover.

But, even with adequate cover, a crack in the concrete can allow contaminants and water into direct contact with the reinforcement. And it won’t surprise you to learn that cracks in concrete aren’t all that rare. Most concrete shrinks as it cures which can lead to cracks. Changes in temperature also cause expansion and contraction which can lead to cracking. Concrete can also crack under normal, expected loading conditions due to the way the steel takes up stresses within the material. One way to solve this issue is by prestressing the rebar, a topic I discussed briefly in a previous video and something I’d like to dive deeper into in the future. But today I want to show another option for reducing these cracks. Fiber reinforced concrete is pretty much exactly what you’d expect it be. It’s not a new idea by any means, but our understanding and use of different kinds of fibers within a concrete mix continues to grow. Adding glass, steel, or synthetic fibers to concrete can provide a lot of benefits, but one of the most important is crack control. I constructed three nearly identical reinforced concrete beams to show how this works, and I let them cure for about a week. The first one only has steel rebar as reinforcement. I’m using my hydraulic press to test out the strength of each beam and see how it performs prior to failure. And I’m using tons as a measurement of force on these beams, just because that’s what the gauge says, but the units are completely arbitrary to the demo. If you prefer SI, just pretend these are metric tonnes. As I increase the load on the beam, you see cracks starting at only around 3 tons. These cracks form because steel stretches a little bit as it takes up the tensile stress in the concrete. The beam is holding the load just fine and isn’t even close to failure, but concrete can’t stretch along with the steel so it has to crack. You can imagine how these cracks could let water and air into contact with the reinforcement and eventually deteriorate the concrete. Those cracks are the important part of this demo, but I went ahead and increased the load until the beam failed because, hey, that’s what hydraulic presses are good for right?

For these next two beams, I included fibers in the concrete mix: one beam has steel fibers and the other has glass fibers. The steel rebar and fibers team up to resist tensile stresses in the beams. The rebar provides large scale reinforcement to resist tension across the entire structural member, and the fibers provide small scale reinforcement to resist localize tension that causes cracking. When I load these beams to 3 tons, you can’t see a single crack. In fact, for both of these beams, I didn’t see any cracks form until almost double that. and even then the cracks were much smaller. Both beams failed at about the same load as first, one, which I expected. Like I said, the fibers don’t really add much overall strength to the beam, but you can easily see they could go a long way in preventing corrosion of steel rebar.

You may be wondering why are we even using steel for reinforcement at all? Steel is relatively inexpensive, well-tested, and strong, but there are lots of other materials that with excellent mechanical properties that don’t face this issue of corrosion. For very corrosive environments, we sometimes use epoxy-coated rebar or even stainless steel, but there are some emerging alternatives like Fiber Reinforced Polymers or FRP bars. This is reinforcement made of basalt, remelted volcanic rock forced through tiny nozzles to create fibers that are extremely strong. Options like this often cost cost more than steel rebar, in some cases a lot more. But, the major impediment to the use of these newer, more innovative types of reinforcement isn’t just the cost. It’s easy to see that those additional costs may be offset by the increased lifespan of the concrete. Another inhibition comes simply from the lack of widespread use. Innovation happens slowly in civil engineering because the consequences of failure are so high. Gaining confidence in a design has as much to do with engineering theory as it does to simply seeing how well similar designs have performed in the past. But many engineering disasters have come not at the expense of bad design, but actually bad maintenance, so long-term durability can be just as important to public safety as other design criteria. We’ll certainly be seeing more innovative ways to reinforce concrete in the future, including the options I mentioned in this video. Thank you for watching, and let me know what you think!

Why the Tacoma Narrows Bridge Collapsed

One of the most fundamental jobs of an engineer is to compare loading conditions to strengths. If the loads exceed the strengths, you know you’ve got a problem. Buildings and other structures face a huge variety of loads, including floods, snow, rain, ice, earthquakes, and crowds of people. One of the most interesting forces faced by civil structures is the wind. Hey I’m Grady and this is Practical Engineering. Today we’re diving into one of the classic case studies of engineering failure: the Tacoma Narrows Bridge.

A bridge is a quintessential civil structure. Humanity’s need to get from one place to another without getting wet is as old as history itself. And for so many years, there was one force with which bridge engineers had to contend: gravity. The fundamental question of bridge design was this: how can we hold up the structure itself and all the people and vehicles that may cross against the force of gravity pulling them downward. And secondary to that, how can we do it economically, for the least cost to the public, since most bridges are funded by the taxpayer. So over time, bridge designs evolved with our understanding of structural engineering and ability to produce better construction materials towards lighter and more efficient shapes, one of those shapes being the suspension bridge.

A suspension bridge is essentially just a deck, two towers, two main cables, and connector rods which suspend the deck, hence the name. The primary advantage of suspension bridges is that they can so efficiently span long distances with only two towers, reducing the amount of material required, and more importantly, the cost. This advantage of being able to span long distances while minimizing material gives suspension bridges their iconic slender and graceful appearance. But that same lack of material reduces the rigidity and stiffness of the structure. Where, before, bridges were generally stiff enough that gravity was the only load that needed to be considered, now a new force started to impact their designs: the wind.

In July 1940, the Tacoma Narrows bridge opened to traffic between Tacoma, Washington and the Kitsap Peninsula. At the time, it was the third-longest suspension bridge in the world. Financing construction of the bridge was a major obstacle, which led the state to pursue an innovative design. Rather than the originally-proposed trusses, the bridge used two narrow plate girders to stiffen the deck, giving the bridge its iconic steel ribbon appearance across the Puget Sound. Unfortunately that analogy extended beyond its appearance. Even during construction, it was apparent that the bridge was too flexible even under moderate winds. Construction workers gave it the nickname “Galloping Gertie.” Only four months after it opened, the bridge collapsed in dramatic fashion. In fact, this failure was so dramatic, that there’s a good chance you’ve seen this video before. So what’s happening here?

You’ve probably heard of resonance. This is the phenomenon where a periodic force syncs up with the natural frequency of a system. The classic example is a swing. With resonance, small periodic driving forces, like pushing someone in a swing, can add up to large oscillations over time because the energy is stored. In the case of wind-induced motion, the periodic driving force comes from an effect called vortex shedding. This is where a fluid flowing past a blunt object oscillates as vortices are formed on the backside. When these alternating zones of low pressure occur at a frequency near the natural frequency of the structure, even small amounts of wind can lead to major oscillations. This is why some chimneys are equipped with helical vanes to create turbulence and break up the vortices. The day of its failure, the Tacoma Narrows Bridge did experience resonance from the vortex shedding. You can see this in the vertical undulations for which the bridge was famous. But this resonance isn’t why it failed. About 45 minutes before failure, a different kind of oscillation started.

You can see in the historical footage that, right before failure, the bridge isn’t oscillating vertically, but in a twisting or torsional motion. The reason for this change in oscillation is still debated, but one of the best suggestions has has to do with the aerodynamics of the bridge. Rather than a truss through which wind can flow, this shape of the Tacoma Narrows Bridge with the large steel plates on either side created some strange interactions with the wind. Any amount of twist in the bridge created vortices, or areas of low pressure, in locations that actually amplify the twisting motion. As the bridge returned to its natural state, its momentum twisted it in the other direction where the wind could catch it and continue the twisting. This phenomenon is called aeroelastic flutter. It’s the same reason that a strap or sheet of paper vibrates in the wind. It’s a completely separate mechanism than resonance from vortex shedding, because the periodic forces are self induced from the naturally unstable aerodynamic shape of the bridge. This torsional flutter eventually created too much stress in the suspension cables, and the bridge failed.

One way that modern bridges avoid flutter is to include a gap in the center of the deck so that the pressures on either side can equalize. I cut a slot in my model, and sure enough the vibrations almost completely stopped. Another option is just to make the bridge deck more aerodynamic to avoid creating vortices that push and pull on the structure. Of course, bridges aren’t the only civil structures affected by the wind. Take a look at the very first Practical Engineering video about Tuned Mass Dampers to learn about how wind-induced motion can be mitigated in skyscrapers. For a simpler example, take a look outside at just about any high voltage power line. You might notice small devices hanging near the insulators at each pole. These are stockbridge dampers that help suppress wind-induced vibration on long cables and signs. And of course, other types of engineers contend with flutter as well. I’ve heard that airplanes are designed for wind loads, but I can’t confirm it.

These days, we have a much better understanding of the wide variety of loading conditions that can be faced by buildings and other structures. But, much of our current understanding has come from failures of the past. The case of the Tacoma Narrows bridge is a well-known cautionary tale that’s discussed in engineering and physics classrooms across the world. The main lesson isn’t necessarily that you should make sure to consider aeroelastic effect when you design a suspension bridge (even though you definitely always should), but I think more importantly it’s a reminder of how profoundly capable we are of making mistakes. When you push the envelope, you have to be vigilant because things that didn’t matter before start to become important. Unanticipated challenges are a cost of innovation and that’s something that we can all keep in mind. Thank you for watching, and let me know what you think.

Why Tunnels Don't Collapse

Tunnels play an important role in our constructed environment as passageways for mines, conveyance for utilities, and routes for transportation. But, excavating a tunnel underground in unstable material can lead to some dangerous situations, like the 2010 mining accident in Chile when 33 men were trapped deep in subsurface for more than 2 months. Hey I’m Grady, and this is Practical Engineering. On today’s episode we’re talking how engineers stabilize tunnel excavations to keep them from collapsing.

Rocks are heavy. That may seem self-evident, like many fundamental principles of civil engineering. But when you build things underground, it starts to become a major consideration. Just like atmospheric pressure is created by the weight of air molecules pressing down on each other, pressure exists in the subsurface of the Earth from the weight of the soil and rock above. This pressure compresses the material in the subsurface more and more the further down you go. Building a horizontal passageway, or a tunnel, through this material, interrupts the flow of these compressive forces. Just like if you remove a column from a building, excavating a tunnel takes away the support from the material above. Where you once had compression throughout the subsurface, now you’ve created a zone of tensile stress, where the material above the tunnel is trying to pull away from itself.

Many materials react differently to tension than they do to compression, and soil and rock are no different. You can imagine soil as a collection of individual particles. The only reason a soil mass has any strength at all is because of the friction between those particles. But friction is a function of the force pressing the particles together. So, if you instead reverse that force and pull the particles apart, the soil loses all its strength. Some soils, like clay, do have a certain amount of natural attraction between the particles, called cohesion, but it’s not enough on its own to resist significant forces. In other words, you can’t make a rope out of dirt - it has no strength against tension. If you build a tunnel in soil, you have to replace the support you removed with some other way to transfer the load of the soil above. This is why many tunnels are lined with materials like steel or concrete, to provide support to the tunnel walls and transfer the stresses in the subsurface around the tunnel. These lining systems add a major cost to the tunnel construction.

Rock, on the other hand, behaves a little bit differently in that it does have some tensile strength. You could make a rope from it. Not that it would be particularly useful, but it’s a good way to imagine the difference between soil and rock mechanics. In fact rock generally has more strength than soil for all types of stress. This additional strength gives rock the ability to transfer forces around a tunnel just like the lining discussed before. But, it’s not as simple as saying tunnels in soil require support and tunnels in rock don’t. Geologists use the term “massive” to describe rock that is uniform without layers or joints. Unfortunately, not all rock is massive. In fact, most geologic units of rock in the subsurface have at least some amount of jointing, or natural breaks. In many cases, the jointing of rock follows specific patterns that can be observed and mapped. But, the problem with joints is that they have no tensile strength, and so no ability to transfer tensile stress. You can see that jointed rock starts to behave more like a soil just with much larger particles. So, even tunnels through rock often require some type of support to prevent collapse.

But, what if there was a way to take advantage of the superior strength of rock without going to the added trouble and expense of lining the tunnel to provide support? Well it turns out there is. Rock bolts are a type of reinforcement for stabilizing rock excavations, usually made from steel bars or bolts. I built this demonstration to show how they work. This is essentially the frame of a table, but the top is completely open. I attached a bottom to the frame to represent temporary shoring of a tunnel roof. Even though our permanent support system doesn’t rely on this, it’s necessary until we get the rock bolts installed. My rock bolts are just actual bolts with large fender washers to spread out the load. You can see that I spaced them out in a nice grid pattern. Actual rock bolts are similarly installed in a pattern along a tunnel.

For the rock material of the tunnel roof, I’m using gravel. Of course, there are a few differences from the real world and my demonstration here. First, in the real world, the rock is there first. We don’t get the convenience of adding the rock after the tunnel is already in place. Real rock bolts are installed by drilling into the native material. The other difference is the scale. Although there isn’t a fine line between soil and rock mechanics, gravel really falls into the soil side. It would never be feasible to use this many rock bolts just to stabilize a gravel mass. Rock bolts are most feasible when you’re tunneling through jointed rock where you can put a little more space between the bolts, but this demo is just to show that it can be done.

To tension the rock bolts, I tightened washers and nuts onto each one. Another obvious difference between my demo and the real world is that we don’t normally having access to the top of the bolts to add nuts and washers. Instead, the rock bolts are secured at their ends by some other method. Two of the most common methods of anchoring are a wedge device and pumping in grout. It’s very similar to putting an anchor in concrete or even hanging a picture frame in drywall. Once the bolts were tensioned, it was time to remove the temporary bottom.

You can see I lost a little bit of gravel between the rock bolts, but the majority of the rock is spanning gap. I’ve essentially created a bridge made from gravel. But you know that supporting its own weight isn’t exciting enough for this channel. So I decided to put my own safety on the line as a test subject. The rockbolted gravel could support my weight, even with a few hops. You can see things flexing a bit underneath, but the simulated tunnel ceiling held strong. There are lot of ways to conceptualize what’s happening here. At the most basic level, the bolts are creating a continuous zone of compression in the gravel. I’ve taken a fractured rock mass and knitted it back together, giving it the ability to resist tensile stress. This is very similar to post-tensioned reinforcement used in some concrete structures.

Like I mentioned before, trying to support a gravel ceiling using rockbolts isn’t the most appropriate use. They do have their limitations. But, this simple construction method dramatically reduces the cost of making tunnels through rock safe from collapse. And public safety is priority number one for civil engineers. Do you have questions about tunnels or any other topic in engineering? If so, post it in the comments below. Thank you for watching and let me know what you think.

What is Fluid Cavitation?

If you subject a fluid to a sudden change in pressure, some interesting things can happen. You can cause tremendous damage to moving parts, or you can harness this destructive power in many beneficial ways. From mantis shrimp killing their prey to ultrasonic cleaning, so many things rely on this fluid phenomenon.

You might even call this video a treat especial, because this is the story what may be one of the most inept YouTube collaborations of all time, thanks to me. It all started with a sketch of a venturi. A venturi is a device that constricts the flow of a fluid to take advantage of Bernoulli’s principle. You may have heard of this principle, which basically says that all the energy in a fluid can take one of three forms: kinetic, potential, or internal energy. And the total amount of energy is the same along a streamline. So if you change one - for example you increase the kinetic energy of the fluid by speeding it up - the others have to accommodate - in this example, the fluid’s pressure goes down. Being able to lower the pressure of a fluid (also known as a vacuum) just by constricting the flow area makes a venturi a very useful tool that can be found in all kinds of devices from engines to trombones to scuba diving regulators.

So, I thought, I’d like to have one of these venturis, and I knew just the guy to make it for me. You may have heard of his YouTube channel: Arduino versus Evil, now cryptically shortened to AvE. We’ve never seen his face but we’re pretty sure he’s handsome. He and I had been emailing ideas across the U.S. - Canadian border, and this seemed perfect. I have a channel centered around practical demonstrations of engineering principles - he has a clapped out Bridgeport milling machine. It was a match made in YouTube heaven. So I sent that sketch over to AvE and said, “Could you make something like this.” And he said, “The drawings are never right. There are details left off. The guy doesn’t know his a** from his elbows.” But, he tried to make it anyway, providing us with many excellent lessons about manual machining. “There are three ways to do this…”

And in a second video, the prototype was finished, and we were left with these parting words: “If I was a betting man - and I am - I’d bet that this ain’t going to work.” And it didn’t. Or at least I have to assume it didn’t, because 10 months later I got this in the mail. Instead of giving me the hard truth - that my sketch was poorly considered and I wasted his weekend - he gave me something even better: a care package including a clear acrylic liquid flow meter that was designed by someone who knew what they were doing.

And, if you look closely at this flow meter, you might recognize the shape as a venturi, which is perfect, because I need a venturi to show you this fluid phenomenon. Here’s my setup: I have my garden hose running into the garage and a pressure boost pump feeding a manifold that connects to a pressure tank, a pressure gage, and this flow meter. I modified the meter so it acts like a venturi by gluing the weight to the center post so it can’t slide up and down. And I have a differential pressure gauge to measure the pressure drop across the venturi. The drop in pressure is the whole purpose of this demonstration. To understand why we need to look at the phase diagram of water.

We know that water changes state based on temperature. It’s a solid (ice) when it’s cold, a liquid at room temperature, and a gas (steam) when it’s hot. But, the phase of any substance also depends on the ambient pressure. You can see that, even at room temperature, water can turn to steam at very low pressures. This is true for a lot of liquids. If I force this water through a small enough opening in the venturi, according to Bernoulli, I’m decreasing the internal energy (aka the pressure) and converting it to kinetic energy (aka the flow velocity). And if I get the flow going extremely fast, I can decrease the pressure below the vapor pressure of the water, converting to steam.

Steam by itself isn’t a problem, but the issue comes when the pressure goes back up and the steam collapses back into a liquid. On a larger scale, this collapse can lead to thermal shock. Check out my video on the steam hammer to learn more. But, on a smaller scale, collapsing steam bubbles are called cavitation. And even though the scale is smaller, the damage cavitation can cause can be just as destructive. This is because collapsing steam causes water to speed up and decelerate violently. Water isn’t compressible, so it slams into itself creating a shockwave. It’s like a thousand tiny water hammers. Sometimes where cavitation is occurring, you even can hear these shockwaves, which often sound like gravel moving through a pipe. If I build up enough pressure in this tank and open the valve to the venturi, you can clearly see (and hear) the cavitation occurring. I can’t measure the pressure at the constriction of the venturi, which will be a very strong vacuum, but this gauge measures the total loss in pressure caused by the turbulence and cavitation, just for reference and because it looks cool.

Needless to say, in most cases, cavitation is bad news. It can erode pipes, impellers, and other moving parts, leading to accelerated wear or catastrophic failure. It can even cause damage to the spillways of very tall dams. So engineers generally avoid designs that might subject liquids to sudden changes in pressure. Pipes get smooth bends rather than abrupt changes in size or direction. Boat propellers and pump impellers are carefully designed to match with the speed and power of the motor to which they are attached. And dam spillways are designed to avoid any protrusions into the high-velocity flow.

However, although it is generally avoided in all kinds of industries, cavitation can also be a force for good. Ultrasonic cleaners use cavitation to agitate a solvent and break the strong bonds between contaminants and parts. Some industries use cavitation to mix compounds that are difficult to combine (like paints). Finally, some shrimp can move so quickly, they create a cavitation bubble to kill their prey. As for this flow meter, it seems to be holding up fairly well so far. The acrylic seems to be able to absorb the shockwaves better than metal would. So, it is probably best that our collaboration worked out the way it did. Thanks to AvE for supplying the demonstration for this video. If you like seeing the insides of tools and industrial machinery and don’t mind a little bit of language, check out his channel and tell him I sent you. Also, thank you for watching, and let me know what you think.

What are Cosmic Rays?

Every hour of every day, a thin cosmic rain of charged particles collides with the earth’s atmosphere, some of which eventually reaches the surface. Until recently, observing and measuring cosmic rays was the domain of physicists in fancy laboratories. But now, thanks to a group of scientists at MIT and the National Centre for Nuclear Research in Warsaw, even a dork in the garage like me can be a citizen particle physicist. As soon as I read about this project, I knew I had to build one.

Behold the CosmicWatch Desktop Muon Detector - or at least the pieces of one. This project was designed as an education tool for a “novice high school student,” so I’m a little bit outside of my skill level. But, who am I to deny you the incongruity a civil engineer soldering tiny components to a circuit board while talking about cosmic radiation? Before we get into the engineering behind the device, first we need to know a little bit about cosmic rays (or at least our current understanding of them, because there is still a lot of mystery behind their origin).

Spread throughout our galaxy, and indeed the entire universe, are stars. On occasion, those stars explode creating supernovae, and when they do, they eject a tremendous amount of interstellar material, also known as star stuff. This material is traveling so quickly that it generates a shock wave of superheated plasma. These shockwaves are believed to be the origin of most of the universe’s cosmic rays. The superheated plasma accelerates the particles to unimaginable speeds, and some eventually reach the earth. When they slam into the earth’s atmosphere, they produce a slew of secondary particles with crazy names like pions and kaons that eventually decay into muons that can survive the trip through the atmosphere and even penetrate into the earth’s crust. Scientists observe and measure muons and other cosmic radiation to learn more about the universe with fantastically complex and expensive equipment, but this detector opens that door to any student or citizen with a soldering iron and a good magnifying glass.

As fun as it is to dive into particle physics, the coolest part of the CosmicWatch project is the engineering. The device uses interesting components and clever circuity to make it possible to detect and count these cosmic rays. And it all starts with the scintillator, a piece of plastic with a very special ability to absorb the energy of a radiation particle and re-emit that energy as light. As a muon passes through the scintillator, a burst of light is created. It’s not enough to see with the naked eye, but it can be detected by the attached photomultiplier, which is essentially a super-sensitive solar panel capable of measuring even just a single photon. The photomultiplier converts the burst of light from the scintillator into an electric signal. But, this signal is extremely short - less than a microsecond - which is hard to detect. The CosmicWatch uses an Arduino nano to measure the signal, but it can only take a measurement about once every 6 microseconds. You can see how easy it would be for the Arduino to miss the Muon pulses.

So the CosmicWatch includes a peak detector circuit to amplify and stretch out the electrical pulse so that it can be detected by the Arduino. This shot from the oscilloscope shows the output from the peak detector in yellow. If I zoom in on the time scale, you can see how short the actual pulse from the photomultiplier was. Once the Arduino detects a pulse, it sends a signal to this LED to let you know. The Arduino can count the amplitude and number of pulses to measure the average detection rate, it can record each pulse on a memory card, and it can even send the data over USB to your computer. I built two detectors, which makes it possible to measure the direction of a particle and helps cut false triggers from other types of radiation. When operated in coincidence, a muon is only recorded if it was detected by both devices at the same time. In this shot, the bottom detector is the slave which only blinks if it detects a muon at the same time as the master above.

Now that the detectors are assembled and working, it’s time to do some science, and there is a lot of science that can be done here. This is such a great educational tool because the measurements are so simple. Most of the experiments you can do are really asking the same question: does this particular parameter affect the rate and or intensity of cosmic rays detected? Spencer published some very cool experiments he used to test out the detector, including how the rate changes at ground level vs. down in a mine and how much the detection rate increases during a flight on an airline. But, you know how much I like to make cool graphs, so I also designed a few of my own experiments to test out.

First I know that Muon formation happens in the atmosphere, and I also know that some atmospheric properties like temperature and stability change through the course of the day. So I hypothesized that there might be a measurable difference in muon detection between day and night. To test this out, I left the detectors running in the same spot for 24 hours. I started the count at 6:30 am when I left for work and reset at 8:00 pm to leave it overnight. The rates and measured intensities were almost identical, suggesting that, if there is a difference in detection rate between day and night, it is only a small effect. The null hypothesis prevails.

Next, I wanted to test how the direction affected the detection rate. You can leave these detectors blinking on your desk, but it’s still hard to imagine the cosmic rays passing through your personal space if you don’t know what direction they’re coming from. My guess was that most would come from directly overhead because it’s the most direct path through the atmosphere. I set up the detectors one on top of each other for a day, then side by side for the second day. My results agreed with Spencer’s that the detection rate from side to side was about half of that from straight up. This chart shows the probability that a muon would exceed a certain amplitude, and you can see that the measurements from the horizon had more low-energy detections than from straight overhead.

My last experiment, obviously, needed to be related to concrete because I said I was going to keep making videos about concrete and then made a muon detector instead. My hypothesis was that layers of concrete would provide some shielding and attenuate the detection rate. So, I left the detectors in coincidence mode running in my car and parked on a different level of the parking garage at work for three days. Measuring only the particles coming from straight up, there was a small but obvious reduction in the detection rate for each layer of concrete in the parking garage above my car.

I love this project because it takes something that is not just invisible, but may be unknown to most people and makes it so tangible and approachable. If you’re an educator, this is an awesome tool for exploring the scientific method because the experimental design is fairly simple, the data collection is easy, and the subject matter is fascinating. More advanced students may even be able to develop experiments related to time dilation and special relativity. Huge thanks to Spencer and the other folks associated with CosmicWatch who developed this awesome device and helped me with this video. Check out the link to their website in the description. Thank you for watching, and let me know what you think!

Why Does Concrete Need Reinforcement?

In the last video, we talked about concrete 101, and why concrete is such a great construction material. But, I didn’t mention its greatest weakness.

To understand concrete’s greatest weakness, first, we need to know a little bit about the mechanics of materials which is the fancy way of saying “How Materials Behave Under Stress.” Stress, in this case, is not referring to anxiety or existential dread but rather the internal forces of the material. There are three fundamental types of stress: compression (pushing together), tension (pulling apart), and shear (sliding along a line or plane). And, not all materials can resist each type of stress equally. It turns out that concrete is very strong in compression but very weak in tension. But, you don’t have to take my word for it. Here’s a demonstration:

These two concrete cylinders were cast from the exact same batch, and we’ll see how much load they can withstand before failure. First, the compressive test. (Hand pump gag). Under compression, the cylinder broke at a load of about 1000 lb (that’s 450 kilos). For concrete, that’s pretty low because I included a lot of water in this mix. The reason is my rig to test the tensile strength isn’t quite as sophisticated. I cast some eye bolts into this sample, and now I’m hanging it from the rafters in the shop. I filled up this bucket with gravel, but it wasn’t quite enough weight to fail the sample. So, I added another dumbbell to push it over the edge. The weight of this bucket was only about 80 lbs or 36 kilos - that’s less than 10% of the compressive strength.

All this to say, you shouldn’t make a rope out of concrete. In fact, without some way to fix this weakness to tensile stress, you shouldn’t make any kind of structural member out of concrete, because rarely does a structural member experience just compression. In reality, almost all structures experience a mixture of stresses. That’s no more clear than in a classic beam. This particular classic beam is homemade by me out of pure concrete here in my garage. Applying a force on this beam causes internal stresses to develop, and here’s what they look like: the top of the beam experiences compressive stress. And the bottom of the beam experiences tensile stress. You can probably guess where the failure is going to occur on this concrete beam as I continue to increase the load. It happens almost instantly, but you can see that the crack forms on the bottom of the beam, where tensile stress is highest and propagates upward until the beam fails.

You see what I’m getting at here: concrete, on its own, does not make a good structural material. There are just too many sources of tension that it can’t resist by itself. So, in most situations, we add reinforcement to improve its strength. Reinforcement within concrete creates a composite material, with the concrete providing strength against compressive stress while the reinforcement provides strength against tensile stress. And, the most common type of reinforcement used in concrete is deformed steel, more commonly known as rebar.

I made a new beam with a couple of steel threaded rods cast into the lower part of the concrete. These threads should act just like the deformed ridges in normal rebar to create some grip between the concrete and steel. Under the press, the first thing you notice is that this beam is much stronger than the previous one. We’re already well above the force that failed the un-reinforced sample. But the second thing you notice is that the failure happens a little bit slower. You can easily see the crack forming and propagating before the beam fails. This is actually a very important part of reinforcing concrete with steel. It changes the type of failure from a brittle mode, where there’s no warning that anything is wrong, to a ductile mode, where you see the cracks forming before a complete loss of strength. This gives you a chance to recognize a potential catastrophe and hopefully address it before it occurs.

Rebar works great for most reinforcement situations. It’s relatively cheap, well-tested, and understood. But it does have a few disadvantages, one of the major one being that it is a passive reinforcement. Steel lengthens with stress, so rebar can’t start working to help resist tension until it’s had a chance to stretch out. Often that means that the concrete has to crack before the rebar can take up any of the tensile stress of the member. Cracking of concrete isn’t necessarily bad - after all, we’re only asking the concrete to resist compressive forces, which it can do just fine with cracks. But there are some cases where you want to avoid cracks or the excessive deflection that can come from passive rebar. For those cases, you might consider going to an active reinforcement, also known as pre-stressed concrete.

Prestressing means applying a stress to the reinforcement before the concrete is placed into service. One way to do this is to put tension on the steel reinforcement tendons as the concrete is cast. Once the concrete cures, the tension will remain inside, transferring a compressive stress to the concrete through friction with the reinforcement. Most concrete bridge beams are prestressed in this way. Check out all that reinforcement in the bottom of this beam. Another way to prestress reinforcement is called post-tensioning. In this method, the stress in the reinforcement is developed after the concrete has cured. For this next sample, I cast plastic sleeves into the concrete. The steel rods can slide smoothly in these sleeves. Once the beam cured, I tightened nuts onto the rods to tension them. Under the press, this beam wasn’t any stronger than the conventionally reinforced beam, but it did take more pressure before the cracks formed. Also, this one wasn’t quite as dramatic because instead of failing the actual steel rods, it was the threads on the nuts that failed first.

I hope these demonstrations helped show why reinforcement is necessary for most applications of concrete - to add tensile strength and to change the failure mode from brittle to ductile. Just like the last video, I’m just scratching the surface of a very complicated and detailed topic. Many engineers spend their entire career studying and designing reinforced concrete structures. But, I’m having some fun playing with concrete and I hope you are finding it interesting. I’d love to continue this series on concrete, so if you have questions on the topic, post them in the comments below. Maybe I can answer them in the next video. Thank you for watching, and let me know what you think!

What is Concrete?

Concrete is as much a part of the urban landscape as trees are to a forest. It’s so ubiquitous that we rarely even give it any regard at all. But, underneath that drab grey exterior is a hidden world of complexity.

Concrete is one of the most versatile and widely-used construction materials on earth. It’s strong, durable, low maintenance, fire resistant, simple to use, and can be made to fit any size or shape - from the unfathomably massive to the humble stepping stone. However, none of those other advantages would matter without this: it’s cheap. Compared to other materials, concrete is a bargain. And, it’s easy to see why if we look at what it’s made of. Concrete has four primary ingredients: Water, sand (also called fine aggregate), gravel (aka coarse aggregate), and cement. A recipe that is not quite a paragon of sophistication. One ingredient falls from the sky, and the rest come essentially straight out of the ground. But, from these humble beginnings are born essentially the basis of the entire world’s infrastructure.

Actually, of the 4, cement is the only ingredient in concrete with any complexity at all. The most common type used in concrete is known as Portland cement. It’s made by putting quarried materials (mainly limestone) into a kiln, then grinding them into a fine powder with a few extra herbs and spices. Cement is a key constituent in a whole host of construction materials, including grout, mortar, stucco, and of course, concrete. A lot of people don’t know this, but every time you say cement when you were actually talking about concrete, a civil engineer’s calculator runs out of batteries.

I’m just kidding of course, and you an hardly be blamed for not knowing the difference if you’ve never mixed up a batch of concrete before. Even if you have mixed some concrete, good chance it was in a ready-mixed bag where all the ingredients were already portioned together. But, each ingredient in concrete has a specific role to play, and cement’s role is to turn the concrete from a liquid to a solid. Portland cement cures not through drying or evaporation of the water, but through a chemical reaction called hydration. The water actually becomes a part of the cured concrete. This is why you shouldn’t let the concrete dry out while it’s curing. Lack of water can prematurely stop the hydration process, preventing the concrete from reaching its full strength. In fact, as long as you avoid washing out the cement, concrete made with Portland cement can be placed and cured completely under water. It will set and harden just as well (and maybe even better) as if it were placed in the dry.

But, you may be wondering, “If water plus cement equals hard, what’s the need for the aggregate?” To answer that question, let’s take a closer look by cutting this sample through with a diamond blade. Under a macro lens, it starts to become obvious how the individual constituents contribute to the concrete. Notice how the cement paste filled the gaps between the fine and coarse aggregate. It serves as a binder, holding the other ingredients together. You don’t build structures from pure cement the same way you don’t build furniture exclusively out of wood glue. Instead, we use cheaper filler materials - gravel and sand - to make up the bulk of concrete’s volume. This saves cost, but the aggregates also improve the structural properties of the concrete by increasing the strength and reducing the amount of shrinkage as the concrete cures.

The reason that civil engineers and concrete professionals need to be pedantic about the difference between cement and concrete is this: even though the fundamental recipe for concrete is fairly simple with its four ingredients, there is a tremendous amount of complexity involved in selecting the exact quantities and characteristics of those ingredients. In fact, the process of developing a specific concrete formula is called mix design. And I love that terminology because it communicates just how much effort can go into developing a concrete formula that has the traits and characteristics needed for a specific application. One of the most obvious knobs that you can turn on a mix design is how much water is included. Obviously, the more water you add to your concrete, the easier it flows into the forms. This can make a big difference to the people who are placing it. But, this added workability comes at a cost to the concrete’s strength.

To demonstrate this balancing act, I’m mixing up some ready-mix concrete with different amounts of water. For the first sample, I’m using just enough water to wet the mix. You can see it’s extremely dry. A mix like this is certainly not going to flow very easily into any forms, but you can compact it into place. In fact, dry concrete mixes like this are used in roller-compacted concrete which is a common material in the construction of dams. For the next three samples, I used increasing amounts of water up to what is pretty much concrete soup. After the concrete has had a week to cure, I cut the samples out of the molds. It’s time to see how strong it is.

This is actually more or less how concrete is tested for compressive strength in construction projects. Obviously, I’m not running a testing lab here in my garage, but I think this will give us good enough results to illustrate how water content affects concrete strength, plus these cylinders look like they might attack at any time, and we need to deal with them. I made three cylinders of each mix, and I’ll break each one, watching how much pressure the cylinder was applying at the moment of failure. And this experiment was too cool not to invite my neighbors over to help.

We started with the samples that used the most water. It was no surprise that it took almost no pressure at all to break them, on average about 700 psi or 5 MPa. You can see how crumbly the concrete is even after having a week to cure. All that water just diluted the cement paste too much. The next two samples used the range of water suggested on the premixed concrete bag. These were much stronger, breaking at an average of 1600 psi and 2200 psi or 11 MPa and 15 MPa for the high and low end of the water content range. And you can really see the difference in how the concrete breaks. Finally, we broke the samples with the least water added to the mix. You can see how rough these samples were because there wasn’t enough water for the concrete to flow smoothly into the molds. But, despite looking the worst of the four, these were the strongest samples of all, breaking at an average of around 3,000 psi or 20 MPa. On this shot, you can even see the crack propagating through the cylinder before it fails. It just goes to show how important mix design can be to the properties of concrete. Even varying the water content by a small amount can have a major impact on strength, not to mention the workability, and even the finished appearance of the concrete.

It’s impossible to state how much I am just scratching the surface here. There is so much complexity to the topic of concrete partly because it has so many applications: from skyscrapers to canoes and everything in between. In fact, no matter where you are, you’re rarely more than a few feet from concrete - a fact that is inexplicably a source of great comfort to me. But, I took less than 10 minutes to describe what is literally the foundation of our modern society. So I’m dedicating at least the next few videos to dive deeper into the topic of concrete. The next video will be about its greatest weakness. If you’ve got questions about concrete, put them down below in the comments and maybe I can get them incorporated into the next videos. Thank you for watching, and let me know what you think!

Why do Bridges Move?

We often think of civil engineers as designers of static structures, or things that don’t move. That would be nice, but the reality is that everything moves for one reason or another, and one of those reasons happens to be temperature.

Whether you realize or not, you’re probably already familiar with thermal expansion, which is the property of materials to change their volume depending on temperature. If you’ve used a glass thermometer, you’ve even taken advantage of it. The liquid in a thermometer, usually mercury or alcohol, increases in volume as it heats up. Since we can characterize this expansion, we can put it to use as a measuring device. Maybe you’ve had some experience with a less useful application of thermal expansion. If you’ve ever put glass dishware on a burner or poured cold water on a dish that just came out of the oven, you know it doesn’t go so well. If it’s allowed to expand and contract evenly, stresses in the material don’t build up. But, if you heat or cool it unevenly, certain parts of the glass will fight against each other as they change size. Glass isn’t flexible, so instead of bending, it just shatters.

Thermal movement is something that has to be considered in nearly every field of engineering because there aren’t many places that don’t see fluctuations in temperature. And there are really only two options when designing for thermal expansion: The first one is to prevent the movement by constraining it, a feat that is almost always impractical. Thermal movement can generate tremendous amounts of internal stress. Watch how this wire can lift up a weight just by heating it up, then cooling it down. So, the other way to accommodate thermal movement is just to allow your design the freedom to move as it so desires. But sometimes that’s easier said than done, especially for large civil structures.

This is the formula for thermal expansion. It may look complicated, but it’s really not. It essentially says that the change in size of anything is a linear function of temperature proportional to its length. And the slope of this line is the coefficient of thermal expansion. We’ve measured this property for a whole host of materials, and you can look up tables online. Lucky for civil engineers, the thermal expansion coefficients for steel and concrete are nearly identical, which is why we can combine them into the ubiquitous construction material, reinforced concrete, without worrying too much about fluctuations in temperature pulling it apart. But, even though they expand and contract at the same rate, they still expand and contract. A perfect example of this is a sidewalk.

Let’s look back at our formula and plug in some numbers for a very typical situation. If we enter values for the average length of a city block, the average high and low temperatures in a given year, and the thermal expansion coefficient for concrete, we can see that the total movement of a sidewalk over the course of a year can be upwards of 4 cm or 1.5 inches. Obviously, you can’t leave a gap in the sidewalk that big at the end of every block, so instead, we leave small gaps spaced every so often to accommodate that movement. Most of the joints you see along a sidewalk are just to control cracking, but if you pay attention, every so often you’ll see an actual break in the concrete filled with some kind of flexible material. These are expansion joints that give the walkway the freedom to move from fluctuations in temperature.

But what about structures that are longer than a city block? Thermal movement scales with length, so engineers need to take a lot more care with linear infrastructure. Long runs of pipe, especially if they experience fluctuating temperatures, need expansions joints to prevent damage. The rails for trains can experience “sun kink” where a hot day can actually buckle the steel. One of the biggest challenges for thermal movement is on bridges. So I built a little model to show why.

Unlike sidewalks that can have periodic expansion joints, bridges only have support between spans. You can’t have an unsupported break in the bridge, so that means all of the thermal movement happens at the supports. The allowance can’t be evenly distributed across the length; it happens all in one spot. For bridges with very long spans, that can be a lot of movement. I’ve got my bridge set up with one side pinned and one side free to move. I’ll fire up the sunny day simulator and watch what happens on the dial indicator. The bridge expands along its length as it heats up. This is exactly what happens in real life. Now I’ll try to pin both sides of the bridge so that the movement is constrained. The bridge still expands as it heats up, but now it has to expand in directions it wasn’t meant to go. It’s a little bit hard to show on camera, but the entire bridge has buckled side to side. I’m using a flexible rod for this demo, but if a real bridge was constrained like this, the forces generated by thermal expansion would probably lead to failure of the structural members.

Expansion joints on bridges not only have to allow the bridge to move while still being supported, they also have to bridge the gap in the road deck so that cars can safely drive over it. So, if you look closely, you’ll see lots of creative ways engineers manage the thermal expansion. These are some photos collected from the web and sent to me by viewers of different bridge bearings that allow thermal movement. My inspiration to make this video came when I was looking through some vacation photos. Take a close look at this steel catwalk over the river. The shorter, cantilevered beams are welded directly to their anchor plates. They’re free to move because they’re only connected to the rock on one side. But look at the beams that stretch across both sides of the canyon. At first glance, it seems like they’re constrained on both sides and we know that’s bad engineering. But if you look closely, you can see that they’re bolted to the anchor plate using slotted holes to allow the beams to expand and contract. I hope this video gave you a little more insight into the dynamic nature of structures we normally consider static. Keep your eyes out and you’ll notice allowance for thermal movement everywhere you look. Thank you for watching, and let me know what you think.

How to Collapse a Pipe by Closing a Valve

In the last two videos, we’ve looked at phenomena that cause high-pressure spikes in pipes. But a lot of people pointed out that very low pressure in pipes can be just as dangerous.

If you watched the water hammer video I made a few months back, you’ll know that slamming a valve shut on a flowing pipe can cause a huge spike in pressure. That’s because the fluid inside a pipe has a lot of momentum, and fluids aren’t compressible enough to absorb sudden changes in velocity. Spikes in pressure aren’t always bad, but they can be dangerous if a pipe bursts or expensive by requiring stronger pipes with higher pressure ratings.

But in that video, I didn’t talk about what happens on the other side of the valve. So, I’m revisiting that demonstration with a few modifications so we can get the full picture. Here’s the setup: valve, clear pipe, pressure gage, more clear pipe, 50-foot garden hose, tree. The tree’s not important but I don’t want anyone to think I’m wasting this water. You won’t be surprised to learn that flowing fluid in a pipe downstream of a valve also has momentum, and that fluid also has a hard time stopping without a big fluctuation in pressure. But, unlike upstream where the momentum is carrying the fluid toward the valve, on the downstream side, the fluid is trying to flow away from it. So, the spike in pressure is negative - in other words, it creates a vacuum.

You may have noticed something different about this pressure gage. It only measures pressures that are below atmospheric - it’s a vacuum gauge. Watch what happens when I slam this valve shut. We get a very strong vacuum in the pipe, and then some fluctuations as the pressure wave propagates back and forth through the pipe. The momentum of the fluid in the water hose is pulling away from the valve. That fluid tension sharply lowers the pressure in the pipe. This trapped bubble gives a pretty good indicator of what’s happening as well. This is pretty far from a laboratory setting (no offense to the backyard scientist), but I’m seeing a peak of more than 30 inches of mercury or 100 kilo-pascals below atmospheric pressure. That’s a lot of vacuum. In fact, it’s enough to pull dissolved gas out of the water.

Take a look at the spot just downstream of the valve when I slam it shut. A spontaneous cloud of fine bubbles forms as the vacuum pulls. This is dissolved gases coming out of solution with the water. When the pressure returns, the bubbles shrink, but they don’t immediately go back into solution with the water, so you can still see a light haze in the water, especially when I turn the valve back on. Very cool in this demonstration, but bad news if your pipe wasn’t designed to withstand these types of pressures. Just like positive pressure spikes from water hammer, this phenomenon has caused numerous failures of pipe systems from implosions due to vacuum.

So, how can this be avoided? If the risk of failure is significant, like for very large pipelines or costly equipment, engineers will specify vacuum relief valves that will allow air into the pipe if the pressure gets too low, reducing the vacuum to protect the equipment. But, the simplest solution is the same as discussed in the other water hammer video: avoid sudden changes in velocity. Ask any firefighter and they’ll tell you: you gotta close valves slowly. You still get a vacuum downstream, but much less of one. Hope you liked this quick follow up. Thank you for watching, and let me know what you think!

What is a Steam Hammer?

Last month we talked about the damaging effects of water hammer, but there’s another state of H2O equally if not more dangerous when put in pipes.

Unless you live in a home with an older radiator or work in certain industrial settings, you probably aren’t as familiar with pipes that carry steam as those that carry water. We don’t normally need access to steam in our everyday lives like we do to its liquid analog. That’s not to say, though, that we don’t rely on steam. In fact, it plays a critical role in our modern society. We use steam for heating, cleaning, cooking, and a vast array of industrial processes. About 90 percent of all electric power produced in the world is through the use of steam turbines.

If you didn’t see my previous video about water hammer, here are the basics: water is heavy and incompressible. If you suddenly stop water while it’s moving through a pipe, it can create a massive spike in pressure and break stuff like this pressure gauge. Unlike water, steam is compressible. It’s “springy” and can absorb sudden changes in velocity without a big change in pressure. The danger with steam is when it doesn’t want to be steam anymore. In most places on earth, water exists naturally as a liquid. Under the ambient temperature and pressure conditions we consider habitable, most steam that happens to exist will condense. In a steam pipe, the water that forms from condensation (also known as condensate) is the real danger. And I mean danger in the truest sense of the word. Many lives have been lost in tragic accidents resulting from misunderstanding or misapplication of good engineering principles for steam systems. There are several problems that condensate can create, and we’ll talk about two of them in this video. The first one is "thermal shock".

Imagine this: you open a valve allowing steam to flow into a steel pipe. As the steam comes into contact with that cold steel, it condenses. The problem is that steam takes up about 1600 times more volume than its equivalent mass as a liquid. So, when it condenses, it shrinks. In a closed container like a pipe or this glass bottle that just came out of my microwave, that collapsing steam can lead to catastrophic damage. Water rushes to fill the vacuum created by condensation, cooling the steam even further and creating a runaway situation. This can happen extremely fast, and all that water can accelerate and decelerate violently, hence the name steam hammer. If it’s violent enough, it can rupture the pipe leading to an explosion like the one that happened in New York City in 2007. Check out Nick Moore’s video linked below if you want to see this demo in slow motion.

A thermal shock is a dangerous form of a steam hammer, but it’s easy to mitigate. When starting up a steam system, engineers and operators expect condensation as the pipes warm up. So start-up procedures will include running at reduced pressure with bleed valves open to make sure that condensation can’t form a vacuum. The bigger danger happens during normal operations, but to show how it works, first, we need a steam pipe.

Condensation in a steam pipe is always occurring just from the normal transfer of heat to the outside air. And this is roughly what that might look like. I’m using compressed air here in lieu of steam for the obvious safety implications. Engineers manage this condensate by sloping steam pipes and by installing devices that can get rid of condensate from the pipes called steam traps. Steam traps are a fascinating topic on their own, but occasionally they can get clogged or malfunction, allowing condensate to build up.

When water and steam flow together in the same pipe, it’s known as biphase flow. In this situation, the velocity of the steam is usually much higher than the velocity of the flowing liquid water. If there’s only a little bit of condensate in the pipe, that’s really not a big issue. But, if condensate is accidentally allowed to pool up, things can get dangerous. The steam passing over the top of the liquid can create turbulence and waves. If those waves get high enough, the liquid can create a complete seal inside the pipe with the full pressure of the steam behind it. This seal of water becomes a slug or piston and accelerates down the pipe like a barrel of a cannon, picking up more condensate as it travels. This slug of liquid eventually slams into the end of the pipe, resulting in a dangerous pressure spike known as differential shock. Just like thermal shock, many people have tragically lost their lives in steam pipe explosions caused by this phenomenon.

Engineering of steam systems is an incredibly complex topic in mechanical and chemical engineering, and I’ve just scratched the surface in this video. Whether you realize it or not, many of our modern conveniences are a direct result of steam systems, most notably electricity. So it’s critical that engineers can design steam systems to be safe from dangerous phenomena, including thermal and differential shock, also known as a steam hammer. Thank you for watching, and let me know what you think.

What is a Water Hammer?

You might know that most liquids are incompressible (or least barely-compressible), which means no matter how much pressure you apply, their volume doesn’t change. This can be useful, like in hydraulic cylinders, but that lack of “springiness” can also lead to catastrophic failure of pipe systems.

It’s easy to forget how heavy water is since we hardly ever carry more than a few ounces at a time. But if you add up the water in the pipelines of your city or even the pipes in your house, it makes up quite a bit of mass. And, when all that water is moving through a pipe, it has quite a bit of momentum. If you suddenly stop that movement—for example, by quickly closing a valve—all that momentum has nowhere to go. Since water isn’t compressible or springy, it can’t soften the blow. You might as well be slamming concrete into the back of the valve and the walls of your pipe. Instead of being absorbed, that sudden change in momentum creates a spike in pressure that travels as a shockwave through the pipe. Sometimes, you’ll even hear this shockwave as banging in your walls when you close a faucet or run the washing machine, hence the superhero-esque nickname, Water Hammer.

Banging pipes inside your walls can sound a bit spooky, but for large diameter pipelines that can be hundreds of kilometers long, that surge in pressure from a change in momentum can cause major damage. Let’s do a quick calculation: if you have pipeline carrying water that is 1 meter in diameter and runs for 100 kilometers (a fairly average-sized pipeline), the mass of water in the pipe is about 80 million kilograms. That’s a lot of kilograms. In fact, that's about 10 freight trains. Imagine you’re an operator at the end of this pipeline in charge of closing a valve. If you close it in a short amount of time, you’ve basically slammed those trains into a brick wall. And the pressure spike that results from such a sudden change in momentum can rupture the pipe or cause serious damage to other parts of the system. There’s actually another term for when a large spike in pressure ruptures a sealed container: a bomb. And water hammer can be equally dangerous. So, how do engineers design pipe systems to avoid this condition? Let’s build a model pipeline and find out. [Construction montage].

Here’s my setup. I’ve got about 100 feet (30 meters) of PVC pipe connected to the water on one end and a valve on the other. I also have an analog and digital gauge so we can see how the pressure changes and a clear section of pipe in case anything exciting happens in there. I mean civil-engineering-exciting, not like actual exciting. Watch what happens when I close this valve. It doesn’t look like much from the outside but let's look at the data from the pressure gauge. The pressure spikes to over 2,000 kilopascals or 300 psi. That’s about 5 times the static water pressure. It’s not enough to break the pipe, but way more than enough to break this pressure gage. You can see why designing a pipeline or pipe network can be a little more complicated than it seems. These spikes in pressure can travel through a system in complicated ways. But we can use this simple demonstration to show a few ways that engineers mitigate the potential damage from water hammer.

This is the equation for the pressure profile of a water hammer pulse. We’re not going to do any calculus here, but the terms of this equation show the parameters that can be adjusted to dial back these damaging forces. And, the first one is obvious: it’s the speed at which the fluid is moving through the pipe. Reducing this is one of the simplest ways to reduce the effect of water hammer. Velocity is a function of the flow rate and the size of the pipe. If you’re designing a pipeline, the flow rate might be fixed, so you can increase the size of your pipe to reduce the velocity. A smaller pipe may be less expensive, but the flow velocity will be higher which may cause issues with water hammer. In this case, my pipe size is fixed, but I can reduce the flow rate to limit the velocity. Each time I reduce the velocity and close the valve, the resulting spike in pressure decreases.

Next, you can increase the time over which the change in momentum occurs. One common example of this is adding flywheels to pumps so they spin down more slowly rather than stopping suddenly. Another example is to close valves more slowly. If I gently shut the valve rather than allowing it to snap shut, the pressure changes are more subtle. On large pipelines, engineers design the components and develop the requirements for operation of the equipment. This will almost always include rules for how quickly valves can be opened or closed to avoid issues with water hammer.

The final parameter we can adjust is the speed of sound through the fluid, also known as the wave celerity. This describes how quickly a pressure wave can propagate through the pipe. The wave celerity is an indirect measure of the elasticity of the system, and it can depend on the compressibility of the fluid, the material of the pipe and even if it’s buried in the ground. In a very rigid system, pressure waves can reflect easily without much attenuation. I’ve got flexible PVC pipe sitting on the ground free to move which is already helping reduce force of the water hammer. I can increase the flexibility even more by adding an anti-surge device. This has an air bladder that can absorb some of the shocks and reduce the pressure spike even further. Anti-surge devices are very common in pipe systems, and they can be as simple as a spring-loaded valve that opens up if the pressure gets too high. In water distribution systems for urban areas, water towers help with surge control by allowing the free surface to move up and down, absorbing sudden changes in pressure.

Plumbing is one of the under-acknowledged innovations that has made our modern society possible. When you harness the power of water by putting it in pipes, it’s easy to forget about that power. Water can be as hard as concrete when confined, and if you bang two hard things together, eventually something’s going to break. If you’re an engineer, your job is to make sure it’s not the expensive infrastructure you designed. Part of that means being able to predict surges in pressure due to water hammer and design systems that can mitigate any potential damage that might result. Thank you for watching, and let me know what you think!

How Soil Destroys Buildings

When most people think of property damage, they think about natural disasters. But what if I told you, there’s a slow-moving geologic phenomenon that causes more damage in the United States than earthquakes, floods, hurricanes, and tornadoes combined.

If you’ve ever been to a place where the ground looks like this, or if you’ve been in a building that looks like this or this, there’s a good chance you were in a place that had expansive soils. Just like these dinosaur toys, certain types of clay soils change their volume depending on moisture content. They swell when they get wet, and shrink as they dry. This is a microscopic mechanism where the shape and arrangement of the molecules actually change according to the amount of water mixed in. And, large portions of the U.S. Gulf coast and great plains have these kinds of soils. If you’re starting a foundation repair or road paving business, this is an important map for one very important reason: expansive soils break stuff.

Movement on its own and especially very slow movement is usually not a problem for structures. This is why we can lift buildings and even move them to new locations. What causes damage is differential movement. This is where certain parts of a structured move relative to each other. Differential movement leads to sticking doors and windows, cracked walls, and just general out-of-plumpness. And this is why expansive soils are so insidious because they don’t expand and contract evenly. For example, if your house sits on a concrete slab and you haven’t had any rain, the soils around the edges of the slab that are more exposed will dry out and shrink while the interior remains moist. Now you’ve got a foundation with no support around the edges. This breaks one of the fundamental laws of civil engineering, which says, and I quote, “You gotta have dirt underneath your concrete.”

Expansive clay isn’t just an issue for buildings. All kinds of infrastructure are at risk of damage from a shifting foundation. Leaking pipes can cause swelling of the soil, pulling apart joints and eventually leading to issues like sinkholes. Rainwater infiltrating through the cracks in roadways causes localized areas of swelling. This makes the road bumpy and uneven. Not even sidewalks, and by proxy rollerbladers, are spared. When designing to account for expansive clays, engineers not only have to know how much the soil can change in volume but also how hard it can push on anything sitting above, also known as swell pressure. So I’ve rigged up a little test so that we can see not only how soil swells, but also how much pressure it can exert. This apparatus called an odometer. It’s similar to a hydraulic cylinder, except I’m using dirt instead of oil, and I’ll use a dial indicator to measure how far the sample is able to move the piston. If you work in a soil laboratory, I’ll just apologize now for the rest of this video.

For my first test, I’ve got some soil straight from my own backyard. After all, there’s no place like a geologic unit containing abundant clay with high swelling potential. I put this in the oven to dry it out first, don’t tell my wife. Just kidding she knows whom she married. Now let’s put it in the apparatus and watch what happens. As it saturates, the soil expands over time, eventually reaching a 10% increase in volume over its dry state. Trust me, that’s enough to put a crack in the drywall. But, it’s really not that dramatic on video. So, to help illustrate these concepts a little better, I’ve got a bag of the instant viral video. That’s right I’m talking about Superabsorbent Polymer Beads, also known as Orbeez. These beads behave very similarly to expansive soils, except they’re way cooler than dirt in almost every way, even for a civil engineer.

First I tested these with no confining pressure and went a bit overboard. You can imagine if you built a house on this, you might get motion sickness every time it rains. It would wreak havoc on your structure. I tried it again with fewer orbeez, but it was still too much. This is an exaggerated view of what happens as water penetrates the subsurface and saturates an expansive soil. It’s hard to imagine anything that could avoid damage in this environment. So, let’s add some weight - and fewer orbeez this time so I don’t max out the range of my dial indicator. You can see that these fishing weights hardly make a difference. And that makes sense, right? A house probably puts more pressure on the ground below it than a few fishing weights. What about ten times that weight? It takes them a lot longer, but the orbeez are still able to swell to their full dimensions under this 20lb barbell, which is about the most my little acrylic oedometer can handle.

This is not just the case for orbeez by the way. Some clay soils have swell pressures on the order of megapascals (that’s hundreds of pounds per square inch). So you can see how big of a challenge these expansive soils can pose. There are lots of ways that engineers try to mitigate damage from these kinds of soils. You can simply remove all the expansive clay and bring in better soils for your project. You can grade the site so that water drains away from your structure, keeping moisture fluctuations down. You mix chemicals into the soil that limit its ability to absorb water. Finally, you can simply to build heavy enough to counteract the swell pressure and keep the soil from expanding. But as we saw in the demonstration, even a small amount of soil or in this case a colorful soil surrogate can lift a lot of weight.

I’m leaving out the simplest solution, which is simply to avoid expansive soils because it’s generally not feasible. It may be true in the parable that the wise man built his house on rock, but some civil engineer had to build a road to that guy’s house, and the engineer didn’t get to choose what kind of soil was on the way. Expansive soils are not a particularly newsworthy or exciting hazard (unless you’re the type of person who makes videos about dirt in your garage), but they still cause a tremendous amount of damage to buildings and the public infrastructure we rely on every day. They are one of the many factors taken into account when designing civil structures and the subject of ongoing research to find cost-effective and sustainable practices for mitigating the damage they cause. Thank you for watching, and let me know what you think!

How Quicksand Causes Dam Failures

When you think about quicksand, you probably don’t feel much fear or uncertainty. What was once a popular plot device in island and jungle movies of the 50s through the 80s has begun to fade from our collective curiosity. But, in civil engineering, quicksand is more than just a puddle of mud. It has a specific, and potentially more catastrophic, definition that can affect all kinds of infrastructure.

If you ever tried to build a dam in a creek or even in the storm gutter on your street, you know how hard it is. Water is not only powerful, but it’s elusive too. It seems to always find a way through. Like all fluids, water flows from areas of high pressure to low pressure, and a dam is a structure that separates those two conditions. This is a prime circumstance to induce flow, whether it’s through the dam itself, or underneath (through its foundation). Water flowing through soil is called seepage, and it follows some interesting and somewhat unexpected rules. So, to give you a better understanding, I’m building this demonstration out of clear acrylic sheets and some plumbing fixtures. This will essentially let us see a cross section through a dam so we can observe how the seepage behaves.

All dams have some seepage, so engineers need to be able to characterize it. Flow through soil follows Darcy’s Law, which is simple to understand, but complicated to calculate in two dimensions. Before computer models, engineers used an analytical tool called a flow net to estimate the behavior of seepage. Now software can do the work much more quickly and accurately. Darcy’s law says that the rate of seepage depends on the length of the flow path and the difference in pressure across it. The combination of these two factors is called the gradient. If the difference in pressure is small, like in this example, the seepage will be very slow. The bigger the difference in pressure, the greater the seepage flow rate will be. But, if you increase the difference by too much, eventually some strange things start to happen.

What’s happening here? I’ve talked about shear strength of soils in a previous video. Check that out if you want more detail, but here’s the gist: Soil is a granular material that has one main way of holding itself together: friction. Gravity pushes the soil particles together, creating friction which gives the soil strength. Seepage is the enemy of friction. The water gets between the soil particles and pushes them away from each other, reducing the friction and thus reducing the strength. This is important, because shear strength is really the only thing separating a solid from a fluid. In fact, when water pressure within the soil gets high enough to eliminate its shear strength all together, it’s called liquefaction, otherwise known as the “quick condition”. The soil is literally behaving as a liquid rather than a solid… it’s quicksand!

You can see why seepage is bad for dams and levees: engineers generally try to avoid building civil structures out of liquids. Soil liquefaction can be the starting point for a specific type of erosion called piping. As soil is carried away from the dam’s foundation, the seepage path gets shorter. Remember that the seepage depends on the gradient, which is a function of the length of the seepage path and the difference in pressure. If the pressures stay the same, but the path gets shorter, the gradient goes up, creating more seepage. This is a dangerous positive feedback loop. The erosion shortens the seepage path while the increasing seepage creates more erosion. Eventually the erosion pipe reaches the reservoir, leading to catastrophic failure of the dam’s foundation. This isn’t just speculation; piping is the number one cause of failure for earthen levees and dams.

How do engineers deal with seepage? One common ways is called a cutoff wall, a subsurface wall constructed in the foundation of dam. This is some footage from a previous demonstration I built that shows a cutoff wall in action. Notice the length of the seepage flow paths. They’re longer, which means the gradient is lower. That means less seepage and less chance for erosion. Thank you for watching, and let me know what you think!

How to Super-Size the Eclipse - Sun Funnel

If you live in North America, you know that this upcoming Monday is a big day for aficionados of astronomical phenomena: it’s a solar eclipse. And if you haven’t already, you need to get some of these glasses that let you look at the sun. But, the sun’s angular diameter in our sky is only half a degree. It’s smaller than your thumbnail at arm's length, and that’s pretty small. I won’t be in the path of totality this time, but I will be at an elementary school here in San Antonio enjoying the eclipse with students. And as usual when I talk to kids, I’m feeling compelled to try and make this experience a bit more visceral. Like mythbusters with explosions, I figure the best way to increase the excitement of a solar eclipse is to make it bigger.

The one job of a telescope is to gather a large amount of light and concentrate it so that you can see astronomical objects more clearly. That’s perfect for faint subjects like planets or nebulae, but not ideal for our nearest star, the sun. Even looking at the sun with no magnification can damage your vision, and magnifying those harmful rays is going to make the damage worse. So how can we get a bigger view of the sun safely? There’s two common ways: first you can use a solar filter that works just like eclipse glasses, blocking out the majority of the light so that you can safely see the sun through your telescope. But this way has a disadvantage: only one person can see at a time. Not ideal if you’re trying to share the excitement with a crowd. The second way is to use the telescope to project an image of the sun on a screen, and that’s the basis of the sun funnel.

Telescopes and binoculars use lenses or curved mirrors to concentrate light. The optics are usually aligned to focus the light onto the retina in your eye. But, if you move your eye out of the way, the light keeps going. Put a screen in front of it, and now you’ve got a projection. It really is as simple as holding up a screen in front of the eyepiece, but the sun funnel solves two practical problems: (1) it automatically keeps the projection screen in the correct orientation to the eyepiece even as you move your telescope around to follow the sun, and (2) it keeps anyone who doesn't know better from looking through the telescope at the sun and accidentally blinding themselves.

My design is based on a guide published on NASA’s website that I’ll link down below. They used a plastic funnel, but I decided to make my own using this thin-gauge aluminum sheet. I used pop rivets to fasten the edges. You can size the funnel based on your telescope, eyepiece, and how large you want the sun to be using a fairly simple formula. The eyepiece to my telescope slipped into the small end of the funnel and I connected it with a hose clamp. On the other end, I stretched a piece of vinyl shower curtain to act as a rear-projection screen. The last part of the project is to stop down the aperture on my telescope. Most reflector telescopes aren’t meant to focus the strong rays of the sun , and I didn’t want to overheat my secondary mirror. I’ll also be covering up the aperture altogether at regular intervals to make sure I don’t damage my telescope.

Here’s a test I did in my backyard. The sun is plenty bright enough to see, and big too! I think the kids are going to be impressed. I’ll post some pictures of the real eclipse in the sun funnel on my instagram next week but for now I can show a simulation of what it will probably look like. Even if you’re not looking at an eclipse the sun funnel is a fascinating way to observe an actual star up close and personal. I hope you liked this quick project, and for those of you in North America, I hope you get a chance to see the eclipse next week. Thank you for watching, and let me know what you think.

https://eclipse2017.nasa.gov/make-sun-funnel

How does land surveying work?

We use all kinds of smaller measuring tools in our everyday lives such as rulers, protractors, and tape measures. These tools work fine for home projects, but what if we need to layout something large like a road, bridge, dam, or pipeline?

Surveying is the science of taking big measurements. You’ve probably seen these guys on the side of the road looking through fancy equipment on a tripod. Almost any civil engineering project starts with a survey. This is to determine the legal boundaries between parcels of property. Surveying is also used to determine the location of existing infrastructure, and the topography and slopes of the land. Humans have always had a penchant for building big stuff. This means surveying is a career full of history and tradition. Behind every wonder of the ancient world was an ancient geometry nerd who laid out the angles and alignments during construction. Surveying is also how we created accurate maps of the continents like the Great Trigonometrical Survey of India. This took almost 70 years to complete. Everyone should aspire to accomplish something in your life that can be prefixed with the words “great trigonometrical.”

The ubiquitous tool for a survey is called a theodolite, and it’s one job is to measure the horizontal and vertical angles between points. Combine those angles with distances from a chain or tape measure, and you can triangulate the location of any point using trigonometry. Modern theodolites, called total stations, cannot only measure angles, but distance as well, and they have onboard computers to do the calculations and record the data for later use. When you see a surveyor peering through a funny telescope, it’s probably a total station, and he or she is probably sighting a reflector to record the location of a point. For long distances, these measurements have to be corrected for variations in earth’s gravity, refraction by the atmosphere, and yep, even the curvature of the earth. But don’t tell the flat-earthers. We’re sworn to secrecy along with NASA employees and airline pilots.

That’s just scratching the surface of sophistication with modern surveying equipment. With GPS and unmanned aircraft, surveying can get a lot more complicated. But I’ve got a few ways you can do your own topographic survey with fairly basic and inexpensive tools. Maybe you’ve got a drainage issue on your land or you’re planning a landscaping project. Or maybe you just want to exercise your God-given right to take measurements of stuff and write those measurements down on a clipboard. That’s my kind of recreational activity. My goal is to perform a leveling survey of my front and backyard, which is just a way to get the relative topography for an area. I laid out a grid of points on a map of my house and then transferred those points to real life using pin flags. Now I just need to pick my datum or base point and measure the relative difference in height between that point and all the others. I tried a few ways to do this and there are no sines, cosines, or tangents required.

First, a sight level which is essentially a combination of a telescope and a spirit level. To use it, first get a buddy or a willing spouse to hold a surveying rod on the point of interest. Now, look through the sight at a surveying rod and raise or lower the end until the bubble is centered on the line. Once it’s centered you know that you’re looking at a point that is exactly level to your eyes. Simply subtract the height of your eye-line with the height measured on the rod and that’s your elevation. It’s not a precision technique, but it is cheap and simple which the most you can usually hope for in any part of a home improvement project.

The next way I tried is a water level which is literally just a length of clear vinyl tubing filled with a liquid. As long as there are no bubbles or kinks in the line, the free surface at each end of the tube will self-level. I kept one end at my datum a fixed height and measure the height of the water at the other end as I carry it around to each of my points. It’s a little more unwieldy but it does have a distinct advantage, no line of sight required. You can use this method around corners or behind trees with no problem, and again, it’s a cheap and simple solution.

The third method to take a level survey worked best for me: my laser level. Here’s the thing: I really like lasers. I relish any chance I get to use them in a constructive way, and this is perfect. The laser level creates a perfect horizontal line that can be used to line up cabinets or tile, but it is also super easy to read on a surveying rod. You don’t need a helper, but you do probably need to wait until dusk unless your laser is really bright, or you have these sweet laser enhancement glasses. This isn’t the cheapest solution for a DIY land survey, but it is the fastest one I tried, and it’s a tool a lot of people already have.

Surveying is one of the oldest careers in the world, and also one of the most important. Why? Because land is important. If you own some, it’s probably your most valuable asset, and even if you don’t, you're pretty much stuck to it no matter where you go. As a career, surveying is a fascinating mix of legal knowledge, fieldwork, and technical challenges. And since most civil structures are too big for conventional measurement tools, the surveyor is one of the most important companions for the civil engineer. Thank you for watching, and let me know what you think!