[Note that this article is a transcript of the video embedded above.]

Almost immediately after I started making videos about engineering, people started asking me to play video games on the channel. Apparently there’s roughly a billion people who watch online gaming these days, and some of them watch silly engineering videos too! And there’s one game that I get recommended even more than minecraft: Polybridge. So I finally broke down one evening after the kids went to bed and gave it a try. I’m really not much of a gamer, but I have to admit that I got a little addicted to this game (hashtag not-an-ad). I admit too that there really is a lot of engineering involved. You have different materials that give your structure different properties. The physics are RELATIVELY accurate. You get a budget to spend on each project. And your score is based on the efficiency of your design. But there’s one way this game is not like real structural engineering at all: if your bridge collapses, you get to try again!

In the real world, we can’t design a dam, a building, a transmission line pylon, or a bridge, spend all that money to build it, watch how it performs, tear it down, and build it back better if we’re not happy with the first iteration. Structures have to work perfectly on the first try. Of course we have structural design software that can simulate different scenarios, but it’s only as powerful as your inputs, which are often just educated guesses. We don’t know all the loads, all the soil conditions, or all the ways materials and connections will change over time from corrosion, weathering, damage, or loading conditions. There are always going to be differences between what we expect a structure to do and what actually happens when it gets built. Hopefully engineers use factors of safety to account for all that uncertainty, but you don’t have to dig too deep into the history books to find examples where an engineer neglected something that turned out to matter a lot, sometimes to the detriment of public safety. So what do you do?

We can’t build a project then watch the cars and trucks drive over with the pretty green and red colors on each structural member to see how they’re performing in real time… except you kind of can, with sensors. It turns out that plenty of types of infrastructure, especially those that have serious implications for public safety, are equipped with instruments to track their performance over time and even save lives by providing an early warning if something is going wrong.

I love sensors. To me, it’s like a superpower to be able to measure something about the world that you can’t detect with just your human senses. Plus I’m always looking for an opportunity to exercise my inalienable right to take measurements of stuff and make cool graphs of the data. So I have a bunch of demonstrations set up to show you how engineers employ these sensors to compare the predicted and actual performance of structures, not just for the sake of delightful data visualization, but sometimes even to save lives. I’m Grady, and this is Practical Engineering. In today’s episode, we’re talking about infrastructure instrumentation.

And what better place to start than with a big steel beam? In fact, this is the biggest steel beam that my local metals distributor would willingly load on top of my tiny car. One of the biggest questions in polybridge and real world engineering is this: How much stress is each structural member experiencing? Of course, this is something we can estimate relatively quickly. So let’s do the engineer thing and predict it first. Beam deflection calculations are structural engineering 101, so we can do some quick recreational math to predict how much this thing flexes under different amounts of weight. And we can use my weight as an example: about 180 pounds or 82 kilograms. The calculation is relatively simple. You can choose your preferred unit system and pause here if you want to go through them. Standing at the beam’s center, I should deflect it by about 2 thousandths of an inch or about 60 microns, around the diameter of the average human hair. In other words, I am a fly on the wall of this beam (or really a fly on the flange). I’m barely perceptible. In fact, it would take more than 100 of me to deflect this beam beyond what would normally be allowed in the structural code. And it would take a lot more than that to permanently bend it. But 2 thousandths of an inch isn’t nothing, so, let’s check our math.

I put my dial indicator underneath the beam, and added some weight. I started with 45 pound or 20 kilogram plates. Each time I add one, you see the beam deflect downward just a tiny bit. After three plates, I added myself, bringing the total up to around 315 pounds or 143 kilos of weight. And actually, the deflection measured by the dial indicator came pretty close to the theoretical predictions made with the simple formula. Here they are on a graph, and there’s the point at my weight, with a deflection of around 2 thousandths of an inch or 60 microns, just like we said. But, we can’t always use dial indicators in the real world because they need a reference point, in this case, the floor. Up on the superstructure of a bridge, there’s no immovable reference point like that. So an alternative is to use the beam itself as a reference. That’s how a strain gauge works, and that’s the cylindrical device that I’m epoxying to the bottom flange of my beam.

A strain gauge works by measuring the tiny change in distance between two parts of the steel. You might know that when you apply a downward load to a beam, it creates internal stress. At the top, the beam feels compression, and at the bottom it feels tension. But it doesn’t just feel the stress, it also reacts to it by changing in shape. Let me show you what I mean. When I put one of the plates on top of the beam, we can see a change in the readout for the strain gauge. (Of course, I had the gauge set to the wrong unit, so let me overlay the proper one with the magic of video compositing.) For each plate I add to the beam, we see that the flange actually lengthens, in this case by about 3 microstrain. That’s probably not a unit of measure you’re familiar with, but it really just means the bottom of the beam increased in length by 0.0003%. When I add another weight, we make it 0.0003% longer again. Same with the third weight. And then when I stand on top of the whole stack, we get a total strain of about 0.002%, a completely imperceptible change in shape to the human eye, but the strain gauged picked it up no problem.

Imagine how valuable it would be to an engineer to have many of these gauges attached to the myriad of structural members in a complicated bridge or building and be able to see how each one responds to changes in loading conditions in real time. You could quickly and easily check your design calculations to make sure the structure is behaving the way you expected. In my simple example in the studio, the gauge is measuring pretty much exactly what the predictions would show, but consider a structure far more complicated than a steel beam across two blocks, in other words, any other structure. What factors get neglected in that simple equation I showed earlier?

We didn’t consider the weight of the beam itself; I’m not actually a one-dimensional single point load, like the equation assumes, but rather my weight is spread out unevenly across the area of my sneakers; Is the length exactly what we entered into the equation? And, what about three-dimensional effects? For example, I put another strain gauge on the top flange of the beam. If you just follow the calculations, you would assume this flange would undergo compression, getting a tiny bit shorter with increased load. But really what happens in this flange depends entirely on how I shift my weight. I can make the strain go up or down simply by adjusting the way I stand on top, creating a twisting effect in the beam, something that would be much more challenging for an engineer to predict with simple calculations. Putting instruments on a structure not only helps validate the original design, but provides an easy way to identify if a member is overloaded. So it’s not unusual for critical structures to be equipped with instruments just like this one, with engineers regularly reviewing the data to make sure everything is working correctly.

Of course, we don’t only use steel in infrastructure projects, but lots of concrete too. And just like steel, concrete structures undergo strain when loaded. So I took a gauge and cast it into some concrete to measure the internal strain of the material. This is just a typical concrete beam mold and some ready-mix concrete from the hardware store. And even before we applied any load, the gauge could measure internal strain of the concrete from the temperature changes and chemical reactions of the curing process. Shrinkage during curing is one of the reasons that concrete cracks, after all. Luckily my beam stayed in one piece. Once the beam had cured and hardened for a few weeks, I broke it free from the mold. Compared to steel, concrete is a really stiff material, meaning it takes a lot of stress to cause any kind of measurable strain. So I got out my trusty hydraulic press for this one. I slowly started adding force from the jack, then letting the beam sit so the data logger could take a few readings from the strain gauge inside. After the fourth step, at just over 50 microstrain, the beam completely broke. Hopefully you can see how useful it might be to have an embedded sensor inside a concrete slab or beam, tracking strain over time, and especially when you know about the amount of strain that corresponds to the strength of the material. This is information that would be impossible to know without that sensor cast into the concrete, and there’s something almost magical about that. It’s like the civil engineering equivalent of x-ray vision.

One of the most amazing things about these sensors is their ability to measure tiny distances. 1 microstrain means one millionth of the original length, which on the scale of most structures, is a practically impossible distance for a human to perceive. But in addition to tiny distances, they also are excellent in measuring changes that happen over a large period of time. A perfect example is a crack in a concrete structure. You can look at grass, but you probably can’t perceive it growing, and you can watch paint, but you won’t perceive it drying. And, you can watch a crack in a concrete slab, like this one in my garage, but you’ll probably never see it grow or shrink over time. So how do you know if it’s changing? You could use a crack meter like this one, and take readings manually over the course of a month or year or decade. But in many cases, that’s not a good use of any person’s time, especially when the crack is somewhere difficult or dangerous to access. So, just like strain gauges measure distance, you can also get crack meters that measure distance electronically. I put this one across the crack in my garage slab and recorded the changes over the course of a few months.

And, I know why this crack exists. It’s because the soil under the slab is expansive clay that shrinks and swells according to its moisture content. I thought it would be fun to use some soil moisture sensors to see if I could correlate the two, but my sensors weren’t quite sensitive enough. However, just looking at the rainfall in my city, you can get a decent idea about what might be driving changes in the width of this crack, which grew by about half a millimeter over the course of this demonstration. Cracking concrete isn’t always something to be concerned about, but if cracks increase in size over time, it can be a real issue. So, using sensors to track the movement of cracks over long durations can help engineers assess whether to take remedial measures.

And, there are a lot of parameters in engineering that change slowly over time. Dams are among the most dangerous civil structures because of what can happen when one fails. Because of that, they’re often equipped with all kinds of instruments as a way to monitor performance and make sure they are stable over the long term. One parameter I’ve talked about before is subsurface water pressure. When water seeps into the soil and rock below a dam, it can cause erosion that leads to sinkholes and voids, and it also causes uplift pressure that adds a destabilizing force to a dam. Instruments used to measure groundwater pressure are called piezometers. They often resemble a water well with a long casing and a screen at the bottom, but instead of taking water out, we just measure the depth to the water level. That’s made a lot easier with electronic sensors, like this one, but I don’t have a piezometer in my backyard. So, to show you how this works, I’m just hooking my pressure transducer to the tap so we can see how the city’s water pressure changes over time. I hooked this up to a laptop and let it run for about a day and a half, and here are the results.

The graph is a little messy because of the water use in my house throwing off the readings every so often, but you can see a clear trend. The pressure is lowest when water demands are high, especially during the evenings when people are watering lawns, cooking, and showering. In the middle of the night, the pumps fill up the water towers, increasing the local pressure in the pipes. This information isn’t that useful, except that it gives you a new perspective of thinking about real-world measurements. Recently I had a plumber at my house who took a pressure reading at the tap, which seemed like a totally normal thing at the time. But now, seeing that the pressure changes by around half a bar (or nearly 10 psi) over the course of a day, it seems kind of silly to just take a single measurement. And that’s the value of sensors, giving engineers more information to make important decisions and keep people safe after a structure is built.

By the way, the engineering of these instruments is pretty interesting on its own. Most of the sensors I’ve used in the demos were sent to us by our friends at Geokon, not as a sponsorship but just because they enjoy the channel and wanted to help out. These devices rely on a wire inside the case whose tension is related to the force or strain on the sensor. The readout device sends an electrical pulse that plucks the wire and then listens to the frequency that comes back. You can see the pluck and the return signal on my oscilloscope here. Just like plucking a guitar string, the wire inside the instrument will vibrate at a different frequency depending on the tension, and you can even hear the sound of the vibration if you get close enough. Of course civil engineers use lots of different kinds of sensors, but vibrating wire instruments are particularly useful in long-term applications because they are incredibly reliable and they don’t drift much over time. They’re also less vulnerable to interference and issues with long cables, since they work in the frequency domain. In fact, there are vibrating wire instruments that have been installed and functioning for decades with no issues or drift.

And the demos I’ve shown in this video just scratch the surface. We’ve come up with creative ways to measure all kinds of things in civil engineering that don’t necessarily lend themselves to garage experiments, but are still critical in performance monitoring of structures. Borehole extensometers are used to measure settlement and heave at excavations, dams, and tunnels. Load cells measure the force in anchors to make sure they don’t lose tension over time. Inclinometers detect subtle shifts in embankments or slopes by measuring the angle of tilt in a borehole along its length. Engineers keep an eye on vibrations, temperature, pressure, tilt, flow rate, and more to make sure that structures are behaving like they were designed and to keep people safe from disaster.