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

A lot of engineering focuses on structural members. How wide is this beam? How tall is this column? But some of the most important engineering decisions are in how to connect those members together. Take a column, for example. You can’t just set it directly on a foundation, at least not if you want it to stay up. It needs a way to physically attach to the foundation. This may seem self-evident, maybe even completely obvious to most. But in that humble connection that’s so ubiquitous you rarely even notice it, there is so much complexity. Baseplates are the structural shoreline of the built environment: where superstructure meets substructure. And even understanding just a little bit of the engineering behind them can tell you a lot of interesting things about the structures you see in your everyday life. I’m Grady, and this is Practical Engineering.

Let me start us out with a little demonstration. If you’re a regular viewer, you know how much you can learn from our old friends: some concrete and a benchtop hydraulic press. I cast two cylinders of concrete about a week ago, and now it’s time to break them for science. These were cast from the exact same batch of concrete at the exact same time. For this first one, I’m pushing with a fairly narrow tool. I slowly ramp up the force until eventually… it breaks. I had a load cell below the cylinder, so we can see the force required to break this concrete. This scale isn’t calibrated, so let’s say it broke at 1400 arbitrary Practical Engineering units of force. Practicanewtons? KiloGradys? What would you call them? Now let’s do the same thing with a wider tool. At that same loading, this concrete cylinder is holding steady. In fact, it didn’t break until 3100 units. Here’s a trick question. Was the second cylinder stronger than the first one? Hopefully it’s obvious that the answer is no.

Most materials don’t care about force. I mean, in the strictest sense, most materials don’t care about anything. But what I mean is that the performance of a material against a loading condition usually depends not on the total force, but how that force is distributed over an area. It’s pressure; force divided by area. Increase the area, lower the pressure. And pressure is what breaks stuff. So that’s what a lot of baseplates do. They transfer the vertical forces of a column to the foundation over a larger area, reducing the pressure to a level that the concrete can withstand.

And that’s the first engineering decision when designing a baseplate. How big does it need to be? If you know the force in the column and the allowable pressure on the foundation, you can just divide them to get the minimum area of the plate. That’s the easy part. Because steel isn’t infinitely stiff. If I put this column on a sheet of paper, I think it’s clear that there’s no real load distribution happening here. The outside edges of the paper aren’t applying any of the column’s force into the table; I can just lift them. But this can be true for steel too. I filled up an acrylic box with layers of sand to make this clearer. If I use a thin base plate, the forces from my column don’t distribute evenly into the foundation. You can see that the baseplate flexes and the sand directly below the column displaces a lot more. I can try this with a thicker, more rigid baseplate, and the results are a lot different. Much more even distribution of pressure. So the second engineering decision when designing a baseplate is the stiffness of the plate, usually determined by the thickness of the steel, based on the loads you expect and how far the plate extends beyond the edges of the column. And in heavy-duty applications like steel bridge supports, vertical stiffeners can be included to make the connection even more rigid.

So far, though, the baseplate isn’t really much of a connection. That’s the thing about compressive loads: gravity holds them together automatically. There are no bolts in the Great Pyramid of Giza. The blocks just sit on top of each other. And that could be true for some columns too. The main load they see is axial, along their length, pressing the plate to the ground. But we know there are other loading conditions too. A perfect example is a sign. Billboards and highway signs are essentially gigantic wind sails. They don’t actually weigh all that much, so the compressive force on their base isn’t a lot, but the horizontal forces from the wind can be significantly higher than that. Those horizontal forces can increase the compression force on one side of the base plate, so you have to account for that in the design. But they also can result in shear and tension forces between the baseplate and foundation, so you’ve got to have something in place to resist those forces too. That’s where anchors come in.

There are a lot of ways to attach stuff to concrete. There are anchors that epoxy into holes, screw into place, or use wedges to expand into the hole. And of course, if you’re extra careful and precise, you can even embed anchor rods or bolts into the concrete while it’s still wet. There’s a huge variety of styles and materials that offer different advantages depending on your needs. Here’s just one manufacturer’s selection guide for the anchors and epoxies they provide. But like third year engineering students, all of those anchors can fail if they’re overloaded. And they can fail in a lot of different ways under tension or shear forces. The anchor rod itself can fracture or deform. It can lose its bond with the concrete and pull out. It can break out the surrounding concrete. Or if it’s too close to the edge, it can blow out the side. Calculating the strength of the anchor bolt and concrete connection against each of these failure modes is a lot more complicated than just dividing a force by a pressure to determine the baseplate area. So most engineers use software that can do the calculations automatically.

But, there’s another challenge about baseplates I haven’t mentioned yet, and it has to do with tolerances. Concrete foundations can be pretty precise. As long as you set the forms accurately and make them strong enough to avoid deflection while the concrete is being placed, you can feel confident in the dimensions of the structure that comes out of them. But there’s usually one surface that isn’t formed: the top. Instead, we use screeds and trowels and floats to put a nice finish on the top surface of a concrete slab or pier. But it’s rarely perfect enough to put a column directly on top. That’s not to say it can’t be done. I’ve seen concrete finishing crews do amazing work. But it’s usually not worth the effort to get a concrete surface perfectly level at the exact elevation needed for every column, especially when you have the time pressure of concrete setting up. And those tolerances matter. Just one degree off of level will put a 16-foot or 5-meter column out of plumb by more than 3 inches or 80 millimeters. Unless you’re in certain parts of Tuscany, that’s not gonna work. It’s more than enough to misalign some bolt holes. And that only magnifies for taller columns like signpoles. So, we usually need some adjustability between the plate and the concrete.

Sometimes that means shimming the baseplate to get it perfectly level. And the other primary option is to use leveling nuts underneath the plate. I welded up a custom-branded column and and baseplate that was laser-cut by my friends at Send-Cut-Send to show you how this works. These parts turned out so nice. By adjusting these nuts up or down, I can get the column to point in the exact direction required. And I can get it to the exact right elevation too. But maybe you see the problem here. All the work we did to make sure the baseplate distributes the vertical load even across its area is lost. Now the vertical loads are just being transferred through some shims or through the bolts directly into the anchors. So, in a lot of cases, we add grout between the plate and the concrete to bridge the gap. Grout is basically concrete without the large aggregate, mixed with a low viscosity so it flows more easily into gaps. And it often includes additives to prevent it from shrinking as it cures, making sure it doesn’t pull away from the surfaces above and below. When it hardens, the grout can transfer and distribute the loads into the foundation. So if you pay attention to baseplates you see out in the built environment, you’ll notice it’s pretty common that they sit on a little pedestal of grout and not directly on the concrete below. But even this comes with a few problems.

First is load transfer. Even with the grout, some of the vertical loads are still going into the anchor bolts that might not have been designed for compression. So now we’ve added a few more new potential failure modes to the laundry list: punching through the bottom of a slab, and buckling of the rod itself. Sometimes contractors will use plastic leveling nuts that can hold the column during construction, but will yield after the column’s loaded so the grout supports all the weight. Second is fatigue. Especially for outdoor structures that see wind and vibrations, the grout under the baseplate might not hold up to repeated cycles of loading. Third is moisture. Grout can trap water, leading to problems with corrosion, especially for hollow columns like sign poles where condensation needs a way out. And the grout can hide that corrosion, making it difficult to inspect the structure. And fourth, adding grout below a baseplate is just an extra step. It’s kind of fiddly work to do it right, and it costs time and resources that might otherwise be spent somewhere else. In fact, there are a lot of cases where it’s an extra step worth skipping.

You can design anchor bolts strong enough to withstand all the forces a column will apply, including the compressive forces downward. And you can design a base plate stiff enough that those forces don’t have to be distributed evenly across the entire area. And if you do, you have a standoff base plate. It just floats above the concrete with only the anchors in between. It looks like a counterintuitive design. We think of a baseplate as kind of a shoe, so it should be sitting on the ground. And a lot of them are designed that way. But for other structures, a baseplate is really just a way to connect a foundation to a column through an anchor. So if you pay attention, you’ll see these standoff baseplates everywhere. A lot of state highway departments have moved away from using grout to make signs and light poles easier to inspect. And they often install wire mesh to keep animals out from hollow masts.

Clearly there’s a lot more to baseplates than meets the eye, and that means there’s also a few myths going around grout there. A common misconception is that standoff baseplates are meant to break away in the event of a collision. And I totally understand why. If an errant vehicle hits a signpost, a relatively minor deviation from the road can turn into a deadly crash. Smaller signs installed near roadways often do use breakaway hardware or features. You’ll often see holes drilled in wooden posts, bolts with narrow necks meant to snap easily, or slip bases like this one to make sure a sign gives way. But for larger structures like overhead signs and light poles, that’s generally not the case. Having one of these break away and fall across a highway could create an even bigger danger than having it stay upright. So, even though they might look similar, standoff baseplates are distinct from sign mounts designed to break loose in a collision. Instead, larger structures installed in the clear zones of highways are protected from crashes using a guardrail, barrier, or cushion.

Baseplates are like bass parts in music, it’s easy to overlook them at first, but once you notice them, you can’t stop paying attention to how important a role they play. And just like bass lines, they might seem simple at first, but the deeper you dig, the more you realize how complex they really are.