Why 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

move

s for one reason or another, and one of those reasons happens to be temperature. Hi, I'm Grady and this is Practical Engineering. Today we talk about thermal expansion. This video is sponsored by Blue Apron. More on that later. Whether you know it or not, you are probably already familiar with thermal expansion, which is the property of materials to change their volume depending on temperature. If you have used a glass thermometer, you have even taken advantage of it.

The liquid in a thermometer, usually mercury or alcohol, increases in volume as it is heated. Since we can characterize this expansion, we can use it as a measuring device. Perhaps you have had some experience with a less useful application of thermal expansion. If you've ever put glassware on a stovetop or poured cold water on a dish fresh from the oven, you know it doesn't turn out that great. If it is allowed to expand and contract uniformly, stresses in the material do not 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 is not flexible, so instead of bending, it simply breaks. Thermal motion is something that must be taken into account in almost all engineering fields, because there are not many places where temperature fluctuations do not occur. And there are really only two options when designing for thermal expansion: The first is to prevent movement by restricting it, a feat that is almost always impractical. Thermal movement can generate enormous amounts of internal stress. Notice how this cable can lift a weight by simply heating it and then cooling it. So the other way to accommodate thermal movement is to simply allow your design the freedom to move as you wish.

But sometimes this is easier said than done, especially in the case of large civil structures. This is the formula for thermal expansion. It may seem complicated, but it really isn't. It basically 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 have measured this property for a large number of materials and you can search for tables online. Luckily for civil engineers, the thermal expansion coefficients of steel and concrete are almost identical, so we can combine them in the ubiquitous building material, reinforced concrete, without worrying too much about temperature fluctuations separating them.

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 go back to our formula and add some numbers for a very typical situation. If we enter values ​​for the average length of a city block, the average maximum and minimum temperatures in a given year, and the coefficient of thermal expansion of concrete, we can see that the total movement of a sidewalk over the course of a year can be more than 4 cm or 1.5 inches. Obviously, you can't leave that big of a gap in the sidewalk at the end of each block, so we leave small gaps spaced out from time to time to accommodate that movement.

Most of the joints you see along a sidewalk are just to control cracking, but if you pay attention, every once in a while you'll see an actual break in the concrete filled with some type of flexible material. These are expansion joints that give the walkway the freedom to move due to temperature fluctuations. But what about structures that are longer than an apple? Thermal motion increases with length, so engineers must be much more careful with linear infrastructure. Long runs of pipe, especially if they experience fluctuating temperatures, need expansion joints to prevent damage. Train rails can experience "solar bending," where a hot day can warp the steel.

One of the biggest challenges for thermal movement is

bridges

. So I built a little model to show why (assembly?). Unlike sidewalks that may have periodic expansion joints,

bridges

only have support between spans. You cannot have a break without support in the bridge, which means that all thermal movement occurs in the supports. The allocation cannot be distributed evenly throughout; everything happens in one place. For bridges with very long spans, this can involve a lot of movement. I have my bridge set up with one side fixed and the other free to move. I'll turn on the sunny day simulator and watch what happens on the dial indicator.

The bridge expands along its length as it warms. This is exactly what happens in real life. Now I will try to secure both sides of the bridge to limit movement. The bridge still expands as it heats up, but now it has to expand in directions it wasn't supposed to go. It's a little difficult to show on camera, but the entire bridge has twisted from side to side. I am using a flexible rod for this demonstration, but if a real bridge were restrained in this way, the forces generated by thermal expansion would likely cause failure of the structural members.

Expansion joints in bridges not only have to allow the bridge to move while still being supported, but they also have to close the gap in the road deck so that cars can pass over it safely. So if you look closely, you'll see many creative ways engineers handle thermal expansion. These are some photos collected from the web and sent to me by observers of different bridge supports that allow for thermal movement. My inspiration to make this video came when I was looking at some vacation photos. Get a close look at this steel walkway over the river. The shorter cantilever beams are welded directly to their anchor plates.

They are free to move because they are only connected to the rock on one side. But notice the beams that extend on both sides of the canyon. At first glance, it looks like they are limited on both sides and we know that's bad engineering. But if you look closely, you can see that they are bolted to the anchor plate using slotted holes to allow the beams to expand and contract. I hope this video has given you a little more information about the dynamic nature of structures that we normally think of as static. Keep your eyes open and you'll notice that there is room for thermal movement wherever you look.

Thanks for watching and let me know what you think. Thanks to Blue Apron for sponsoring this video. Blue Apron delivers all the fresh ingredients you need, right to your door, in the exact proportions to create delicious recipes at home. No trips to the supermarket and no waste of unused ingredients. We love being in our house and have so much fun cooking these meals together, not to mention eating them. If that sounds like something you'd be interested in, the first hundred people to click the link in the description will get $30 off their first order. Again, thanks for watching and let me know what you think!