how do architects calculate if a structure like a bridge is stable?
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I’m an engineer and have worked on the foundation for bridges. When we design the foundation, we get loads (weights, etc) from the structural engineer. We use these to check different possibilities that could cause things to fail. For most of the ones I’ve done this ends up being 4 main ways.
The first one is having a super heavy truck drive over it, this is the strength condition. The max weight it will take. The elementary school experiment is putting a ruler between two books and setting something on the ruler, when it breaks, that’s the strength.
Next would be service, if you ever drive over a bridge and feel it flex, this is service. It may not fail, but if it flexes/bounces too much is is uncomfortable. For foundations this includes settlement. For the ruler, now you only want the ruler to bend so much.
We have earthquakes here, so we need figure out how big it will be. The geologic survey does that for us. Then we look at what the dirt is like. Now sit you books and ruler on something. Something like pudding will shake more than rock. This is called an extreme event, we don’t want it to fall down, or if it’s important, it needs to be usable after the earthquake. In a building this would be the difference between you being about to get out of your house and not be dead vs the hospital that still needs to be the hospital after the earthquake.
After this there are a bunch of other extreme events. For rivers we look at how bad the flood will be. The geologic survey gives us an idea too. We have to see that it will be ok for a 500 year flood, a flood that has a 0.2 percent to happen each year. Will that flood wash away all the dirt around our foundation? This actually is usually our worst case in our area when we cross a river, even though we expect a 7.0 earthquake.
Other possibilities include:
What if there’s ice in the river in the spring that hits the bridge? Or gets stuck and won’t let the water by?
What if a truck runs into the bridge?
What if the wind makes the bridge vibrate like a guitar string? (This is what killed the Tacoma narrows)
What if the steel used for the foundation rusts?
Bridges are hard, but the challenge is fun. On the other hand, you are not going to do it alone. We always have someone else in our office independently check our “homework.” Then, in most cases, the state department of transportation checks it themselves. Even if it’s not for the state or feds, cities and counties just don’t have the expertise.
One super nice thing is the building code for bridges. It is both very complex and very simple at the same time. The old binder copy I have is 6 inches thick, and the new one is bigger. But it is very step by step. And has lots of footnotes and references. It’s very step one,step 2. If you have something unusual, get this book out.
Most of the math is not super involved, but you need to understand what it’s doing. Design for this heavy of a truck, the tires go here, the beam can only bend this much. Now check if the next thing happens. Most of it can actually be done with paper and a calculator if it’s a simple bridge. Computers help you do the hard math, but you need to be able to tell if the answer makes sense.
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