Inspired by Calvin’s same question where his Dad answers “They drive bigger and bigger trucks over the bridge until it breaks. Then they weigh the last truck and rebuild the bridge,” how do engineers determine the actual load limit on bridges in the real world?
In: Engineering
He is kind of right but obviously we don to build an entire bridge. Instead we take a single beam and bend it until it breaks. That gives us the load limit for that beam. We then multiply by how many beams we use for the bridge. [Here is a video showing this process on an airplane wing.](https://www.youtube.com/watch?v=zcaaznmp4hQ)
This is fine when building a simple bridge. But it gets expensive when you need big long beams just to test the strength of them, especially if you find out that the beam is too weak and have to order bigger ones. So instead we have made tables of different beam sizes, types and lengths. And we have found the patterns for how these parameters affect the strength and have made mathematical models. So if you want a certain strength and length of a bridge we can simply calculate what size beams you need.
For bridges where a single beam is not efficient we can instead calculate the forces applied. You want each beam to stay still so all the forces on it needs to cancel each other out, and the forces going through the beam needs to be smaller then its maximum strength. So you can calculate how the forces of a load on the center of the bridge transfers through all the beams and therefore calculate how much force each beam needs to handle. You then look up in the tables above or use the models to calculate these same values. Then you know the size of the beams you need.
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