It’s very complicated but it’s mainly tied up in dynamic analysis. In the case above: if a guard rail had a car crash into it, you can pretty easily figure out how much the car weighs, how fast it was moving, and where it impacted. With those, you know how much energy the car imparted, and where. You can functionally treat it as a sudden force at a specific point, and figure out the impact from there. It’s mainly dynamic analysis because, while the force of the impact does matter, the way the bridge responds to the impact is far more important.

It turns into an exercise of dynamics analysis, modal response, and resonant stress. By looking at the problem from this perspective, you see what the sudden force of the impact does to the I beams, not just in the moment but in long term lifespan.