You have abstractions and models for those things. I was formally trained as an EE, so I'm just guessing at how structural engineers do it.

I would expect someone building a bridge to keep the average/peak winds into consideration - and then feed it to CAD or whatever modeling software they use to design the structure. They don't need to know the exact force a screw was tightened with - they do need to give the specs of what range they should be tightened to. Again - considered in CAD. They don't need to know that theory is right - they just need to know it's not wrong to an unacceptable degree.

I'm sure there's some guessing, but a lot of these things are actually factored in.