They are guessing much more than computer scientists would think, typically . A structural engineer does not know: the peak wind force, what the ground under the bridge is really made of, what the actual tensile strength at the weakest piece of material is, what the exact force on the screws were at time of fastening (and after), etc... Heck, they don't even know if euler bernoulli beam theory is actually right about the existence of a neutral axis..They just take their best guesses, add generous safety factors and have the bridge inspected regularly ..

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.