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 ..