I'm not sure if I want to reply to AC's, but I forgot to mention I'm a structural engineer myself by education... Most structures of respectable size fall back on Finite Element Analysis to gauge the response to a variety of loads. [The estimation of loads is a research topic in itself, where the factors of safety comes from a rigorous stochastic-based reliability analysis].
Once analysis has been performed, design is a bit of intuition, but certainly not estimation - it's more of heuristics... so you say, "this worked last time, let me try this option and **analyse** if it'll work this time too."