One of the hardest things we have to do as structural engineers is explain to builders/contractors why something that they know they can build in the field will not work when we design it on paper. The following is a typical situation, we size a ridge beam for a cathedral ceiling and the framer tells me that he has never needed to do this before, either followed or preceded by, I have been doing this for at least 20 to 30 years. So here is the simplest explanation for why it seems that at times we make life difficult for contractors.
One main reason is the fact that we have to design for lateral loads, such as wind events and earthquakes. Gravity is present 100 percent of the time so a contractor is used to building for that force. Design lateral events however, may span years between events or may never occur during a building’s life, so there is no intuitive development of what is needed to resist those forces. One look at the devastation in Nepal shows that structures can withstand gravity loading for years, yet crumble under lateral loading.
The other variable we have to deal with is the actual connection of material; we model connections using either a pinned or a fixed condition. In the field there is really no such thing as a pure pin, there is always some type of rotational restraint. For example, with a nailed wood connection we can’t model a fixed connection until someone develops a way to weld wood together; I don’t think a pine tar welding rod is in our future.
So when a structural engineer tells you something won’t work it is usually because the total lateral and gravity loads over-stress the structure or our model of the structure does not work. We are here to prevent things from falling down and to remain usable even after design loading occurs. The end goal is to work with builders and contractors to make sure that the buildings we create are usable and safe for years to come.