Modeling and Understanding, Part 3

Part 1 and Part 2 of this series give, I hope, some idea of what I’m talking about.

This discussion began with the idea of intuition in engineering and I want to approach it from the investigation side. Unless a building is very simple, you don’t know what it’s structural system is when you start looking at it. You figure it out as you go along.

If you had x-ray vision, you could see structures in the manner of the cutaway view of the Roseman Bridge in Iowa that’s above. But it is unlikely that you are Superman.

If you were patient and methodical, and had a client that had the time and the budget for you to be patient and methodical, you could simply examine every square inch of the building looking for structural clues. Most buildings have some places where there are no finishes or gaps in the finishes, and so it is likely that being patent and methodical would get you an answer, eventually.

Or, if you’re willing to rely on intuition, you can take a step back and just look at the building as a whole. Planar structural elements – walls and floors, most often – are much stiffer than linear elements and stiffer than assemblies of linear elements. But certain types of planar elements – plastered wood-stud partitions and gypsum block partitions, for example – are not necessarily strong enough to carry the loads that their stiffness picks up. So a good starting point for investigation is to look for the kinds of damage associated with, for example, partitions that have cracks from foundation movement, or floor movement, or building sidesway. I don’t even think about doing this: it’s automatic because I know, without any calculations or modeling, that partitions are stiffer than their strength can handle, which is the reverse of steel beams, which are strong enough to bend severely before overload destroys them. This is not a blind process, but rather relies on my experience to differentiate structural elements based on strength and stiffness.

Of course, once you intuit something, you’d better then go check out your idea, using actual analysis, to see if it makes sense. I hesitate to call it testing a hypothesis, because that is giving this a veneer of scientific predictability that I don’t think it deserves. It’s constructing a narrative and then seeing if the story makes sense. And stories are an appropriate place for intuition. Since analysis is still required, what did intuition gain us? A shortcut s to where to look for structure and what to analyze, rather than the brute-force method fo methodically examining everything.