It’s a bit confusing to see a structural diagram for the first time. How exactly does an entire bridge turn into a straight line? I’ve tried to show photos and diagrams side by side throughout Monthly Mechanics, to smooth the mental leap required. This month I’d like to discuss this critical step of structural analysis at length.
For example, here’s a table:
How would you make a structural diagram of this table? In the words of Thoreau: simplify, simplify. Ignore for a moment the bracing, the exact shape of the parts, and all decorations. Think in terms of rectangles and straight lines.
A table has basically three parts: the tabletop (a rectangle), the apron (four horizontal lines), and the legs (four vertical lines). When you put something heavy on the table – say, a Thanksgiving turkey – the weight travels through the tabletop to its perimeter where it’s supported by the apron. Then, the weight passes through the apron to the ends where it’s supported by the legs. Finally, the weight moves through each leg to the floor
Modeling is the anti-art. The goal is to strip an object down to its essential structure. Tables come in many shapes, but their function is always the same: to transmit the weight of that turkey to the floor. All that’s really important is the number of legs and where they’re located – a one-legged pedestal table has a different load path than our four-legged coffee table.
Structural engineers like to model things as simply as possible at first, so they can calculate the reactions and internal stresses by hand. Later they might perform a more complex analysis with a detailed computer model (which includes bracing for example).