This might be familiar to you…sitting at a café table, you realize it rocks back and forth and stuff a bunch of paper napkins under one leg to stop it rocking. If so, then it was a table with four legs.
So, why does this happen? Because, structurally, four legs is one leg more than the minimum needed to support a plane (the tabletop).
Taking things down one dimension level, three supports is one support more than the minimum needed for a beam. With two supports I can tell you immediately how much load they’re carrying. We call this set-up structurally determinate. If we add a third support then I can’t tell you the load at the supports without knowing more about the stiffness of the beam and the supports themselves. We call this structurally indeterminate. Both have their place in designs and both have their advantages and disadvantages, including beyond just structural thinking.
But back to the humble café table, I’m always surprised so many (though not all) come with four legs. You always see people putting cameras on top of tripods because tripods don’t wobble. Ahh, if only structural engineers designed all the café tables in the world…think of all the table legs and paper napkins we’d save!
I'm Paul Kassabian. I'm a structural engineer and a Principal at SGH in Boston, MA. I taught graduate students at MIT for nine years and currently teach on/off at Harvard's Graduate School of Design (GSD). These are videos based on my years of teaching structures to students.