The twisting action of the bridge has since become known to engineers as “torsional instability,” which means that a structure has the capability to twist freely in the middle. I think of torsional instability as the movement no one expects. Most structures don’t have the right combination of size and length to twist noticeably, so torsional instability is forgotten about until a new construction dips just below the threshold where it manifests and then, suddenly, it’s back!
