The Inflexible, Bent

We informally divide the world into flexible objects and inflexible ones. That’s a gross simplification based on a bad idea. Everything is flexible; the only difference is degree. The example that I have used roughly one per year since the late 1990s, while teaching, is that the Great Pyramid of Giza bends when the wind blows on it. It just doesn’t bend very much compared to, say, a daisy loaded by wind pressure.

Gabi took this picture in a church in Spain. At first glance it’s a groin vault in a gothic structure. The second glance shows a series of thin steel rods cross from left to right. They’re modern interventions intended to stop the vault from spreading in that direction. The third glance shows a potentially serious problem: the vault is seriously deformed. The rib running lower-left to upper-right, for example, has a long straight section in the middle, extending almost to the central round hub, that used to be curved.

We think of stone as inflexible, and it is in the manner of the Great Pyramid: under load, it deforms so little that we typically can’t see it. But assemblies of ashlar masonry are far more flexible because the mortar moves easily and small rotational deformations of the joints are enough to cause large deformations in the assemblies.

This kind of deformation of a stone structure can mean one of two things: (1) the stone has reacted to external conditions (typically loading and support settlement) by moving to a stable position and all is now well, or (2) the stone is reacting to external conditions, is become less stable, and will eventually fail. The early stages of both look exactly the same.