Two Arches

First, from Providence, a stone arch holding up some of the front facade of a bearing-wall building:

If you look closely, there are some cracks around the individual voussoirs. Given the short span and the relatively large depth of the arch, it should be fine under load. I actually wonder if the cracks are because it’s not loaded heavily enough to create enough compression to prevent the cracks. The reason it might not be heavily loaded is that there is a spandrel panel above it perfectly capable of arching action, and since the spandrel panel is in the load path before the arch, it will carry as much load as its stiffness will allow before the load gets to the arch. The little cracks don’t look particularly serious.

Second, from upper Manhattan, a door in a steel-frame building:

This arch is carrying very little load, since there is a spandrel beam directly above it supporting the facade above the second floor and the second floor itself. But the thing about this arch it not the arch structural action but the 3D decoration. The first arch is decorated, but in a fairly ordinary way for its Victorian era. The sawtooth decoration here, making it look like the voussoirs are turned 45 degrees out of plane with the wall, may not be unique, but I’ve never seen it anywhere else.

I don’t know that there’s any deep message here other than one that is well known: masonry construction allows for great flexibility of geometry and built-in ornamentation as long as the basic geometry – in this case, making sure that the arch thrust line is within the arch material – is satisfied.