I’ve talked before about the phenomenon called “arching action” but you’ll never see a better demonstration than this. Ignore the wood “studs” in that picture, they’re just supports for the now-removed plaster and lath. The real story here is a brick wall, two wythes/8 inches thick, with no lintel at the door. The thin wood planks of the door frame aren’t strong enough to support any significant load and are, in fact, failing. The result is that the brick above the door opening is vertically unsupported.
I am not surprised, and I expect that no one else is surprised, that the masonry above the door is failing. We put lintels over openings in masonry walls for a reason, which is that the masonry requires support against gravity. As is well known, mortar is not glue and will not support the weight of unsupported bricks for long, so without a lintel we expect to see the brick dropping down. And yet…
There’s a pattern to the failure. Measured vertically, the zone of failed brick is thinnest at the left and right ends (the sides of the door opening) and thickest in the middle. The area of unfailed brick directly above the failure looks like an arch. And if we define an arch by its structural action rather than by its appearance, then the area of unfailed brick is an arch. The relevant structural action for an arch is quite simple: supporting vertical loads across a horizontal span through the use of compressive forces. The bricks in the arched line above the failure are in compression in the exact same pattern as the voissoirs in a regular arch. The compression follows the visible curved line down to the upper corners of the door opening, and the resulting forces from there are vertical compression on either side of the door carrying the gravity load down to the masonry foundation below and horizontal compression in the wall on either side of the door.
The brick below that virtual but visible arch has failed because it is not in enough compression from the sides to hold it in place against the downward pull of gravity. But it doesn’t matter in terms of the overall stability of the wall because the arch is carrying the masonry above. In other words, the failure can’t and won’t spread because all the rest of the brick, including the virtual arch, is stable. This is a nice demonstration of a sometimes hard-to-picture idea.
More on this topic: here.
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