What Is Arching Action?

by Don Friedman on April 18, 2016

A phrase that inevitably shows up when we discuss structural action within masonry walls is “arching action.” Usually, when we say that we mean that the masonry (most often brick, but not always) is acting as if there were an arch even though there is not.

Let’s see it:

The hard-burned tan face brick has been removed over the window, exposing the sloppier back-up made of common brick. The metal at the top center is a balcony bracket, the blue metal on the left is part of the scaffold, and the outline of a shallow jack-arch is visible at the bottom of the back-up. But the fascinating part of the picture is the self-supporting partially-demolished face brick.

The face brick in this area is not tied to the back in any way – this building was constructed before metal ties were used, and there are no header bricks visible – and the rough opening is supported against vertical movement only at the sides. The bricks are obviously not shaped or arranged as arch voussoirs, so whatever the “arching action” is, it is not entirely dependent on arch geometry.

In fact, each brick is corbelled: cantilevered half its own length, and held in place by the weight above the non-cantilevered half. This is a false arch, similar to those used in antiquity, but it may well be more than that. A false arch has no thrust, but there is a potential line of thrust in a shallow curve over the corbels of the brick. If the corbels did not exist because there was not enough weight above to hold the brick cantilevers in place, that line of true arch thrust would not work because the bricks would fall out of position; the false arch action that holds the bricks in position enables the true arch thrust. The actual stresses within the brick, which we did not attempt to measure for such a small amount of work, probably represent a combination of the true and false arch mechanisms.

To use a popular phrase: it’s complicated.

More thoughts on this topic: here.

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