A Weird Analogy

From a small 1940s or 50s apartment house, a probe exposing the loose lintel over a window:

The window is at the bottom right, with a pair of 2x4s shoring the lintel at the extreme bottom right corner. The form of the lintel is a pair of channels, within the back-up wythes, toed toward one another to form a rough box; the outboard channel has a shelf angle welded to it to carry the veneer brick.

So far, so good. The reason I took this picture of a pretty boring lintel is the diagonal cut of the channel end. That’s not normal. That kind of diagonal cut (although closer to vertical) came into use in the nineteenth century as a “fire cut”. It was intended to solve a specific problem: the collapse of brick walls when a large fire burned through wood joists at mid-span. The fire damage would turn a series of simply supported beams into a series of cantilevers, as the burn-through destroyed structural continuity in the wood, and that series of cantilevers loaded with dead load and the live load of furnishings would create a large-out-of-plane moment on the supporting masonry wall. The idea of fire cuts is that they allow the stub of a burned joist to freely rotate and fall, saving the wall from being pried apart by those moments.

Fire-cutting steel beams in general doesn’t make much sense. The beams are spaced much farther apart than wood joists, so there is no combined prying action, and the beams fail by bending freely as their yield stress drops from heating. The kind of cantilever moment that fire cuts are meant to prevent doesn’t exist. Fire-cutting a five-foot-long lintel makes even less sense, as the lintel is so much stiffer than it needs to be that even a loss of capacity is unlikely to lead to failure worse than a little sagging. And in addition, the heat at a lintel will be very high on a human scale, but not so high compared to the heat of a beam at an interior floor. It should be noted that loose wood lintels, when they were used, were generally not fire cut.

I have no idea what happened here.