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Anatomy Of An Arch

Just your basic street wall in SoHo… Putting aside the unfortunate paint (probably there to cover graffiti) there’s some interesting design in those window head, particularly at the basement level. For the sake of clarity, here’s a labelled close-up:

A is the first floor window; B is a decorative stone band; C is the field of the wall at the basement level; D is a relieving arch for the basement window; E is decorative fill between the relieving arch and lintel; F is the basement-window lintel; G is the basement window; and H is the sidewalk. (An unrelated note: there’s a vault at the basement or cellar level below that sidewalk. The dark portion of the sidewalk adjacent to the building is steel diamond plate where there used to be cast-iron vault lights, the lighter outboard portion is a series of granite slabs that were originally, and possibly are still, supported on cast-iron beams as the vault roof.)

The most obvious point I have to make is that the actual load that has to be supported over the basement window is small, consisting of the first-floor window and the masonry from its sill down to the basement window. Very roughly, based on guessing the wall thickness and figuring the other geometry by counting bricks, that rectangular prism of masonry weighs about 5000 pounds. That might seem big, but it is not on the structural scale. The one-piece stone lintel might be able to carry that load acting as a beam, but it would never have to: even if the relieving arch was not there, arching action in the field of brick would carry most of the load to the piers on either side. Even with the relieving arch, there might be some arching action taking place in area C. Whether or not there is arching action depends on the stiffness of the relieving arch: it appears to be built of common brick, which means the curve of the arch was made by having the head joints vary in width. If there’s any real give to the mortar (i.e., if it’s a lime mix or a lime-cement mix rather than all portland cement) then that arch is more flexible than an arch made with shaped brick and may well be more flexible than the theoretical arching-action arch of the brick above. If it’s more flexible than the arching-action arch, then arching action is carrying some of the load.

The stone lintel is supporting its own weight and the diagonal-brick decorative fill and really not much else. But I do like the detail of taking a decorative stone band and having it turn into the lintel. I particularly like having the corners of the lintel nipped off to allow the brick of the relieving arch to run by.