Historic Structural Detail: A Composite Arch

by Don Friedman on March 16, 2016

One of the earliest challenges for structural engineers – long before the profession formally existed – was how to support masonry walls over openings. The tight column spacing of Greek and Egyptian temples, for example, was based in part on the limited spanning capacity of stone beams. Masonry arches, as used by the Romans, could span further and carry more load but had fairly strict rules on geometry in order to work.

For the most part, mid-nineteenth-century New York was built of small buildings with short spans and few masonry-wall openings bigger than a four-foot-wide window. There was, however, one common exception to that: the rear of industrial loft buildings. These warehouse and manufacturing buildings, concentrated in lower Manhattan, were built using the same wood-joist and brick-wall structure as almost every other building, but they tended to have a geometric peculiarity. They filled their lots entirely at the ground floor, but had a small set-back of the rear wall to create a light-court at the floors above. That meant that the upper, main portion of the rear wall, some four or five stories high, had to be supported over a void at the ground floor ceiling. A masonry arch wouldn’t work because there was not enough vertical space between the ground floor ceiling and the bottom of the second-floor windows, and because there was no structure at the sides capable of taking an arch’s horizontal thrust.

Mona took some pictures of a demolition site that show the solution. (Unfortunately, the pictures had to be taken though a plexiglass window, so there’s some ghosting.)

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Demolition of an old building is in progress, with only remnants left of the steel and iron framing and the brick walls. Once again, looking at a demolition site shows what we can’t see in a normal building. What’s that in the background?

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It’s the answer to how you support a brick wall before steel beams were available: a cast-iron arch with a wrought-iron tie-rod. Because iron is so much stronger than masonry, the arch rise could be kept shallow, to fit below the second-floor windows above. By using an iron tie-rod, the thrust problem was eliminated. This arch, a composite of the two readily-available metals of the time, is a self-contained unit used almost like a beam. The cast-iron portion is shaped like an upside-down rail, with a bulbous bottom and a flat top, with the top serving to support the brick of the wall above.

Where have we seen this type of composite beam before? Of yeah, the Bow Bridge deck beams, built circa 1862.

 

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