Steel

Aslant

by Don Friedman on January 8, 2018


That’s the ceiling – which appears to be the structural roof – at the north entry to the Whitehall Street station on the R/W. Specifically, this is the stair leading down from the entry mezzanine to the platform. The excavated volume of space is a sloped rectangular prism, with the roof following the slope.

The standard roof for the subways built before 1940 is a series of concrete vaults supported on steel beams. Usually, as you can see here, the web of the beam is filled in with concrete that’s integral with the vaults. This is a very strong system that needs little or no reinforcing in the concrete to function properly and that strengthens the beams through partial composite action with the concrete vaults.

The roof over the stair is the standard system on a slant. Obviously the vaults are tilted, but so are the beams (even though they don’t have to be) and better still, so are the base plates below the beams. You can see the base plate for the beam in the center of the photo sticking out of the wall.

The use of a steel base plate in what has to be a reinforced-concrete wall is a little odd, but this station was built in 1918 and concrete in the New York area was usually pretty low strength at that time. Having a base plate on an angle is much odder, since gravity only pulls down. The roof may be geometrically following the slope of the stair, but the earth pressure is directed downward, and can be broken (for ease of analysis) into two smaller forces: one perpendicular to the roof and one parallel to it. The force perpendicular to the roof works on that base plate the way it’s designed, but the force parallel to the roof has the side of the beams bearing on the concrete directly. It’s functioning, so it’s obviously okay, but I don’t quite get the logic that says a base plate is required for one force but not the other.

It’s possible that the designers simply took their standard flat roof and tilted it in this area and called it a day. It’s over a hundred years ago, so I doubt anyone will ever know.

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That’s a picture from the new west concourse at Penn Station, phase 0.01 of the plan to convert the Farley Post Office into a new headhouse for the station. I had a picture up a while back showing a newly-exposed column at an office building on Rector Street; the exposed columns above, which support the […]

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Changing Fashions

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The black paint and lighting make the picture a little hard to read, but this is an old gentleman dressed up in the latest fashion. The building is 2 Rector Street, now known as 101 Greenwich Street*, a 1905 office building that was expanded vertically in the 1920s. It’s a fairly typical building of its […]

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Different Types of Efficiency

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I seem to keep coming back to train station roofs as I write. That’s happening partly because I travel by train a good amount so I’m looking at those roofs, and partly because they tend to have exposed structure designed by engineers with little or no separate architectural design, which may not make them beautiful […]

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Detailing For The Forces

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A big part of structural design is “detailing,” which is drawing the way various pieces of a building  are connected. The bridge above (click to enlarge all of the photos) is in Wrocław and has a shorter span than the average American suspension bridge of any era. I walked over it a number of times […]

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Composite Structure

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That’s the side wall and an oblique view of the front of a small building on Nassau Street. The front appears to be early twentieth century, but the odds are good that this is an older building that has been modified multiple times. If nothing else, the storefront – visible as sheet metal at the […]

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Sidewalk Vaults and True Obsolescence

August 14, 2017

For people unfamiliar with sidewalk vaults, the illustration above, from 1865, might seem plausible, but it’s actually Daniel Badger’s fantasy of how he could sell more iron. The left-hand side is reasonably accurate for mid-1800s vaults: the facade columns extend down past the plane of the sidewalk, marking the separation of the cellar proper from […]

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Weathering Metals

August 9, 2017

Beautiful, right? Thanks to some old alteration work, we get to see the results of a nice little experiment in exposing metals to water. You’re looking at three beams here: a modern steel beam that had been directly supporting a sidewalk (middle left, outlined in purple below), the original cast-iron girder that supports that beam […]

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