Construction

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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