This Is Not An Architectural Critique Of The New PATH Station, Part 2

by Don Friedman on January 4, 2017


(Click on any image to expand.)

In part 1 (last March!) I mentioned that I was interested by what I could see of the roof of the PATH station itself, which is due west of the Oculus. The Oculus has received most of the press, is visible aboveground, and is (by virtue of the stores within it) more heavily traveled by tourists, but the “whale rib cage” roof over the station proper is fascinating.

It’s also so geometrically complex that it’s nearly impossible to photograph in such a way that it makes sense. The panorama photo above is amusing but grossly distorted. The roof is supported by a series of curved ribs running north-south, at right angles to the main axis of the space of the station and Oculus. It’s symmetrical around its own east-west center line. That centerline is generally low (each rib is a bit different from those adjacent, which is how the overall three-dimensional shape is formed), with a wide arch up to the east and west.

Those wide arches descend to a row of girders and free-standing columns on each side. The ribs then arch upwards again and descend to the floor as a second row of columns, engaged with the side walls of the space.

Note the hinges to the left and right of the column-line girder:

In addition to the girders at the column lines, there’s an arch running across the ribs, connecting them at the centerline.

I mentioned in March that the central arch seemed too small to be supporting the roof, and it still looks that way, although steel is very strong and that arch might be strong enough.

Photographing the space, the curve of the ribs – roughly a double M shape – suddenly seemed familiar. It’s the curve of a cantilever truss. The picture below was used circa 1890 to demonstrate the cantilever principle used in the Firth of Forth rail bridge:

The two men representing the main spans are not extraordinary strong. Rather, their arms are in tension, the wood poles running from their hands diagonally down to their chair seats are in compression, and the counterweights on the outer ends of the structure keep them from tilting in toward the load of the third man in the middle.

Now if I highlight (very badly) some of the structure…there’s our double M curve:

I don’t know if the upper legs of a true cantilever truss exist here. The arched roof is below the open plaza west of the Oculus and north of the museum, so there’s a lot of space unaccounted for between the visible ceiling in these photos and the ground level above. The “arch” that connects the centerline of each rib can be pictured by having a few dozen trios of suited and hatted men reenacting this diagram, with the men in each trio slightly different heights from their neighbors so that each trio is at a slightly different elevation above the ground, and the middle “load” man in each trio extending his legs to kick the underside of the seat of the middle man in front of him. The Busby Berkeley effect of the kicking would be amusing but would not add any significant strength to the combined structure.

My guess as to the nature of the roof structure is just a guess. But I doubt that center arch carries much load.

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