Partly Hidden and Accidentally Photographed

I took the picture above just before the West Thames Street bridge was put in place. The mesh-enclosed tower in the middle is the elevator that will lead to the west side of the bridge, the gray-steel west pier of the bridge is just to its right. The center pier of the bridge, where the two spans meet, is on the left between the woman with the backpack and the high-reach truck.

The building behind the center pier, light blue with a slatted facade, is a municipal garage, next to and partly over the entrance to the Brooklyn-Battery Tunnel. The north wing of the garage spans over the tunnel road where it intersects West Street. It is far from the longest or most-heavily-loaded bridge embedded within a building in New York, but it’s the most visible.

Much of the garage is concrete-framed, but the portion of the garage over the road is supported on parallel steel trusses. The yellow thing you can sort-of see through the blue slats? That’s one of the trusses. It’s two stories high, or about 16-18 feet, which is a pretty good depth for a truss spanning about 120 feet. The truss form is a modified Warren with verticals, which works fine with a generally uniform load and with heavy H-section diagonals.

The one detail of the truss that’s visible from this distance and through the slatted facade is the size of the gusset plates that connect the truss diagonals and verticals to the upper and lower chords. From this angle the lower-chord gussets aren’t really visible, but I’ve seen them up close (from within the garage) and they’re mirror images of the upper-chord gussets. The big yellow rectangles are gusset plates connecting the upper chord, two diagonals, and vertical; the big yellow triangle is a gusset plate connecting the upper chord and a vertical. The gussets are so big that almost half of the total length of each diagonals is covered by the plates. This is partly because the forces are so large and partly because the connections are reviewed rather than welded.

A lot of buildings have transfer trusses hidden within. This unremarkable garage just happens to be giving us a glimpse of the kind of structure that’s normally completely hidden.

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