Odd One Out

Two views of the Queensboro Bridge… First, a HAER photo I’ve used before:

Second, a view from an early 1900s edition of King’s Views of New York, showing a rendering of the bridge before it was completed (on top), paired with a photo of the just-completed Williamsburg Bridge (below).

The rendering makes the Queensboro look lighter than it does in reality, and I suspect that is the fault of people’s unfamiliarity with its type. It’s the only major cantilever truss in the city and there were not a lot in the surrounding area when it was built. Like most cantilever trusses, the depth of the truss varies to match the level of bending moment, giving it a profile that looks – a little, if you squint – like a three-span suspension bridge. Non-engineers might well think it was some kind of monstrously clunky suspension bridge.

(Digression about bridge design… Truss bridges are all, in the end, beams. Most truss bridges, like most beams, are a constant depth from end to end. You can design a simply-supported truss bridge where the truss depth matches the moment diagram and you get an arch truss, a fish-belly truss, or a lenticular truss, depending on whether the top chord, bottom chord, or both chords curve. Cantilever trusses are deep at the towers because those are the supports and so that is where the moment is greatest. Suspension bridges are a whole different creature and are deep at the towers to keep the tension in cables – which becomes greater as the cable “droop” gets smaller – reasonable.)

Given that you can build a suspension bridge using eye-bar chains that look a lot like the tension chords of Queensboro, and people did, the most telling physical difference that shows it’s not a suspension bridge is the lack of anchorages at the ends of the main spans.