“It’s Alive!”

by Don Friedman on June 23, 2017

That beautiful specimen of design is the Pulaski Bridge (not to be confused with the Pulaski SkywayGeneral Pulaski was a fellow worth memorializing) over the Newtown Creek, running between Greenpoint in northwest Brooklyn and Long Island City in western Queens.

The bridge is a double-bascule, otherwise known as a drawbridge with two movable leafs. It has a pedestrian walkway, recently improved by the addition of a separate bike lane, so that walkers and bikers are no longer sharing the same path; the walkway provides easy access to the 7, E, and M trains in Queens to people on the Brooklyn side. Since the bascule leafs rotate upward when the bridge is (rarely) opened to allow a tall boat through, there are three related joints in the roadway: one at each end of the movable leafs where they abut the non-moving portions of the roadway, and one at the center of the span, where the two movable leafs almost touch. Here’s that center joint:

Assuming the video loaded correctly in your browser, you just watched seven seconds of the two edges moving up and down relative to one another as the weight of cars driving by changed the loading of the two movable leafs. Other than proving, to anyone who didn’t already know this, that there are places you can safely walk by but where you shouldn’t stick your fingers, why bother?

The important point here is one that can’t be emphasized enough: all structures move when loaded. The amount of movement may be small, it may be imperceptible to us, it may be difficult to measure, but it’s there. If you load a structure, it moves. The fact that the bridge is visibly moving in two different directions at the joint is not a defect or a sign of a problem. It is, rather, evidence that the bridge is working as designed (each leaf is supported separately) and that al structures move when loaded (in this case, by the cars driving past). If you want to see a much more dramatic and larger-scale version of the same effect, here’s a video of the Manhattan Bridge moving as subway trains travel over it.

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