An overly bouncy footbridge in Brooklyn, N.Y., reopened last month after a nearly three-year shutdown, as a lawsuit looms over its award-winning designer and national chief bridge engineer for HNTB, Ted Zoli.
Opened in March 2013, the $3.9-million, 450-ft-long Squibb Park Bridge was designed to bounce mildly. Later, residents began to complain that the bridge was bouncing too much. The bridge was closed in late 2014.
David Farnsworth—principal with Arup, which engineered the $2.5-million bridge fix—says the closure was not due to the bounciness per se but because “the bridge was basically slowly deforming over time. The main fixes we implemented were targeted at stabilizing the bridge and keeping it from deforming.”
Construction Manager Pavarini McGovern oversaw the retrofit, which began in 2015 with initial installation of some clamps to stabilize the bridge. Crews installed bridge support scaffolding, which enabled the workers to straighten and retension the cables.
To prevent future deformation, Arup turned the underslung suspension bridge into a truss, installing clamps to the cable that runs beneath the bridge to the diagonal timber trusses of the two center spans. “The original design allowed the cable to slide,” says Farnsworth. “We put clamps on either side of the saddle and another beneath to fully enclose it.” Metropolitan Walters fabricated and installed the clamps and sleeves that strengthened the diagonal connections, which might go into tension due to their eccentric positions.
The bridge marries durable black-locust timber with draped steel-cable supports. “We drilled through-holes into all the steel cups so that water would drain out and the bridge could dry more quickly,” says Farnsworth. Tuned mass dampers ameliorated the bouncing, which is inherent to lightweight bridges, he says.
The bouncing was purely vertical, unlike in the case of London’s Millenium Bridge. “There, [the bouncing was] primarily lateral in nature, not vertical. When enough people walked together in sync, the bridge started moving.” On opening day, the thousands of people who lockstepped into the frequency of that movement exacerbated the swaying. “We learned a lot about bridge dynamics from that project,” says Farnsworth.