BOUNCING BACK IN BROOKLYN BRIDGE PARK
It’s not often that a company known for building high rises gets to work on a piece of public infrastructure. But that was exactly the case when Pavarini McGovern joined the team working to modify Brooklyn’s Squibb Park Bridge.
Squibb Park Bridge first opened in March 2013 and was an instant hit with the surrounding neighborhood. An efficient pedestrian overpass from Brooklyn Heights to Brooklyn Bridge Park, the bridge gives residents and visitors a scenic connection to the park over its 450ft timber and steel zigzag with a playful underfoot bounce. But after some structural instability concerns arose in 2014, the bridge was closed until retrofits could be made to safely reopen it to the public without sacrificing its whimsical aesthetic.
In 2016, Brooklyn Bridge Park Corporation (BBPC) hired top engineering firm Arup to design and oversee modifications to the bridge, and Pavarini McGovern as the construction manager to execute the plan. Pat Kirshner, VP of capital projects for BBPC, was excited to see Pavarini McGovern win the assignment since the two had successfully teamed on the Sea Glass Carousel when she worked for the Battery Park Conservancy. “I appreciated Pavarini McGovern’s enthusiastic and attentive approach,” she said of the carousel project—another undertaking which was intricate, peculiar and dear to the community. Kirshner was confident that Pavarini McGovern would supply a team with both the skill and sensitivity to bring the Squibb Park Bridge back into use and make it the neighborhood jewel it was meant to be.
After thorough performance testing and review, Arup developed a solution to address the bridge’s stability issues: Pavarini McGovern would have to make several modifications to the existing structure. First, PMG installed special scaffolding below the two main spans to support the bridge above Furman Street and allow realignment of the bridge structure. With the scaffold in place, the bridge’s own support cables were de-tensioned. Next, the team carefully repositioned the bridge into its ideal geometry using adjustable supports attached to the scaffolding. They stiffened crucial timber-to-steel connections, reconnected lateral bracing and installed tuned mass dampers to reduce the vibration of the bridge as pedestrians walk across it. Once these modifications were complete, the cables were pulled back into tension and clamped into place at each truss connection to keep the bridge in the right shape.
With the bridge once again carrying its own weight, the PMG team removed the scaffolding and put the tuned mass dampers to the test. The dampers reduce vibration by absorbing energy as the bridge moves. To confirm they were working, the team placed accelerometers on the bridge while test walkers marched across it to the beat of a metronome. “We had nine people bouncing together at fundamental frequency,” said David Farnsworth PE, principal at Arup. The team took measurements first while the dampers were disengaged, and then again while engaged. Once final adjustments were made, the bridge retained approximately 10% of its former bounce—just enough to put a spring in your step.
The bridge is open once again, and public response has been very positive. The long-awaited reopening coincided with the first warm weather of the year, helping people access Brooklyn Bridge Park at just the perfect time. In a morning visit to the bridge, Kirshner stood at its center with a contented smile, watching the people walk by. “The best thing is just seeing people use it,” she said.
The bridge has undergone regular monitoring to test the efficacy of the improvements and is fitted with vibration sensors which will signal an alert if it vibrates too much. But the outlook is clear: Arup and Pavarini McGovern have made this bouncy bridge safe. As development continues in the area surrounding Brooklyn Bridge Park, the bridge will continue to enhance residents’ quality of life as a vital pedestrian shortcut with quirky Brooklyn charm.
Client: Brooklyn Bridge Park Corporation
Services: Construction Management
Completion: April 2017