The unusual pedestrian bridge at Florida International University had an undetected design flaw that probably contributed to its collapse in March, the Miami Herald reported, citing a review by three independent structural engineers.
At the newspaper’s request, the engineers reviewed nearly 2,000 pages of mathematical calculations for the bridge over Tamiami Trail that collapsed March 15, killing one construction worker and five people in cars at a stoplight.
The engineers said a diagonal strut was inadequate to support the load at the north end of the bridge, where cracks appeared before crews installed the bridge over eight-lane Tamiami Trail on March 10.
Figg Bridge Group, which designed the FIU pedestrian bridge, and university officials declined to discuss the bridge collapse, citing an ongoing investigation of the incident by the National Transportation Safety Board (NTSB).
The NTSB will announce the official cause of the bridge’s collapse after it finishes investigating the incident, which could take as long as two years.
A spokesman for the federal agency said outside experts who draw conclusions about the cause of the bridge’s failure do a disservice to the public.
Figg Bridge Group designed the pedestrian bridge to look like a cable-stayed bridge, with a deck suspended by cables that fan out from a tall mast.
But diagonal struts beneath the bridge provided its structural support, not the “cables,” which actually were metal pipes intended to provide visual flair, especially at night, when lights on the pipes would be lit.
Figg and Munilla Construction Management (MCM) won the contract to build the bridge by proposing the faux cable-stayed design, an unusual version of a traditional truss design.
A competing proposal by a construction company called Fachinna lacked visual appeal, according to Tom Gustafson, an FIU administrator and a former speaker of the Florida House, who served on the FIU selection committee that awarded the contract.
In an audio recording of a selection committee meeting, Gustafson said the design that Fachinna proposed was “a 12-foot-wide sidewalk across a busy highway, and I don’t think that’s what we wanted.”
Figg’s unusual design made calculations of stress more complicated and led to different lengths and thicknesses for each of the bridge’s 12 diagonal struts, according to the structural engineers who reviewed Figg’s work.
The Herald got the structural calculations and design plans through a request for public records and shared them with structural engineers Ralph Verrastro, David Beck and Linwood Howell.
The engineers said the collapse of the FIU bridge could have been prevented if the diagonal truss at the north end of the span had been planted deeper in the ground and had more concrete and more rebar at its connection with the deck.
The engineers also cautioned that other factors could have contributed to the bridge’s collapse.
Beck and Howell said the so-called ABC technique for building the bridge in sections and assembling them may have contributed to its collapse, but the rapid construction technique itself probably wasn’t the cause. [Miami Herald] – Mike Seemuth