This story was originally published in Concrete Construction.
After a concrete pedestrian bridge that was under construction collapsed last week, everyone is looking for answers. The 174-foot, 950-ton concrete span fell onto six lanes of traffic on March 15, killing six people and injuring several others.
The $14.2-million Florida International University (FIU) project was being delivered by the design-build team of FIGG Bridge Engineers of Tallahassee and Munilla Construction Management (MCM) of South Miami.
The focus so far has been on a FIGG engineer who left a message alerting the Florida DOT of cracks visible in the concrete. Although he said he didn’t believe the cracking posed a safety issue, its significance remains unclear. Some experts believe the cable-stayed structure was weakened by crew members tightening cables following a stress test. Over-tightening cables to reinforce the concrete could’ve ultimately lead to a collapse.
As investigators start recreating the accident, it’s important to remember that determining the cause could take as long as 18 months. Engineers say most bridge collapses happen because of work errors, not design flaws. Below are other considerations.
- Accelerated bridge construction (ABC) using a special crane called a self-propelled modular transporter (SPMT). ABC refers to Federal Highway Administration-approved construction methods that reduce inconvenience to the public by minimizing road closures. SPMTs slide components fabricated offsite into place at the jobsite. FIU's bridge was touted as the “largest pedestrian bridge moved via SPMT in U.S. history.”
- Cracks in the concrete. Not uncommon during construction, they could be merely cosmetic or a sign of a serious problem.
- Stress tests. These tests typically involve placing calibrated weights on the span and measuring how the structure responds. If the cables were over-tightened during the stress test, then it could have caused torquing that would create a cracking pattern in the concrete on impact with the road.
- Using concrete trusses over steel. Most bridges are built with steel trusses because of the pull-push pressures. Also, because concrete is much heavier than steel, the extra weight could've factored into the bridge's collapse.
- Mix design. According to one report, this was the world's first bridge made with “self-cleaning” concrete. Photocatalytic cement contains an additive that activates when exposed to sunlight to resist pollutants that gather on the surface and cause discoloration.
Forensic engineers are picking through debris, looking at designs, and piecing together inspection results to get to the bottom of what happened. We'll keep you posted.
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