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 post-tensioned concrete truss structure may have collapsed during tensioning adjustments following a stress test. Over-tightening cables to close up the cracks in the concrete could’ve ultimately led to a collapse. More speculation has been collected in this Wikipedia article.
As investigators start recreating the accident, it’s important to remember that determining the cause could take as long as 18 months. Here are some other points about this bridge:
- The bridge was built using 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 stress tests were ongoing while the cables were being adjusted, that could have been a contributing factor.
- 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. This is highly unlikely to have had any contribution to the collapse.
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.
THE FIU BRIDGE COLLAPSED OH MY GOD pic.twitter.com/JO7jfx5AoN— Gabriela Collazo (@GabrielaRose12) March 15, 2018