On the morning of last yearâs Florida International University pedestrian bridge collapse, when the engineer of record assured project team members that there were no safety risks related to cracks propagating across a part of the unusual single-truss structure, other project team members voiced mild concern, but no alarm. In hindsight, considering that the bridge had no inherent structural redundancy as it sat, incomplete, straddling a busy highwayâand would suffer a sudden, catastrophic and deadly collapse just hours laterâthe teamâs lack of urgency remains puzzling, say engineering experts contacted by ENR for comment.
Minutes of the meeting in the contractorâs field office recently released by the Florida Dept. of Transportation show that attendees offered modest suggestions and questions to FIGG Bridge Engineers.
Bolton Perez & Associates, the projectâs construction engineering and inspection contractor, asked, âDo we need temporary shoring?,â for instance. FIGG officials responded that it was not necessary. Instead, the minutes show that FIGG staff suggested that steel channels and post-tension bars would âcapture some of that force which is better than vertical support. The diagonal member is what needs to be captured.â To the suggestion that another engineer should peer review the bridgeâs cracks, FIGG concurred.
An official with FIU asked a representative with Bolton Perez their opinion of FIGGâs presentation analysis. Bolton, Perez said they could not comment at the moment, but would âexpediteâ a response in 2-3 days, according to the notes.
An FDOT representative asked FIGG to supply a copy of the presentation for the agencyâs records.
Engineers asked by ENR to review the meeting presentation and minutes for this story donât believe that it shows exactly what errors or mistakes precipitated the sudden collapse.
Designed with a single central, open truss, the pedestrian bridge structure featured a narrower top chord. The top chord was to serve as a canopy over the wider bottom chord, which would be the walking surface. Cables from a 109-ft-high central pylon, not yet built at the time of the collapse, would add stability, according to the design-build proposal. The concrete deck was designed with two-way post-tensioning tendons.
At the time of the collapse, contractors were apparently adjusting a tension rod in one of the diagonal struts between the chords at one end of the bridge. It is possible that the projectâs prime contractor, MCM, and its post-tensioning subcontractor, in attempting to fix the problems, made an error that caused the bridgeâs single truss to crack and give way. According to the National Transportation Safety Board, crews reportedly had been re-tensioning diagonal member 11 at the time of the collapse. Lacking redundancy, the truss failed at that end and fell to the ground, smashing autos and claiming six lives.
Just days before the meeting, the truss structure cast alongside the road was loaded onto its permanent supports and inspections showed no distressed members. But two days before the collapse, MCM emailed FIGG about cracks. FIGG responded by instructing MCM to install temporary shims in the base of a pylon directly below the portion of the bridge with the cracks, between the permanent support shims.
Then, on March 15, 2018, engineers, contractors, consultants, state DOT representatives and officials with FIU, the project owner, gathered to hear why the bridge designer thought cracks were occurring around part of the structureâs walking surface. The section in question was the bottom chord of the concrete truss comprising the bridge, at one of the diagonal web members at the structureâs north end.
Meeting notes indicate that it was known that cracks were âgrowing daily.â
Despite that, FIGG Bridge Engineers assured project team members that they saw âno safety concernâ due to the cracking. According to the meeting notes, FIGGâs lead technical designer Denney Pateâwho led the presentation, according to FIUâand bridge engineer Eddy Leon were on site for the presentation. Dwight Dempsey, FIGGâs design manager, joined by phone.
Team members in attendance probably held Pateâs opinion in high regard that morning. FIGG-MCMâs design-build proposal lists numerous accolades for Pate in support of its description of him as â