Where Was That Crack Before FIU Pedestrian Bridge Collapse?

Where Was That Crack Before FIU Pedestrian Bridge Collapse?

Where was the crack that an engineer reported to the construction crew working on the pedestrian bridge that collapsed at Miami’s Florida International University?

Now that emergency responders have removed the six victims who were trapped under the bridge when it fell, the investigation is moving into higher gear.

Given the news that an engineer working on the project had reported a crack in the concrete prior to the collapse — albeit noting it wasn’t a life-safety issue, a key question will be whether the crack led to the bridge failure.

Watching the video from a nearby driver’s dash cam (posted farther down), which shows the minute the bridge collapsed, you can see a section near the left giving way, pulling the rest of the structure down with it.

As the NYTimes.com reported, two days before the collapse, “an engineer on the project left a voice mail message for the Transportation Department about ‘some cracking that’s been observed on the north end’ of the bridge, according to a recording from the department released on Friday. At both the meeting and in his message, [the engineer] said the cracking did not present any safety issues.” The story continues:

Construction crews were working on a diagonal beam at the north end of the structure at or about the time of the collapse, according to information the National Transportation Safety Board provided to local members of Congress. Workers were tightening cables that ran inside the beam.
Such adjustments, which engineers call “post-tensioning,” are common in concrete designs to fine-tune the structure once it is in place. In this case, however, it was not clear whether the cable-tightening was routine or an urgent undertaking in response to the discovery of the crack in the bridge.
Witnesses said the collapse appeared to start near the north end. But no one, including the N.T.S.B., has so far placed any blame for the collapse on the cables or cable-tightening work.

Florida Sen. Marco Rubio (R), in a post on Twitter on March 15th, the day of the collapse, said the “cables that suspend the bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today.”

A few things about concrete. Small cracks are common, given its properties. As most engineers will explain,  concrete has weak tensile strength, which is why we have rebar, reinforcing steel added to help improve its ability to withstand more compression and load-bearing stress.

Clearly, load-bearing stress led to the bridge failure. It is early yet in the probe. But watching the video provides visual clues that suggest the extent of that reported crack — and how crews addressed it — will be a major question to be answered.

Marco Rubio’s tweet:

FIU’s Pedestrian Bridge Collapse Probe, ABC and Safety