Building and Construction

Bridge Cracks Were Found Earlier Than First Reported: Miami Herald

17 May 2018

A newspaper reports that a support truss in pedestrian bridge that collapsed in mid-March at Florida International University showed signs of cracks 10 days before the structure was lifted into place.

Documents were released in response to public records requests from the Miami Herald newspaper. The paper says the documents show that the university's construction and engineering team found potentially problematic cracks in the bridge earlier than officials have acknowledged. (View an engineer's drawing of the bridge.)

The bridge collapsed March 15, killing six.The bridge collapsed March 15, killing six.The cracks were found in late February at the base of a diagonal support member at the north end of the span. That was the point where the structure failed on March 15 while under construction. The 950-ton bridge fell onto a roadway below, killing six.

(Read "After Bridge Collapse, Investigators Search for Root Causes.")

Three independent engineers who looked at photos, records and bridge blueprints at the newspaper's request agreed the cracks were a red flag signaling potentially critical structural problems. They say that a truss member, identified in plans as No. 11, was "under-designed" and not strong enough to withstand the pressure from the weight of the bridge.

The location and diagonal shape of the cracks shown in photos support that theory, the engineers reportedly said. They said the cracks should have prompted work on the bridge to stop for an in-depth review that likely would have resulted in the truss connection being re-engineered and reinforced.

(Read "NTSB Looks at Tensioning Rods as Bridge Collapse Probe Continues.")

In a February 28 memo, Jose Morales, a consulting engineer for FIU, reportedly notes one crack in particular "merits special attention." Morales urges that the bridge engineer of record be consulted "to provide a response." That engineer of record is named by the Herald as W. Denney Pate of FIGG Bridge Group, which designed the bridge. The memo was sent to a project manager at Munilla Construction Management, the bridge project's builder, and reportedly copied to Alberto Delgado, a construction project manager at the university, and other members of the project team.

(Click to enlarge.) Image of the bridge being lifted into place.(Click to enlarge.) Image of the bridge being lifted into place.The National Transportation Safety Board, which is investigating the bridge collapse, reportedly told the university and the Florida Department of Transportation not to release records dated after February 19, so there are no available public records to document any response from FIGG or other team members to Morales' memo. The Herald says it has sued to obtain other records related to the bridge collapse. The bridge collapse is also the subject of a Miami-Dade police homicide investigation and families of some of the victims have filed lawsuits.

The cracks mentioned in the February 28 memo apparently were discovered while the span was still resting on the ground. That may be important because the span at that point was resting only on a support on either end, mimicking the way it would stand once installed over the roadway. The newspaper says that the cracks thus could have been a sign of shearing pressure — a sideways stress — that the No. 11 truss could not handle once under the full load it was meant to carry.

Speculation is that the connection between the No. 11 truss and the bridge deck was poorly designed, lacking sufficient steel and concrete to bear the load. All the engineers interviewed by the Herald emphasized that a clear-cut cause for the collapse may not be established until the NTSB publishes its conclusions, and that their analysis could change based on new information.

The newspaper said that the cracks were discussed at a meeting of project team members the morning of the collapse, though no safety concerns were raised, according to the Florida Department of Transportation. The bridge failed while crews were on top of a walkway canopy, adjusting the tension on steel support rods inside the No. 11 truss member.

One early report, by U.S. Sen. Marco Rubio, said the crews were tightening the rods in an effort to close the cracks, leading to speculation that over-tightening could have caused the truss to shatter.

NTSB investigators take core samples as part of their probe into the cause of the bridge collapse. Source: NTSBNTSB investigators take core samples as part of their probe into the cause of the bridge collapse. Source: NTSBBut other experts, citing released bridge plans, reportedly said crews were more likely loosening the rods. Because a transport machine could not traverse that roadway edge, the point where it would lift the structure was moved toward the center of the bridge. That means the end would sag when lifted. To prevent that, the rods were added to provide extra compressive support.

The rods were tightened before the bridge was lifted off the ground. The plans called for them to be loosened after the bridge was in place because that extra compression was no longer needed. That's the operation some experts interviewed by the newspaper believe was underway when the bridge failed.

(This story was updated May 21 to include a link to an engineer's drawing of the bridge.)

To contact the author of this article, email david.wagman@ieeeglobalspec.com


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