Errors were made in the design of the collapsed Florida International University Bridge (FIU), a second investigative update by the American National Transportation Safety Board (NTSB) has found.
The bridge collapsed while it was under construction in March this year, killing six people and injuring a further eight.
The design review was conducted by the Federal Highway Administration’s (FHWA) Office of Bridges and Structures in support of the NTSB’s ongoing investigation.
The NTSB said since the first update was published in August, new information on the bridge design had been developed and additional forensic examination/evaluation of bridge structural components had been completed.
The report claims that the errors were made in the design of the northernmost nodal region of the span where two truss members were connected to the bridge deck.
The design errors then resulted in an overestimation of the capacity of a critical section through the node, comprised of diagonal member 11 and vertical member 12 [see diagram below], and apparent underestimation of the load on the same critical section.
FIU bridge showing numbering of truss members
Additionally, it said that the FHWA evaluation determined that the cracking observed in the node prior to the collapse was “consistent with the identified errors”.
Tests carried out by the Turner-Fairbank Highway Research Centre on materials taken from the bridge, showed that the concrete and steel met the minimum requirements specified in the project’s designs.
It said two types of specimens were tested during evaluation of the post-tensioning rods: machined round bar specimens and full-size bar specimens. For all tension tests, it said the specimens met the specified minimum yield strength, tensile strength, and percent elongation at fracture.
Furthermore it said that steel reinforcing bars collected from the collapsed structure included several different sizes all of which met the minimum yield strength, tensile strength, and percent elongation at fracture for their respective sizes.
Samples of rebar recovered from the collapsed FIU pedestrian bridge
The investigation is now continuing to examine the design, review, and construction processes as well as the actions taken once the cracking was observed.
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