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FIU
BRIDGE COLLAPSE—A TRAGEDY AT MANY LEVELS
01/02/2020
I suspect that almost everyone in the concrete
industry has heard about the March 15, 2018,
collapse of the pedestrian bridge that was under
construction over a busy roadway at Florida
International University (FIU) in Miami, FL. The
collapse killed six individuals and injured at
least 10 others. The National Transportation
Safety Board (NTSB) issued its report detailing
the probable cause of the collapse on October
22, 2019.
Click
here for the full text of the
report.
In full disclosure, I was retained as an expert
for one of the parties, but in the end, I did
not provide any opinions to the attorney for
that party.
The NTSB wrote a blistering account of the
failures by various parties. It stated that the
probable cause of the collapse was design errors
by the Engineer of Record (EOR) and that
contributing to the collapse was an inadequate
peer review that failed to detect the
calculation errors in the bridge design. Severe
cracking was observed during construction in the
nodal region at the intersection of concrete
truss members, and this was characterized on
multiple occasions by the EOR not to be
significant. In fact, at the time of the
collapse, the structural member in question was
being retensioned at the direction of the EOR,
presumably in an effort to impart compression in
the nodal region of the cracking. Notably, the
NTSB also stated that the severity of the
outcome of the collapse was due to the failure
of various parties, including the EOR, the
inspection engineer, the contractor, the
university, and the Florida Department of
Transportation (FDOT).
Further, the NTSB noted that the EOR
underestimated the demand that was imparted in
the nodal region at the intersection of the
truss members, and the NTSB faulted the EOR for
not computing the capacity of the nodal zone
where the concrete truss element intersected the
bottom flange. In load and resistance factor
design of structural concrete such as used in
the ACI 318 Structural Concrete Building Code
and in the AASHTO Specifications, an
underestimation of demand and an overestimation
of capacity drives the capacity-to-demand ratio
(C/D) toward a value of 1.0. A value less than
1.0 indicates that there is a high probability
of failure of the structural element. Without a
redundant structural system that would allow a
redistribution of forces, structural collapse
will occur. The structural system used for the
FIU concrete truss pedestrian bridge had no
redundancy that potentially would have allowed a
redistribution of forces in the nodal region
that failed.
Various news reports and even the NTSB suggest
that the FIU pedestrian bridge was “complex” in
accordance with the FDOT requirements for the
project. However, the FIU bridge was not a
complex structure. I will agree that the bridge
design was unique, as noted in the NTSB report,
because concrete was being used for a truss in
which some members are in tension and others are
in compression. What is clear is that the
significant cracking and distress that had been
noted by various parties during construction of
the nodal region presaged imminent collapse. As
documented in the NTSB report, the cracking
observed by various parties was the type and
magnitude observed when failure occurs.
While the EOR, the peer-review engineer, and the
Federal Highway Administration on behalf of NTSB
all used structural analysis programs to
facilitate the rote determination of forces in
the members for the multiple load cases that
needed to be considered, the forces can be
determined by simple hand computations. This
truss bridge is an exemplar of using the
strut-and-tie method as detailed in Chapter 23
of the ACI 318 Structural Concrete Building
Code. There are very detailed requirements in
ACI 318 concerning design of nodal zones, and
had these provisions been followed, there would
not have been a failure of the FIU bridge.
The FIU bridge project was clearly a failure at
many levels, including an apparent lack of
communication between all parties involved and
inadequate focus on the safety of workers and
the public during critical stages of
construction. Although ACI has published
numerous papers and special publications and
held seminars specifically on the utility of the
strut-and-tie method since its introduction into
the 318 Code in 2002, it apparently has not been
enough. ACI and universities need to make a
significant effort to better educate engineers
on the strut-and-tie method, especially in the
detailing of nodal regions, and practicing
engineers need to step up and learn the method.
Randall W. Poston
ACI President
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