On Thursday, March
16, workers on a newly installed span of a pedestrian bridge across the busy
seven-lane Southwest Eighth Street that divides Florida International
University (FIU) from the adjacent town of Sweetwater were adjusting some
tensioning cables in the span. The
Saturday before, Barnhart Crane & Rigging Company had lifted the span from
the side of the road where it had been built over the preceding months, carried
it a short distance along the closed street, and lowered it into place
successfully. FIU officials were
proud that a novel technique called accelerated bridge construction (ABC) was
being used for the bridge, because the school itself has a center that promotes
and studies that technique.
Instead of blocking traffic for weeks or months while hazardous lifting
operations put a bridge into place piece by piece, workers using accelerated
bridge construction build the bridge offline, so to speak, and then shut down
traffic only briefly as entire spans are lifted into place.
The technique has
been used frequently in the last few years with no major problems, so no one
expected any issues this time. As
it turns out, though, those expectations were disappointed.
At 1:47 PM Thursday,
without apparent warning the entire bridge collapsed onto the busy highway,
crushing cars and killing at least one construction worker who was on the
bridge at the time. Six people died
in the accident.
Some of the
questions raised by this tragedy can’t be answered yet. For example:
Why wasn’t the road
closed during an operation such as cable tensioning that might have endangered
the stability of the bridge?
The whole idea of
ABC is to keep transportation going as much as possible. Evidently the engineering firm which
designed the bridge, FIGG Construction of Tallahassee, had reason to believe
that the span was going to support itself safely during tensioning operations,
so no one issued orders to close the road.
What is the
significance of some cracks that one engineer reported seeing on Tuesday, two
days before the collapse?
Right now, we don’t
know. Cracks in concrete mean that
wherever the crack shows up, tensile (stretching) forces locally exceeded
compressive (squeezing) forces.
Concrete is a material that can withstand a lot of compression, but
hardly any tension on its own.
That’s why large concrete structures contain carefully designed and
placed steel reinforcement such as “rebar” and sometimes cables,
which were evidently used in the ill-fated structure as well. Some cracks are only skin-deep, so to
speak, and do not indicate a structural problem, just a cosmetic one. Others may go all the way through a
structure and are signs of a major problem. Until forensic experts piece together the remains of the
174-foot span and figure out where the trouble started, we won’t know whether
the cracks were superficial or significant. At a meeting of construction personnel Thursday a couple of
hours before the collapse, the cracks were discussed and the consensus was that
they were not a safety issue.
If the final design
of the bridge included a tower and suspension-bridge-like pipes connecting it
to the span that collapsed, why weren’t those parts of the structure in place
before the road under the span was opened to traffic?
I am not a
mechanical or civil engineer, and so I’m strictly an amateur compared to
someone with professional training in those fields. Reportedly, the university website about the bridge stated
that the tower and suspension pipes were not needed for static support, and
were there simply to cut down vibrations and add esthetics. Allow this amateur to beg to
intended completed bridge can be viewed here, and
shows that several concrete struts or trusses that connect the concrete roof of
the bridge to the lower walkway part are straight in line with the planned
suspension pipes. It certainly
looks like the trusses were intended to carry the weight of the walkway through
tension (probably by inner tension cables) up through the roof to the
It’s possible (more
than possible, if the FIGG engineers knew what they were doing) that even
without the tower suspension structure, maintaining proper tension on cables
inside the trusses would keep the whole concrete structure in compression sufficiently
to counteract the tension that the walkway part would experience once it was
put in place over the roadway.
What may have happened (and this is purely my speculation) is that when
a construction worker began to adjust one of the tension cables, he might have
done something as simple as turning a nut the wrong way. Such an action might have sent an
already marginally stable structure over the edge of failure, and once such a
delicately balanced system has one part fail, the rest of it goes too.
I do not envy the work of forensic engineers who now have to
transport the messy wreckage somewhere so they can pore over every identifiable
piece, figuring out what was where and what the exact positions of tensioning
adjustments were. From such small
details a picture should emerge that will let us figure out what went wrong
last Thursday that led to such a dismal outcome of something that was supposed
to be a point of pride.
In the meantime, this disaster should serve as a warning for
every construction firm doing accelerated bridge construction. Maybe we should ease off on the
accelerator a little, at least until we find out what happened at FIU last