Unanswered Questions About the FIU Bridge Collapse

Last March 15, an
innovatively-designed pedestrian bridge installed for less than a week suddenly
collapsed onto a busy roadway in Florida International University, killing six
people and injuring several more. 
Although many bridges have been erected successfully using the technique
known as accelerated bridge construction used for this bridge, this was one of
the first times such a bridge failed during construction, and so both the
engineering community as well as the public at large would like to know what
went wrong.  But answers have been slow
in coming.

Accident investigations are
tedious, painstaking tasks, and it’s understandable that the U. S. National
Transportation Safety Board (NTSB), which is the primary agency charged with
the investigation, is going to take as long as it takes to find out what
happened.  On May 23, the NTSB released a
preliminary report on the accident.  But
those hoping to read about a metaphorical smoking gun in the report will be

This is not unusual for
preliminary reports.  Depending on how
accessible the raw data is that has to be examined, preliminary NTSB reports
can come close to answering all the relevant questions.  But a bridge is a large physical object that
doesn’t yield its secrets easily, and it’s quite possible that the agency is
conducting tedious and lengthy examinations of the pieces recovered from
certain sections of the bridge to reconstruct exactly what happened.

The bridge was a concrete
truss, and I learned just now the technical definition of a truss, which “consists
of two-force members only, where the members are organized so that the
assemblage as a whole behaves as a single object.”  (That’s from Wikipedia.)  Trusses are everywhere in constructed
objects.  Your house or apartment
probably has roof trusses in it.  If a
sign support arching over a highway isn’t a single large tube or pole, it’s
probably a truss.  You can tell a truss by
its triangles, each side of which is one of the aforementioned two-force members.  And all that means is that the force on each
member (strut) is applied at only two points, generally the ends.

The NTSB report focuses on
two particular truss members of the bridge, especially one designated as No.
11.  If you picture the bridge as a concrete
floor and ceiling connected by slanting concrete beams (truss members), these
two members were near each end, slanting downward from near the end of the ceiling
to the far edge of the floor at the end of the bridge.  Each slanting member formed the diagonal of a
triangle, the other two sides being the end part of the ceiling and a vertical
beam connecting the ceiling and floor at each end. 

Clear?  Maybe not. 
Anyway, the point of making all these triangles in trusses is that a
triangular shape made of straight sides does not change its shape easily.  You can push on the opposite sides of a
square made of four bars tied together with pivots at the corners, and the
pivots will let you squash the square flat. 
But even if a triangle is made with pivots at the corners, it won’t
change its shape until one of the sides actually bends or breaks.  And that may be what happened to the FIU

There has been much attention
paid to some cracks that showed up near the bottom of No. 11 several weeks
before the collapse.  Photos of these
cracks were accidentally released, the NTSB griped about it, and then the
agency included them in their report later. 
There are good reasons why certain touchy information about accidents
shouldn’t be released before an NTSB report is completed, but it’s not clear if
these reasons apply in this case.  The
Miami Herald commented in an article on the report that the newspaper is trying
to get more information about the bridge released from the Florida Department
of Transportation under Florida’s public-records act, but the lawsuit is still
in progress.

As many people know,
concrete can withstand a lot of compression (squeezing), but hardly any
tension.  That is why steel reinforcement
bars (rebars) are embedded in concrete structures of any size, and why the FIU
bridge included adjustable tensioning rods in some of its members, including
No. 11.  A couple of sources indicate
that construction crews were re-tensioning these rods after it was moved in
place when the bridge collapsed.  The
Herald report speculates that the member might have been compressed too much by
these tensioning rods in an attempt to close the suspicious cracks.  Every concrete structure has a limit as to
how much compression it can stand.  If
the worker got carried away and put too much stress on an already compromised
member, it might have simply crumbled from the pressure.  Because it was in such a vital location,
failure of that member would have caused exactly the kind of accident that

Especially if the worker who
made this mistake was the one who died, the NTSB is reluctant to draw any
conclusions in this direction that are not supported by abundant evidence.  It’s up to them to figure out what traces of
such a mishap would remain in the rubble that was collected from the site, as
well as whatever preliminary evidence such as photos that are available.  So rather than any nefarious conspiracy to
cover up systematic wrongdoing, the delay and refusal to share information may
simply be out of concern that premature release of information could lead to
unnecessary agitation, hurt feelings, and even more lawsuits.  The legal system has grown to accommodate the
NTSB’s role in accident investigation, and anything that would upset that
particular applecart may not be helpful.

All the same, it would be
good if the root causes of this very public and tragic event could be
unearthed, if there are any to be found. 
And other things being equal, it would be nice to have that happen
sooner than later.  Clearly, something went
wrong, and everyone using accelerated bridge constructions stands to learn
something potentially useful from the final report on this accident.  But as the process may take months longer, we
may simply have to wait. 

Sources:  The
Miami Herald’s article on the NTSB preliminary report appeared on May 23, 2018
at https://www.miamiherald.com/news/local/community/miami-dade/article211735504.html.  The NTSB report itself can be accessed at
https://www.ntsb.gov/investigations/AccidentReports/Reports/HWY18MH009-prelim.pdf.  I also
referred to the Wikipedia article on trusses (the engineering kinds).      

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