Design Flaw Identified in FIU Bridge Collapse


Back
on Mar. 15 of this year, a new pedestrian bridge across a busy highway running
through the Florida International University campus suddenly collapsed, killing
six people and injuring eight more.  The bridge
was fabricated as a single long concrete truss consisting of upper and lower
decks connected by a series of diagonal and vertical struts.  Trusses are familiar elements of steel-bridge
construction, but there are special design issues involved in making a truss
out of concrete.  And according to an
update issued by the U. S. National Transportation Safety Board (NTSB) on Nov.
15, it looks like someone may have made a fatal error in part of the design.

When
we blogged on this accident back in March, it was already known that some
cracks had shown up at the north end where the northernmost vertical member and
the adjacent diagonal strut went into the bottom deck.  At the time, the construction supervisors held
a meeting about the cracks, but the NTSB has successfully prevented publication
of the meeting minutes before their final report on the accident can be issued,
which probably won’t be till some time next year.  The Miami
Herald
reports that after the meeting, a construction worker was sent out
to tighten tension rods inside the diagonal strut.  This worker appears to be the one who died
when the bridge collapsed.

The
modern civil engineer has abundant design resources at his or her
disposal:  computer-aided modeling and
stress calculations, three-dimensional visualization and planning tools, and other
computational aids that take a lot of the former drudgework out of mechanical
and civil engineering design.  Such aids
have made possible many recent designs that would have been difficult or
impossible to create using the old manual slide-rule and design-table
approaches. 

But
even with all the computer assistance in the world, the information about a
given design has to be understood and checked by human beings.  That is why most public civil engineering
projects must have their designs approved by a registered professional engineer
(PE), whose stamp or signature appears on the drawings.  That stamp puts the reputation of the
engineer on the line:  it is a guarantee
that the design will do what it’s intended to do. 

Long
chains of reasoning and responsibility lie behind every decision to approve a
set of drawings.  Those chains may pass
from person to person, or from computer output to person.  Computer-aided calculations answer such questions
as, “If this particular junction of a strut and a vertical member is under
that kind of stress, will it be able to withstand the stress with a reasonable
margin of safety?”  Given that the
inputs to tried and tested software are correct, the software should give the
correct answer, assuming that the person using the software knows how to use it
and interpret the results correctly. 
Furthermore, the chain of engineering integrity requires that when the PE
responsible for the overall design, the person whose stamp of approval appears
on the plans, asks underlings if this or that part of the design is good, the
underlings must give an honest answer.  And
the PE must trust that answer, or rather, the persons answering for the
integrity of the plans.

In
any human organization, there is always the possibility of error.  Sometimes errors can be traced to a
particular person, and sometimes they can’t. 
The NTSB has made sure that all available sample materials from the
wreckage of the FIU bridge were tested to see whether they met the minimum
specified strength and other standards. 
And so far the results are all positive, so it doesn’t seem that the
collapse can be based on defective materials. 

The
death or injury of bystanders in a bridge collapse is a tragedy regardless of
whether the accident could have been prevented or not.  But if a design flaw really is the reason for
the collapse, it will be ironic that the design, which has been termed
“unorthodox” in the Herald
report, was before its installation a point of pride for FIU’s civil
engineering program, which specializes in accelerated bridge construction of
the type that was used on this bridge. 

Back
when universities were smaller and more personal institutions, engineering
faculty members would sometimes contribute their professional expertise to
campus projects, helping in the design of new buildings or consulting
professionally with regard to campus technical issues.  The FIU civil engineering professors do not
appear to have been personally involved in this particular design, however,
other than to give their informal approval of the general approach and construction
methods.  In fairness, many bridges have
been successfully built using on-site accelerated bridge construction, which
does not appear to be implicated in the collapse.  But in this case, it might have been a good
idea to have qualified faculty members go over the plans, and they might have
caught any errors that contributed to the collapse.

However,
that is not the way most universities operate these days.  Each professor has his or her own irons in
the research and teaching fires that are lit under them, and to ask one of them
to stop what they’re doing and check some plans for a new building or bridge
would be regarded as an unfair imposition on their time, and rightly so.  They might reply that there are professionals
being paid to do that, and they would be correct.

But
when professionals are paid to do a job, it’s up to them to do it right.  According to the latest update from the NTSB,
someone (or possibly something, if we include computers) failed in that responsibility.  And physical objects are not forgiving.  The warning signs were there:  cracks in the location that subsequently
failed.  We hope that the NTSB will use
the embargoed meeting report to figure out what went wrong, not only in the
original design, but also in the management process that led to the fatal decision
to try tensioning the strut without stopping traffic underneath the
bridge.  But until the final report on
the accident is issued, this accident stands as a reminder to everyone who
deals with technology that could kill or injure someone—a reminder that the
lives of innocent people depend on how well you do your job.



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