Monday, December 31, 2018

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.

Sources:  The NTSB update of Nov. 15, 2018 can be found at  I also referred to the Miami Herald report on the update carried at  My original blog on this accident at had an incorrect date for the accident, which has now been corrected.

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