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 https://www.ntsb.gov/investigations/AccidentReports/Reports/HWY18MH009-investigative-update2.pdf.  I also referred to the Miami Herald report on the update carried at https://www.miamiherald.com/news/local/community/miami-dade/article221706575.html.  My original blog on this accident at http://engineeringethicsblog.blogspot.com/2018/03/the-fiu-bridge-collapse-more-questions.html had an incorrect date for the accident, which has now been corrected.

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 disappointed.

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 bridge.

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 happened.

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).      

The FIU Bridge Collapse: More Questions Than Answers

On Thursday, March 15, 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 differ. 

A sketch of the 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 suspension pipes.* 

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 week.

*NOTE added after posting:  A comment posted by the Happy Pontist, a professional bridge designer, contradicts my admittedly amateur opinion that the tower and suspension pipes might carry a load.  He confirms what the university website claims, which is that they were for vibration damping, and their alignment with the truss members was for esthetic reasons only.  Thanks to the Happy Pontist for this correction. 

Sources:  I referred to reports on the accident carried by ABC News at http://abcnews.go.com/US/pedestrian-bridge-florida-international-university-collapses/story?id=53774444, the Miami Herald at http://www.miamiherald.com/news/local/article205661039.html, and the Washington Post at https://www.washingtonpost.com/news/grade-point/wp/2018/03/16/recovery-efforts-continue-following-florida-bridge-collapse-at-least-6-dead/?utm_term=.6019ddf2639a.

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