The Morani Bridge Collapse: Style Over Substance?

Riccardo Morani (1902-1989) was an Italian civil engineer and bridge designer who was one of the earliest proponents of designs that used mainly prestressed concrete, rather than mostly steel.  In 1967, a bridge he designed was put into service in Genoa, Italy.  It spanned a river, some railroad tracks, and other portions of the city with three tall pylons, each of which had concrete stays reaching diagonally down to the roadway, which was suspended some 145 feet (44 m) above the ground.  It came to be known as the Morani Bridge, after its designer.

On Tuesday, August 14, during an intense rainstorm one of the tower-supported sections of the bridge suddenly collapsed.  As of today (Aug. 26), a total of 43 people have died as a result of the accident, not to mention injuries and property damage, which will total in the millions.  Government officials have called for the revocation of the contract with Autostrade per l’Italia, the private firm that handles highway maintenance in Italy.  One mourner at the state funeral held for many of the victims said that “In Italy, we prefer ribbon-cuttings to maintenance.” 

Engineering experts consulted by the media all said it was too soon to draw any conclusions about what might have caused the bridge to fall.  Bridges designed by Morani have a history of requiring more maintenance than more common designs do.  The stark elegance that may have appealed to clients around the world who were looking for something distinctive to add to a city skyline was achieved at a cost of asking a lot of the material that was used in the bridges Morani designed.  As we have mentioned before, pure concrete has almost no strength in tension, so to use it as a structural material, it has to be reinforced with steel “rebars” and other components that can withstand pulling stresses.  This would be especially true of the stays that slanted down from the tops of the towers to support the roadbed.  Over time, corrosion can attack these tension members, sometimes invisibly deep within a vital member of the structure. 

The evidence of why the bridge collapsed is buried inthe huge piles of rubble that workers will need to clear meticulously and carefully, and because such work is both a huge project on its own and demanding of attention to detail, it may be months or even years before we have an answer to the question of why the bridge collapsed.  After a bridge in Minneapolis collapsed in August of 2007, it took over a year for the U. S. National Transportation Safety Board to issue its final report on the accident, which attributed the collapse to a design flaw that made a gusset plate too weak. 

The problem with forensic investigation of prestressed-concrete bridges is that concrete is a much more complex material than steel.  Unlike steel, which is fabricated under carefully controlled conditions in a steel mill, concrete is often formed onsite, and the way it is mixed, poured, and treated after pouring can influence its ultimate strength and other properties.  Nevertheless, most prestressed-concrete bridges withstand the stresses they were designed for, and so the reasons for the Morani collapse will be interesting to discover, if they can be found.

While we still do not know whether the collapse was due to an initial design flaw or faulty maintenance, the question of maintenance for bridges and other vital pieces of infrastructure is an urgent one that industrialized nations all around the world are struggling with.  In 2017, the American Society of Civil Engineers (ASCE) gave the U. S. a D+ in its “infrastructure report card,” saying that 56,000 bridges (about 9% of the total) were “structurally deficient” in 2016.  While the situation has not reached such a crisis that we see bridges falling down every month, tragedies like the Morani collapse remind us that the price of deferred maintenance is sometimes much higher than anyone would like to pay. 

It’s a little bit like preparing for war.  The only way you know you didn’t spend enough money on preparing for a war is if you lose it.  You can win with barely enough resources, or with three times more resources than you need, and the result is the same.  The art and science of maintenance consists in doing enough to prevent nearly all major tragedies and to do something about minor problems fast enough, while not simply wasting resources on painting a wall that doesn’t need painting, for example. 

Judging by the rarity of bridge collapses, most bridges were either built well enough to start with to survive many decades with whatever maintenance they’ve received, or have been maintained well enough to keep standing.  But the shock value of a major bridge collapse is one of the main motivators for public funding of infrastructure maintenance, which has none of the appeal of new construction. 

Engineers are mostly used to working out of the limelight, doing dull but necessary things like scheduling expensive maintenance that takes money away from more flashy and popular government activities.  Riccardo Morani was somewhat an exception to this rule, attaching his name to striking bridge designs that caught the eye of the public time after time.  If there’s the equivalent of an Internet connection wherever he is, I’m sure he’s sorry to see what has happened to his creation in Genoa, whether the failure is due to him personally or due to insufficient maintenance over the five decades the bridge has carried traffic since it opened.  But maintenance is a job for the living, not the dead, and engineers in charge of maintenance owe it to their constituent publics to be sure that tragedies such as the Morani bridge collapse don’t happen.  We look forward to finding out what went wrong in Genoa a couple of weeks ago, and applying those lessons to future problems so that they can be avoided before more people get killed.

Sources:  I referred to news reports on the accident carried by Time’s website at http://time.com/5367757/italy-genoa-bridge-collapse-death-toll/ and The Guardian at https://www.theguardian.com/world/2018/aug/14/unusual-span-of-collapsed-genoa-bridge-had-seen-frequent-repair-work.  I also referred to Wikipedia articles on Riccardo Morani and the I-35W Mississippi bridge, and the ASCE report card at https://www.infrastructurereportcard.org/.

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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A Bridge Too Close

Construction accidents that lead to loss of life are always tragic, but it's unusual that anyone other than construction workers are killed or injured in them.  When hazards to the public are involved, most construction organizations take extra precautions to make sure that public safety isn't jeopardized by potentially dangerous operations or conditions.  Last week, a freak accident at a construction site came uncomfortably close to turning me into a statistic, but instead picked off another unfortunate member of the public whose safety engineers are supposed to protect.

Most new interstate bridge construction in Texas these days involves those huge prestressed-concrete beams—the ones you will see now and then rolling down a highway after being turned temporarily into a long trailer by hitching the front to a tractor and the back to a set of wheels.  Whenever one of these beams is hoisted into place across an active roadway, the Texas Department of Transportation (TxDOT) usually waits until late at night and diverts traffic away from the overpass under construction, so that if something goes wrong, no passing vehicle will be squashed like a bug under a falling beam.  But once the beam is in place, especially if the roadway involved is as busy as IH-35 that goes from Laredo on the Mexican border up through Austin and Dallas-Fort Worth, the usual thing is to let traffic pass once again.  And that is usually all the precautions that are needed.  But it wasn't enough on Thursday, March 26.

Somehow—the investigation is still ongoing—a flatbed tractor-trailer carrying a bucket-truck-type load wound up going north on I-35 near the small tourist town of Salado, about 50 miles north of Austin.  As part of a months-long construction project to widen the interstate in that area, TxDOT had recently placed the last of about a dozen of the concrete-beam monsters across both northbound and southbound existing lanes as part of a new overpass.  So the picture was that the beams were bridging the roadway, but the structure wasn't yet a bridge—just a set of beams laid between the end buttresses.  Once the superstructure of the bridge was completed, TxDOT would "armor-plate" the leading edge of the bridge with a thick steel L-beam so that any overheight loads would simply damage the armor plating without threatening the structural integrity of the bridge.  But there was no armor plating in place on Thursday.

Concrete is great for bearing evenly distributed compressive loads.  Concrete beams even perform in tension well if prestressed to be in compression, as these beams no doubt were.  But anyone who has tried to make a hole in concrete with a chisel knows that if you concentrate a lot of pressure at a small point, concrete will crack.  And that's exactly what happened about 11:30 AM on March 26.

Despite height-warning signs of a 13.5-foot clearance (there was actually more room than that), part of the flatbed truck's load was high enough to strike the leading concrete beam with such force that it not only cracked, but slammed forward into the adjacent beam and caused it to fail too.  Once that happened, many dozens of tons of concrete beams plunged onto the busy roadway.  It was just a matter of chance that one of the people driving under the bridge at that moment wasn't yours truly.

Earlier that day, I had left San Marcos by car for Wichita, Kansas—to attend a funeral, as it happens.  The logical route would have been straight under the beams in question, but lately, I have encountered so many traffic delays on I-35 that I've taken to driving a back-road route to the west.  It goes through small towns and beautiful hill-country vistas, and although it takes about an hour longer than you can do on I-35 when I-35 doesn't have any slowdowns, I believe the last time I-35 didn't have any slowdowns was sometime around 2003.  So about the time the beams were crashing down in Salado, I was leaving I-35 in south Austin for my leisurely westerly route.

An unfortunate pickup driver was crushed beneath one of the beams and died, and three other people were treated for minor injuries.  The driver of the flatbed that apparently caused the accident was uninjured and made it through the crash to the other side, but will no doubt face a lot of questions from authorities and investigators.  And TxDOT may start thinking about putting armor-plating on concrete beams the minute they are put in place, rather than waiting until the bridge is finished to do so.

In one of the news reports, a TxDOT spokesman objected to the headline "Bridge Collapse" because the structure in question wasn't yet a bridge.  This is a distinction without a difference to most people, but to engineers, especially bridge engineers, you can't have a bridge collapse until you have a bridge.  Technically, this was a construction accident, not a bridge collapse.  But the man in the pickup is just as dead, and it's possible that compartmentalized thinking has contributed to this accident. 

Clearly, the beam was vulnerable to damage caused by traffic from the moment it was put in place and traffic was allowed to pass.  But somehow, the sequence of construction didn't allow for the armor plating (as I'm calling it) to be attached until later in the process.  Because the circumstances are so unusual, construction crews have avoided such an accident up to now.  It's rare that any type of overheight load gets onto the interstate and goes very far, much less one that has the concentrated mass and speed to actually break a large concrete beam.  But we have seen that it can happen, and I bet that somewhere in Austin engineers are rewriting construction specs to make sure that any beams exposed to traffic have their armor-plating on from the start. 

Besides death and injuries, this accident caused annoyance to thousands of beleaguered denizens of I-35, which has the reputation of harboring some of the worst freeway traffic in the U. S.  Situations like this are exactly why I no longer drive long distances on that stretch of roadway if I can avoid it.  My sympathies are with the family and friend of the man who died, with everyone who got stuck in the 10-mile backup for hours, and for the engineers who are sadder but wiser after experience has taught them a better way to build bridges—a category of engineers that stretches back hundreds of years to the dawn of history.

Sources:  I used information from the online edition of the Austin American-Statesman for Friday, March 27 (subscription only), and from the Austin Fox News TV channel KTBC at http://www.myfoxaustin.com/story/28627602/txdot-investigating-what-led-to-crash-on-i-35-overpass-in-salado.