Why Did the Champlain Towers South in Surfside Collapse?

 

Around 1:30 AM on last Thursday, June 24, about two-thirds of the thirteen-story Champlain Towers South condominium in Surfside, Florida crashed to the ground.  At this writing (Sunday morning), five bodies have been recovered, but there are 156 people still unaccounted for.  As recovery efforts have been hampered by hazardous conditions and persistent fires in the rubble, it may take weeks for rescue and recovery workers to pick through the remaining debris.

 

This tragedy not only took the lives of many unsuspecting people, most of whom were asleep in their beds, but has raised a host of questions that may not be definitively answered for months.  At this point, all we can do is examine the logical possibilities that could explain what happened.

 

The building was erected in 1981, and was constructed of reinforced concrete.  The reinforcing bars ("rebars") in reinforced concrete must be protected from corrosion, especially in a humid salt-water environment such as the Miami area.  The condominium was sited on the Atlantic Ocean, and some news reports have indicated that subsidence of a few millimeters a year was occurring for some time before the collapse.  While building a secure foundation on a beach is an engineering challenge, the fact that hundreds of similar structures have stood for many decades on beachfront properties shows that it is a challenge that competent engineers can easily overcome.

 

At least one video exists of the collapse.  It shows the building viewed from the south, on the inside of the roughly L-shaped structure.  The collapse has already commenced at the start of the video, and it is fairly obvious that the entire central tower is moving downward.  This conflicts with some reports that the collapse began near the top and transferred downward in a pancake-type fashion.  There are lights on in a few condo units near the top, and as the electric service connections are sheared the lights go out and the entire central tower plunges to the ground, leaving the east tower to its right standing for a few seconds.  Then it topples to the west as well, leaving only the western tower standing.

 

An inspection report from more than two years ago indicated that concrete supports in the parking garage underneath one of the towers were showing cracks and spalling (flaking), exposing bare rebar in some cases.  This by itself is not necessarily a sign that serious structural problems are present, because superficial spalling and cracking can occur without affecting the load-bearing capability of a supporting member. 

 

However, reinforced-concrete structures are unforgiving of failure of even one load-bearing member.  For example, suppose for whatever reason (uneven construction of the foundation, for example), one of the support pillars in the garage underneath the central tower began to receive much more than its fair share of the load of the floors above it.  While concrete will withstand a great deal of force in compression (squeezing), it is intolerant of tensile force (pulling) and will come to pieces with just a slight tensile force.  That is why rebars are installed, to take up the tensile stress that is present in any complicated structure.

 

Imagine making a squat cylinder of clay with your hands, setting it on its end, and squeezing down on the top.  It's going to get shorter and the middle will bulge out.  The top and bottom will mainly be in compression, but the middle part is in tension.  Rebar is supposed to handle all the tensile forces that arise in reinforced-concrete structures.  But suppose the rebar in this overloaded column was rusty or weak for some other reason, and let go?  The column would then be free to bulge outward and fail.

 

Once even one column at the bottom of the structure fails, and the floor above it fails, everything above that column is probably doomed.  Floors pull columns out of alignment, those columns fail, and you get the kind of collapse we see on the video. 

 

This pattern is familiar to demolition experts who use explosives to "implode" reinforced-concrete buildings.  Taking out the central support columns at the bottom with properly-sized explosive charges will typically cause the structure to collapse inward on itself, which superficially looks like an implosion. 

 

Another possibility is that the roof of the central tower, which was being worked on, was overloaded by construction equipment and failed.  Too much heavy construction equipment played a critical role in the collapse of the I-35W bridge over the Mississippi River in Minneapolis in 2007.  However, this type of failure would have caused a sequential top-to-bottom collapse as the upper floors collided with the lower ones, and that pattern was not evident in the video referred to above. 

 

A possibility that no one has mentioned, but should be considered at least as a logical possibility, is sabotage.  Many of residents of the condominium were presumably wealthy and included Jews and other ethnic minorities.  Someone familiar enough with explosives and building demolition techniques could have managed to breach whatever security safeguards were in place to keep unauthorized people out of the basement parking garage, installed the explosive charge, vacated, and set it off remotely.  If this was done, the evidence lies beneath tons of fractured concrete and will eventually come to light, but not for quite a while.

 

One survivor recalled first feeling the building shake, then hearing a boom and seeing two-thirds of the building coming down.  This is consistent with an explosive event whose vibrations would be transmitted through the solid structure faster than the speed of sound in air.  But abrupt failures of large stressed structural elements often sound like explosions, so this account should not be treated as definitive.

 

Our sympathy and prayers are with those who lost relatives and loved ones in this tragedy, especially as the long process of recovery continues.  The same process will eventually answer the question of what happened last Thursday, but the wait may be a long one.

 

Sources:  I referred to the video of the collapse, available at https://www.youtube.com/watch?v=KR29pLccutY; a USA Today article at https://www.usatoday.com/story/news/nation/2021/06/26/miami-condo-collapse-inspection-reports-concrete-deterioration-surfside/5355975001/; https://www.fastcompany.com/90650317/why-buildings-collapse; and the Wikipedia articles "Building implosion" and  "Surfside condominium building collapse."

Mexico City Metro Train Crash: Structural Problems?

 

Around 10:30 Monday night, May 3, a train on Mexico City's newest Metro Line 12 was traveling east on an elevated portion of the track from the Tezonco to the Olivos stations.  Line 12 was fraught with problems during and after construction in 2014 and 2015.  Several sections of the line were shut down for repairs in 2014, and some citizens complained that the line was further damaged by the 2017 Puebla earthquake.

 

Just as the last two cars of the train were passing over one of the Y-shaped concrete support structures that hold up the girders supporting the tracks, the support structure failed.  This caused the girders to collapse to the freeway median underneath the track, and dropped the train cars some 5 meters to the pavement.  A total of 24 people died either in the wreck itself or shortly afterwards, and 79 people were hospitalized.  It was the worst Metro accident in over forty years, and the Mexican government declared three days of mourning for victims of the disaster.

 

Mexico City Mayor Claudia Sheinbaum has invited a Norwegian accident-investigation agency to look into the causes of the tragedy.  And definitive answers will have to await the agency's work, which may take months or years.  But there are some things we can say already about what happened.

 

Every engineered artifact has both a physical aspect and a human-relations aspect.  The physical aspect is simply the thing itself:  in this case, the concrete supports and girders that failed.  The human-relations aspect (an inadequate phrase, but I can't think of a better one at the moment) is the history of the human organizations, relationships, and interactions involved in the artifact's design and construction.  The human part of the equation is a "structure" that is equally as important as the physical structure it produces.  Both the physical objects and the human-relations history have to be investigated for a complete picture to emerge about how things went wrong and who might be responsible.

 

As every civil engineer knows, concrete is strong in compression but weak under tension.  You can squeeze it and it resists well, but a pure concrete cylinder with no reinforcing bars ("rebar") in it will fail pretty fast if you pull on it.  That is why support structures such as the flat-Y ones that held up Line 12 have to have extensive rebar networks inside them to handle the tensile stress that shows up on the top of the Y, as the pressure of the girders try to stretch the arms outward and downward.

 

I am not a structural engineer and I have exhausted all my knowledge about reinforced concrete in the preceding paragraph.  But people paid to know these things know how to design such structures so that they can withstand both static loads and also the dynamic loads of things like moving trains and earthquakes.

 

Mexico City is one of the most earthquake-prone large metropolitan areas in the world, and one would expect their engineered structures to show evidence of this fact.  Without additional information, I can't say what special precautions have been taken to ensure that the Line 12's support members can withstand earthquakes.  The 2017 Puebla quake had its epicenter near Mexico City and a magnitude of 7.1, and resulted in over 360 fatalities and the collapse of some 40 buildings.  Although the newly-built Line 12 withstood the quake without collapsing, it is an open question whether the quake caused hidden damage to some parts of it.

 

Admittedly, finding internal flaws in concrete structures is a hard thing to do.  Although non-invasive technology such as ultrasonic testing and X-rays can be used, they tend to be either expensive or inconclusive or both.  The only structural test that many civil engineers will accept as conclusive is to stress something until it breaks.  That's fine with small test samples, but it has obvious drawbacks for testing completed structures.

 

Recent research, including some at Texas State University where I teach, has been focused on building "smart structures" that incorporate electronic sensors which can alert engineers to incipient flaws before they get serious enough to threaten the structure's integrity.  If something like this had been installed in the support members on Line 12, it's possible that authorities would have known about the defective support well before it failed, and could have taken steps to repair the defect.

 

But in the absence of smart sensors, the only way to prevent such tragedies as the one that happened on Line 12 last week is to build supports to be strong enough not to fail.  And here is where the human-relations issues come to the fore. 

 

A Reuters article on the accident points out that the portion of Line 12 that collapsed was built by a consortium of a Mexican business empire controlled by the family of famed businessman Carlos Slim and the Mexican division of a French firm.  Large engineering firms can be as good or better than small local firms, and bigness by itself is not a vice.  But governments and businesses in Mexico have been known to host corruption problems. And any time a public work such as a Metro line spectacularly fails, it is time to scrutinize the history of how the work was contracted, how it was inspected, and whether any shortcuts or graft were involved. 

 

As the contracting firm itself pointed out, it is premature to assess any blame or jump to any final conclusions regarding the cause of this multiple-fatality accident.  And Mayor Sheinbaum may be doing the right thing to call in well-respected experts from another country to perform the investigation, if local or federal inspection services are not up to the job or could not be expected to deliver an unbiased report.  So we will simply have to wait for the investigators to draw their conclusions.  In the meantime, Line 12 remains idle and thousands of Mexico City commuters have to find another way to get to work.  But at least they can be reasonably sure they'll get there.

 

Sources:  I referred to a Reuters article " Mexico promises justice after metro train line collapse kills 24" that appeared on May 6 at https://www.reuters.com/world/americas/mexico-vows-probe-into-terrible-metro-collapse-says-too-soon-point-fingers-2021-05-04/.  I also referred to the Wikipedia articles "Mexico City Metro overpass collapse" and "2017 Puebla earthquake."

Tragedy in Taiwan: Collapse of the Golden Dragon Building

On Saturday, Feb. 6, a magnitude-6.4 earthquake shook the island nation of Taiwan.  Nearly all the high-rises in the southern city of Tainan withstood the quake without serious damage, partly because of new building codes enacted after a more severe earthquake in 1999 killed 2400 people.  But in the Feb. 6 quake, a 17-story building containing 96 apartments completely collapsed, trapping victims inside and causing the majority of the deaths involved.  As of Feb. 11, the confirmed death toll in the quake stood at 55, but many bodies probably remain in the ruins of the Golden Dragon apartment building, which was erected in 1994 before the new building codes took effect.  On Feb. 9, the building's developer and two associates were arrested and charged with professional negligence causing death.  Prosecutors claimed that shoddy construction was responsible for the building's collapse, saying that cans of polystyrene foam were used as fillers in the reinforced-concrete structure and that steel reinforcing bars were too short. 

Building a structure that can resist earthquakes is a challenge that modern structural engineers tackle routinely.  Very few steel-framed high-rises are seriously damaged by earthquakes, because the type of steel used in them has a certain amount of "give" which allows the stresses of a shaking foundation to bend but not break supporting members.  The only exception is the unusual case when an earthquake's period coincides with a building's resonant frequency and vibrations build up until something snaps.

Reinforced concrete is another matter entirely.  Concrete has excellent compressive strength, but it's brittle and doesn't bend easily.  If you try to bend it, parts of it are put into tension, and pure (unreinforced) concrete has almost no tensile strength, so it cracks when subjected to the pulling forces that bending causes. 

Many decades ago, construction engineers figured out how to embed "rebar"—steel reinforcing bars—in concrete to provide the tensile strength that concrete alone cannot provide.  Properly apportioned and applied, reinforcing bars can make concrete-framed structures just as strong as steel ones, with the advantage that setting up molds and pouring beams and floors can be a lot cheaper than assembling a steel frame.  So many buildings for which cost is an issue, such as apartment complexes, are made of reinforced concrete.

However, making such a structure earthquake-resistant is a challenge, especially if it was not originally designed that way.  A personal anecdote will illustrate this.  I attended the California Institute of Technology in Pasadena from 1972 to 1976.  That institution began its existence in 1891 as a vocational school funded by businessman Amos Throop.  By 1912, the main building on campus was Throop Hall, a reinforced-concrete-and-brick structure that stood until a 1971 magnitude-6.6 quake seriously damaged it.  Engineering studies showed that the structure was fatally flawed with regard to earthquake resistance, and would probably collapse in another quake of the same or greater magnitude.  So despite its historic associations, it was condemned and fell to the wrecking ball during my freshman year there. 

The problem of what to do with existing structures when building codes change is difficult, and municipal authorities rarely condemn buildings that are not obvious ongoing hazards simply because of a building-code change.  The Golden Dragon apartment building may have been erected in compliance with the codes as they stood in 1994, but emotions are running high after the disastrous collapse, and the developers will have to argue in court as to whether they behaved responsibly during the construction of their building.

One way to enable reinforced-concrete structures to withstand earthquakes is to make ductile joints between the horizontal and vertical members of the structure.  This will allow the building to "follow" horizontal ground movement without imposing fatal strains on the supporting walls.  The fact that lightweight material such as plastic foam was used as fill may not necessarily indicate shoddy construction.  And the length of rebars is something that may or may not have had anything to do with the building's collapse.

The good news coming out of this tragedy is that more buildings didn't collapse, as for example happened in Haiti in 2010.  What few building codes existed there were not enforced, and although there were few structures more than three or four stories high, over 200,000 of them collapsed and the death toll exceeded 100,000.  As a rapidly developing nation, government officials in Taiwan did the responsible thing following the 1999 earthquake and imposed building codes that required buildings to withstand a certain level of earthquake shocks.  The fact that only one major high-rise collapsed, and that one a pre-1999 structure, says that the new building codes have been largely effective.

In addition to investigating the construction of the ill-fated Golden Dragon, Taiwan officials may want to consider a program of inspections of pre-1999 structures with an eye toward preventing more such tragedies in the event of a larger earthquake.  Even if the conclusion is that things are okay, this would be a reassuring thing to find out.  And if some other structures are like time bombs waiting to be set off by a large earthquake, the time to find that out is now, not when the next big one hits. 

Sources:  I referred to news items from Agence France-Press carried by the Australian Broadcasting Company at http://www.abc.net.au/news/2016-02-11/taiwan-court-hears-of-critical-flaws-in-quake-hit-high-rise/7160954 and UK's Daily Mail at http://www.dailymail.co.uk/wires/afp/article-3439998/Taiwan-developer-grilled-collapse-quake-building.html.  I also referred to the Wikipedia articles on earthquake engineering and the California Institute of Technology.