Around 1:30am 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 millimetres 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 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 unauthorised 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.
This article has been republished with permission from Engineering Ethics.