On June 24, 2021, a portion of Champlain Towers South, a 12-story condominium in Surfside, Florida, near Miami Beach, collapsed around 1:30 am. It was one of the most deadly structural collapses in U.S. history, with nearly 100 fatalities. Forensic investigations of the event will likely take years to complete, but there’s a lot we already know about the collapse. I want to summarize the events of this unthinkable tragedy, talk about a few of the structural engineering issues that may have played a part, and finally explain the process of forensic structural investigations and what might result from learning the technical cause of this catastrophe. I’m Grady, and this is Practical Engineering. Today’s blog is on the collapse of the Champlain Towers South building.

Champlain Towers South was built in 1981 along with a nearly identical North structure just up the street. The 12-story oceanfront tower was constructed of steel-reinforced concrete in the small Miami suburb of Surfside. The building sat atop an underground parking garage that extended below the adjacent common area that residents called the pool deck. The building had 136 condominium units.Unlike an apartment property with a single owner, Champlain Towers South was collectively maintained by those 136 condo owners through an association with board members.

In the early hours of June 24th, we know that there was a failure of the pool deck adjacent to the building. Tourists at a nearby hotel were swimming when they heard a crash. They walked over to the Champlain Towers to see that part of the pool deck had collapsed into the parking garage below. About 7 minutes later, the building began to fall. A security camera nearby caught the entire event on camera. The building collapsed in three sections: first a south portion of the building, immediately followed by a north portion, and finally, the east section. The entire western half of the building remained standing.

The search and rescue operation started immediately to get residents out of the damaged building and the rubble of the demolished area. Crews worked 24/7 to sift through debris for survivors. The western part of the structure, which didn’t collapse, posed a hazard to the rescue and recovery crews, especially with the threat of Tropical Storm Elsa potentially bringing high winds to the area. Town officials made the difficult decision to demolish the remaining part of the building on July 4th to safeguard the crews and avoid the possibility of it falling onto the existing search and rescue zone. As of the recording of this video, 97 people have been confirmed deceased. There were 126 people who were in the building during the partial collapse and survived.

Of course, the most critical question of the collapse is why it happened. Unfortunately it’s a difficult one to answer. The Town of Surfside put all of their records and correspondence about the building online in the interest of public transparency. It was in the process of being recertified, a requirement in Miami-Dade County for all buildings when they reach 40 years in age, and every 10 years afterwards. That process starts with a detailed inspection by a structural engineer. For Champlain Towers South, the inspection was performed in 2018. The findings of that structural inspection have been the focus of most of the early conjecture about the cause of the building’s collapse.

Among the items of concern identified during the inspection, one of the most important was the pool deck. The large concrete slab adjacent to the building also served as a ceiling to the underground parking garage. The inspection report noted major structural damage to this concrete slab, mainly as a result of poor drainage and failed waterproofing. The issue was that rainwater on the pool deck could filter through the pavers above the concrete slab, and then it had nowhere to go. So, instead of flowing along a properly sloped slab to drains, it simply pooled above the slab like a bathtub. Unlike a bathtub, this system wasn’t watertight. Runoff was leaking into the concrete below through joints, cracks, and into the pores of the slab itself. This wasn’t a surprise. It was a well-known problem at the condominium, and there were even plastic gutters installed in various locations along the ceiling of the parking garage to divert these leaks away from cars and walkways. But the water wasn’t just a nuisance to residents. It was also slowly deteriorating the reinforced concrete structure itself.

Reinforced concrete is an extraordinarily versatile building material because it is strong, durable, relatively inexpensive, and can be cast into just about any shape. For better or worse, it is one of the most ubiquitous building materials of the modern world. But like all building materials, it has its weaknesses and is subject to deterioration over time. One of the most prevalent of those weaknesses is the corrosion of steel reinforcement. Embedded steel is usually safeguarded against corrosion by the impermeable covering of concrete and its alkalinity, which creates a protective oxide layer around the steel. However, over time, water flowing through concrete can leach certain constituents out, making the concrete more porous and less alkaline. That makes the steel more subject to corrosion. This is especially true in coastal areas where salt laden air from the sea can carry chloride ions toward inland structures. When these chloride ions saturate the concrete, they accelerate the degradation of the protective oxide coating around the steel.

Corrosion doesn’t just weaken steel, it also causes it to expand in volume, creating pressure within a reinforced concrete structure. Eventually the corrosion can reach a point where the pressure is too much. The surrounding concrete breaks away, leading to cracks, spalls (which are small areas of flaked off concrete), or delamination, where parts of the concrete along mats of reinforcing steel are completely separated. Once the steel is no longer surrounded and protected by concrete, the corrosion can progress much more quickly and may eventually lead to a structural failure.

The engineering and construction industries have made huge improvements in design and construction of concrete structures in the past 30 years thanks in large part to the Federal Highway Administration and the International Concrete Repair Institute. However, Champlain Towers South was designed and built before modern building codes included best practices for concrete structures in harsh coastal environments. The engineer who inspected the tower pointed out the problem with the pool deck in strong language, stating that “failure to replace the waterproofing in the near future will cause the extent of the concrete deterioration to expand exponentially.”

Keeping water, especially salty water, away from reinforced concrete is vital. If inadequate waterproofing turns out to be the cause of the failure, it won’t have been the first time. In 2012, the roof of the Algo Center Mall in Elliot Lake, Ontario collapsed, killing two people. The cause of the collapse was corrosion of the building's steel framework instigated by leaks through the improperly waterproofed rooftop parking deck. The prevailing theories about the Champlain Tower collapse from most of the current investigative journalism centers around the pool deck as a trigger or at least a major factor in the building’s demise, especially because the pool deck failure preceded the collapse. However, there is less certainty about what role in the collapse the failure of the deck slab had, since it does not provide any support to the building itself. What we see in the surveillance video would have required failure of one or more columns below the structure. One possibility is that the deck slab punched through intermediate columns such that it was hanging like a sheet from the columns below the building, sometimes called catenary action. The forces from the hanging slab could have loaded the columns below the building in a way they weren’t designed to withstand, causing them to buckle. However, the exact mechanism in which those columns failed is still unknown.

Tragedies like this are usually the result of many separate factors coinciding, and there are several circumstances that may have contributed to the collapse. A research team studying changes in land and sea levels in the area in 2020 measured some unusual settlement of a few millimeters per year in the area of this building. Although many areas experience significant long-term and large-scale settlement, also called subsidence, it’s possible that if different parts of the site were settling at different rates, the tower’s foundation could experience additional structural stresses. Also, some photos of the rubble appeared to show less reinforcing steel than was called for in the original design drawings, particularly at the column-to-slab connections. There were also regular intrusions of groundwater into the parking garage, the recent construction of an adjacent high-rise building, and ongoing construction to the building’s roof to consider.

All of these factors and many more will be reviewed by the forensics teams who are already investigating the cause of the failure. Many of these investigators have been on site during the recovery and cleanup operation to make sure rubble and debris that may offer clues into the cause of the collapse are documented and preserved. Major parts of the building are being preserved as evidence, so rubble was sorted on site and taken to a nearby warehouse for cataloguing.

It’s important to keep in mind that each one of these forensic teams is trying to answer a slightly different question. The Town of Surfside hired its own investigator, Allyn Kilsheimer, to begin looking into the collapse. Surfside has a number of high-rise condos under their purview, so presumably they felt the need to conduct their own investigation for the safety of their citizens. Another critical service that Mr. Kilsheimer is providing is to satisfy the public’s need for information, which is why you see him doing interviews and talking on news shows on behalf of the Town of Surfside.

At the federal level, the National Institute of Standards and Technology announced that they would launch a full investigation into the collapse. Formerly called the Bureau of Standards, NIST does a lot of research and science around measurements, materials, manufacturing, and engineering. Since 2002, they also have a federal mandate to investigate the cause of failure when a building collapse results in substantial loss of life. Their investigation will likely be the most thorough, including laboratory testing of steel, concrete, soil specimens, and structural modeling. During the recovery operation, they were on site with sophisticated equipment to take detailed records as rubble was hauled away and performing non-destructive testing to locate reinforcing steel and determine properties of the concrete members. They will also review all the reports and photographs from professionals, survivors, and witnesses of the event. Their final report will probably take a year or two to complete. The primary purpose of that investigation will not be to find fault, but rather to make recommendations for improvements to the building code and industry practices in the fields of structural engineering and construction.

Insurance companies, victims, owners, and designers will also be involved in lawsuits to try and establish who is at fault in this tragedy and potentially award damages as a result. Those legal teams will hire their own experts who will be investigating the details of the collapse. However, their focus will be toward establishing professional and organizational culpability more than the technical causes of the failure.

Finally, the county called a grand jury to examine the building’s collapse. A grand jury is essentially a group of citizens used to administer justice in various forms. Most commonly, grand juries are used as a step between accusing a person of a crime and trying them in court. However, they can also conduct their own investigations as representatives of their community. If the grand jury finds serious negligence or wrongdoing, there may even be criminal investigations that result from the collapse.

Was it a poor design, a mistake made during construction, lack of proper maintenance, or a combination of all three? That’s the question the forensic teams will be trying to answer. And just to temper expectations a little, they may not find a final and clearcut cause of the collapse. The difficulty of forensic engineering is that you’re trying to piece together a sequence of events from small and disparate puzzle pieces. Unfortunately, in this case where the failure likely began at the bottom of the structure, most of those puzzle pieces were buried in a pile of rubble.

I want to emphasize that this type of event is extremely rare. The damage to the Champlain Tower South pool deck shown in the 2018 inspection report is severe, but it was not an indication of an imminent collapse of the adjacent building by itself. Although we don’t know exactly how much things worsened between then and the collapse, I think you’d be hard-pressed to find a structural engineer who would evacuate a building based only on the level of deterioration shown in that report. Nearly all buildings, even with moderate maintenance, will last much longer than 40 years without fear of something catastrophic. Modern building codes are designed to ensure that our structures are engineered with redundancies and factors of safety for exactly this reason. My heart goes out to the victims of this unspeakable tragedy. I hope that investigators can get to the bottom of the collapse and its cause so that something like this never happens again.