It was before sunrise Thursday 24 June 2021 that my pinging mobile phone, acknowledging text messages coming in, woke me up. Upon checking the headings of these messages alerting me to the partial sudden collapse of the 12-story Champlain Tower South condominium tower in Surfside, Florida, I realized not only the sobering reality of a tragic and catastrophic event in progress but also the possibility that some of my own research findings about which I have taught, written, presented and published extensively (i.e.: Hurricane Mitigation for the Built Environment (2015) CRC Press, ISBN 978-1-4987-1498-3) might be contributing factors in finding answers to the unfolding drama.

View of the Champlain Tower South condominium building looking east from above Collins Avenue. The red outline identifies the section that suddenly collapsed in the early morning of 24 June 2021 (by Ricardo Alvarez from Google Earth imagery)

I have not been at the site of the disaster, but extensive media coverage, including network TV video footage and zoom-ins from multiple camera angles, security cameras, and from drones, as well as extensive narratives from reporters, first responders, neighbors and passerbyes, and public officials, has provided me with an excellent vantage point to begin to formulate some ideas about possible causes of this building collapse.

Undoubtedly, extensive forensic engineering investigation on site, as well as a thorough and comprehensive review of original building design documentation including site surveys, soil studies, design criteria, structural calculations, as-built plans and specifications, will be required to identify the most probable causes of the building failure witnessed earlier today.

Be that as it may, it is critically important that those with relevant expertise who have dedicated time and effort to study the vulnerability of the built environment and the performance of buildings in this coastal region as they interact with a range of hazards, offer and contribute their nickel’s worth of opinions and ideas. It is in this light that I offer the following thoughts.

What do we know so far? The 12 story building was built in 1981 and consists of a reinforced concrete main structure, and a combination concrete masonry and glazed envelope. The building, located at 8777 Collins Avenue on the east side of the street facing the Atlantic ocean and the beach, was most probably designed to the requirements of the 1979 edition of the South Florida Building Code. Media reports indicate building was undergoing, or had recently undergone, inspection by Florida registered design professional for purposes of recertification of its structural and electrical systems as required by the Miami-Dade County Code (Section 8-11(f)) for all buildings that have been in service for forty years. At this time it is not known what this inspection revealed or what recommendations for remedial action, if any, were included in the signed and sealed Recertification Report, or if such report has been completed and submitted, or not, to the Building Official.

While we may surmise some details about other aspects of the design of this building, based on the requirements of the governing building code at the time, there are important elements of information we are missing. For example, what are the details of the foundations, are there construction control or expansion joints, is the main structure one continuous unit throughout the building or did sections of the buildings have their own independent structures? It is clear we lack information that would help us better assess how this building performs under various combinations of loading conditions.

View toward the northwest showing the collapsed section of the Champlain Tower South condominium building (PC screen shot)

On the other hand, from video footage widely shared by media outlets, we know that the collapse of the building happened in two rather brief sequential episodes, each lasting but a few seconds, separated by an interval of approximately ten seconds. In the images that have been shown we can see how the collapse starts at the center and center-north section of the building to the east of the elevator well. Once this portion of the building has collapsed the entire section fronting on Collins avenue remains standing as also does the northeastern portion, which after about ten seconds starts to lean toward the center of the building and then totally collapses. In less than one minute the entire event is over.

Beyond the above, we have anecdotal information from witnesses in neighboring buildings, bystanders, and from evacuees from the portion of the damaged building itself. From what these individuals have shared via media outlets it appears there were sounds coming from the building as well as some swaying movement that was felt by some preceding what has been variously been described as a “loud bang”, “a sound like thunder”, “boom”, and other similar reports followed by the actual sound of the building collapsing.

From this we have a visual record of the actual collapse as it happened, a good idea of the sequence of events and timeline involved, and information about sounds and collateral effects preceding the collapse itself.

So, why did it happen?

Let us start by accepting we can not exclude any possibility at this time, this includes any human intervention, whether unintended or malicious, that may be shown to have led to the collapse either directly or as a contributing factor. Although a preponderance of the early knowledge appears to preclude the possibility of human causation, the prudent approach is to let the investigation unfold and the evidence speak for itself. We simply don’t know enough at this time to exclude or to include the human factor.

Along the same lines, given the time this building has been in service and a lack of known complaints about defects of deficient performance, it appears design flaws and construction deficiencies might also be excluded as probable causes linked to the collapse, but here again the prudent approach is to let the investigation proceed and gathered evidence speak.

Beyond these caveats, on the basis of my extended experience in research and application related to the performance of buildings while interacting with or under the influence of natural hazards, and what I have learned from the practice of the disciplines of vulnerability assessment, risk management, hazard mitigation, and climate adaptation, I would like to share my opinion about two plausible causes for the partial collapse of the Champlain Tower South condominium building in the early morning of Thursday 25 June 2021.

Chloride-induced corrosion and sea-level rise: the Champlain Tower South building is located a mere 90 meters from the waters of the Atlantic at low tide, and perhaps only 70 meters at high tide. The distance to the water if even less during King Tides that periodically affect the area, and much less under the effects of storm surge generated by approaching or passing tropical cyclones.

In essence, this building is located, and has been in service for forty years, in a marine environment. A marine environment is one where beyond the salt water of the ocean there is salt spray constantly in the air. Sodium chloride, the main component of salt in ocean water, and other electrolytes make sea water highly corrosive. Air in the marine environment of the coastal region is also highly corrosive because of the salt spray it contains. In fact salt water is known to be five times more corrosive of metals, such as steel, than fresh water. And salt spray in the coastal air makes it ten times more corrosive than inland air with regular humidity.

In the context of this coastal marine environment consider the reinforced concrete structure and foundations of the building in question. Reinforced concrete is porous. This porosity allows corrosive marine air to penetrate and eventually oxidize the reinforcing steel embedded in it. A far worse effect takes place below ground where structural members and the building foundations come in contact with salt water that intrudes and penetrates reinforced concrete corroding the reinforcing steel.

Corrosion of reinforcing steel embedded in concrete causes the metal to increase its cross sectional dimensions, pushing against the concrete cover, causing micro cracks that eventually grow into larger cracks until the concrete actually breaks exposing the reinforcing steel to the elements, marine air or ocean water, increasing the rate of corrosion. Over time this process weakens the structural integrity and may lead to failure.

Relative to the building foundations, which penetrate deep into the ground, exacerbating factors make the degree and rate of corrosion much worse. Sea level rise involves two components, a vertical component or increment in water depth and a horizontal one, which is how much the sea penetrates inland as the water rises. The dynamics of these components push salt water intrusion faster and farther inland under buildings in the coastal region. Another exacerbating factor is the acidification of the ocean caused by its intake of carbon dioxide CO2. The more acid sea water becomes the more corrosive it is. In summary, the more sea level rises and the more acidic it becomes, the much more of a corrosive environment adversely affecting the foundation of the building. In the case of the Champlain Tower South this process of corrosion and weakening has been ongoing most probably unseen for forty years.

Could the foundations of this building have reached their limit in terms of capacity to take the loads transferred by the structure, the weight of the building itself, with all its equipment and content, and occupants, as well as external loads constantly applied by the elements (wind, water, rain), and various intermittent impacts, and transfer them to the ground?

In my opinion, failure of the foundations is indeed a plausible cause of the collapse. The absence of contributing preceding factors such as interaction with a natural hazard, a fire, an explosion, accidental impact, or ongoing structural work on the upper floors, plus the suddenness of the collapse clearly shown in video footage, totally point toward failure at the bottom of the building rather than a cascading effect from the top.

Ground failure: another albeit less likely possibility is that the ground under a portion of the building suddenly failed after a relatively long slow process of undermining, either from natural or accidental anthropogenic causes. This plausible cause would be consistent with the observed mode of collapse and associated timeline, as well as with appearance of the pile of rubble.

This hypothesized ground failure would have deprived the building foundations of critically required support leading to the sequence described before.

Geotechnical engineering and soil mechanics are outside my field of expertise, so I will not venture an opinion as to the exact nature of site-specific ground failure that may be involved in this case, but wanted to identify what I see as another plausible cause that would be consistent with the visual evidence shown so far.

What are the implications of what has transpired? It is indeed too early to speak of lessons learned, but not early enough to realize we may be viewing the early stages of a transformational event? Transformational certainly for the victims and survivors of the catastrophic event, but beyond this transformational for the entire community of Southeast Florida in terms of how those who reside here need to view the vulnerability of the place both regionally and at the property level.

Global warming a byproduct of global climate change is causing accelerating sea level rise. Greenhouse gas emissions, especially carbon dioxide (CO2), is a main contributor to global warming and ocean acidification. This combination of factors has generated the corrosive mix that causes chloride corrosion leading to slow, but continuous and unseen damage to building foundations along built-up coastal regions. This is a fact.

It is also a fact that this region of Florida is highly vulnerable to sea level rise and natural hazards exacerbated by the same. The fact that unrelenting urban development has been permitted for decades along the coastal region has continuously increased what is at risk in terms of human life, human activity and property.

The only objective of these lines is to offer my opinion regarding a plausible cause of the partial and sudden collapse of the Champlain Tower South. But I would like to close by posing some of a myriad of questions elicited by the tragic event.

What effect will the tragic event have on how insurers view their risk in insuring buildings and property in these vulnerable parts? What effect will this have on real estate values for the many other forty-year old and older, and for that matter newer buildings as well, situated along the coastal region in close proximity to the water and equally subject to the same unseen damaging hazard? Is there a need for major revisions to the required 40-year old building recertification process? Are we witnessing a first glimpse of a trigger for forced or managed retreat from sea level rise? These and many other similar questions need to be asked from the perspective of potential effects, and need for change, on various sectors such as condominium boards, building management, realtors, design professionals, insurance, county and municipal commissions, building officials, and others.

But, beyond these as of yet unanswered or to be answered questions, the main implication is the human tragedy, the loss of life, the human suffering and the heroic effort by search and rescue teams doing all they can to save those who might still be alive in the rubble or to recover those who did not survive so that their loved ones may have closure. May God bless victims, rescuers, and survivors. We owe it to all of them to find answers and solutions to prevent repetitions of this tragic event in the future.

Food for thought and need for action indeed!

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