The dramatic collapse of the Champlain Towers South Condominium and the resulting loss of life and carnage has triggered widespread concern about the safety of large scale and high-rise concrete buildings, especially in American beach communities. As we gather with our families on this Independence Day, the lives lost and families affected provides us pause to discuss the engineering behind structures similar to the tower that fell and important structural fundamentals that should not be ignored in order to protect the safety of tall structures.
Champlain Towers South Condominium was a twelve-story structure originally built forty years ago, in 1981 by Nattel Construction, a company owned by an embattled developer, the late Nathan Reiber. It has two sister buildings, Champlain Towers North and Champlain Towers East, developed in 1982 and 1994, respectively. The North and South towers are architecturally similar and constructed around the same time, while the East tower is architecturally different and built more than a decade later. The North and East towers remain standing; the portion of the South tower which remained after the collapse were leveled in a planned demolition.
While reports continue to come in and forensic investigations are underway, it is too early to definitively announce the cause of the collapse of the South tower. Nonetheless, a review of the basic causes of failure of such concrete structures in simple terms may be helpful. There are basically two fundamental types of failures of similar tall concrete buildings: (a) foundation failure and (b) super structure failure.
Foundation Failure
Building foundations are critical and must be designed properly. The total load from a building’s superstructure – analogous in some ways to a human body’s skeleton – is borne by a building’s foundation. The foundation functions by transferring the superstructure’s load onto the soil on which the foundation is supported. If the soil type is sand, additional precautions need to be taken during design and construction. That is the case because when sand is saturated with underground water and is shaken by earthquake, nearby highway traffic, or, in the case of the Champlain Towers South Condominium, vibration from adjacent construction, sand loses its load carrying capacity. This character of sand is known as liquefaction and one such example is quicksand. Recent research conducted by Missouri University of Science and Technology has proved that even static load may introduce liquidation in sand when sand is saturated with water. This means, even absent a dynamic load such as an earthquake or traffic loads, liquefaction may occur. Liquefaction is highly unlikely to be the cause of the structural failure here because the foundation of the building was anchored using pile caps into piles extending deep into the limestone.
From the reports we have reviewed, if a foundation failure occurred, it was a failure of the foundation floor of the building – located at the pile caps highlighted by the purple arrow below. Both differential rates of settlement of the piles and standing water over extended periods of time could have caused a failure of the foundation floor.
Super Structure Failure
The super structure of the Champlain Towers South Condominium was built with reinforced concrete and was designed mainly with beams, columns, and slabs, as shown above. In any project, when concrete members are constructed and loaded, hairline cracks appear on the concrete’s surface. While hairline cracks are always present in concrete, when a structure is under-designed, those cracks become significantly larger. A structure can be under designed when the columns or slabs are smaller than they should be, or the steel reinforcement cross section is less (cross sectional area of rebar less than warranted by the weight to be carried).
We know that the air flowing upon the Champlain Towers South Condominium was from the Atlantic Ocean and salty in nature. If the members were under designed – or Nattel Construction did not construct the columns or slabs to the design (by using less steel than the designs called for), the cracks on the concrete would have grown significantly. Those cracks, combined with the blowing salty air around the members for the past 40 years, created rust on the steel bars inside the members, as was evidenced in photographs from a pool contractor taken just days before the collapse. Rust itself causes steel members to expand, causing wider cracks and further separation between the steel and concrete. If the reinforcing steel bars became so rusted as to have become destroyed or they were exposed and disconnected from the concrete as evidenced in the pool contractor’s pictures, the concrete members could no longer carry the loads they needed to carry and could indeed crumble ground like the debris at the Champlain Towers South Condominium.
Conclusion
As mentioned above, while it is too early to reach definitive conclusions regarding the cause of the building failure at Champlain Towers South Condominium, many potential sources of danger were present and unattended to for years at the building. A more rigorous and attentive structural review program could have averted the disaster that ultimately brought the tower down and destroyed the lives of those who perished and many of their families. We grieve their loss while we continue to better understand the specific cause of failure and urge building owners, management companies, and caretakers to take structural issues seriously, especially as buildings enter the fourth or fifth decades of service.
BOSTON ● JULY 4, 2021
About the Author
Dr. Mukti L. Das, Senior Technical Advisor
Dr. Mukti L. Das is Senior Technical Advisor to Troca Global Advisors. As a Fellow of the American Society of Civil Engineers, an honor bestowed upon less than three percent of American civil engineers, Mukti has more than 40 years of engineering experience on nuclear, fossil and hydroelectric power plants, pulp and paper manufacturing facilities, underground structures, road and railroad bridges, high-rise structures and other industrial facilities. Mukti currently serves as the Chairman of the American Concrete Institute Subcommittee on Foundations for Dynamic Equipment.
A leading global structural expert, Mukti serves Troca Global Advisors and its clients for all engineering related issues, including structural forensic engagements, investigating and determining the causes of structural failures of buildings, bridges and other facilities. Mukti renders opinions and provides testimony in court and other judicial proceedings.
Formerly a Principal Civil Engineer at Bechtel Power Corporation and a member of the accreditation board for ABET, Mukti holds a Ph.D. and M.S. from the University of Massachusetts, Amherst and a B.S. from the University of Calcutta. He’s also our German expert (he lived in Germany for years, is fluent in German, and savors his beloved Franziskaner-Bräu).
About Troca Global
Troca Global Advisors and its affiliated companies provides high value advisory services to clients across disciplines and around the world who seek solutions to critical, interesting and often complex issues. The firm is dedicated to achieving pragmatic and efficient results for our clients based on years of multi-disciplinary experience. The exceptionally broad expertise of our network allows us to advise our clients in a large array of disciplines.
Our Structural Services Team, led by Dr. Das, provides guidance to owners and managers of buildings, bridges, roadways, and facilities regarding a wide array of structural stability, health and safety issues.
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