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This building collapse investigation could leave a lasting legacy

The partial collapse of the Champlain Towers South condo building in Surfside, Fla., in 2021 prompted a federal investigation that could leave a lasting legacy on structural engineering.

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Surfside condo collapse

Miami-Dade Fire Rescue Department/Wikimedia Commons

Over the past few years, a number of high-profile partial building collapses have left structural engineers wondering how these tragedies were allowed to occur and what lessons they could draw to reshape the profession. Just last year, two occurred within a month of each other. On April 18, a parking garage building partially collapsed in the Financial District of Manhattan in New York City. On May 28, an apartment building in Davenport, Iowa, partially collapsed, killing three people and injuring at least one. 

However, the structural failure that has arguably garnered the most attention in recent years happened on June 24, 2021, when the Champlain Towers South condominium in Surfside, Fla., partially collapsed, killing 98 people. The event prompted an investigation from the Department of Commerce’s National Institute of Standards and Technology, which is expected to have a draft report ready to be released to the public in May 2025. 

Earlier this month, NIST offered an update on its investigation, and a lot of what the public learned from the presentation wasn’t particularly surprising. Glenn Bell, NIST’s co-lead investigator, reiterated the “no stone unturned” rhetoric we heard about the investigation in 2022. For many observers, NIST has lived up to that mantra, having explored 24 failure hypotheses with varying degrees of effort. While some have expressed concern that the investigation might be taking too long, Bell wants to ensure NIST gets it right, because a lot is riding on it, including the opportunity to reshape building codes and improve processes and methods for investigating building failures. 

“If our findings are not credible; if they are under attack and not accepted, all of that opportunity to make tremendous change just goes away, and that would be a shame,” Bell said. “That’s what keeps me up at night. And that’s why we’re being so thorough in this investigation. We talk about leaving no stone unturned. We want these findings to be ironclad. Because the potential for good and positive change to come out of this tragedy is large.”

NIST’s comprehensive review of the collapse of the World Trade Center buildings on 9/11 and the structural collapse of two overhead walkways at the Hyatt Regency Hotel in Kansas City, Mo., in 1981 left an enduring legacy that changed the profession. Here’s where the Champlain Towers South investigation stands and how it could have a similar impact on structural engineering.

What we know and what we’ve learned

NIST noted that there were several original design and construction errors in the building dating back 40 years before the collapse. In general, the analysis indicates that the tower columns were heavily loaded axially and not substantially affected by applied bending moments. However, several columns had moderate strength deficiency and, as noted in past updates from NIST, their design strength does not comply with the original codes and standards of the time (the 1979 edition of ASCE-7 and the 1977 edition of ACI 318). Reinforcement in some columns exceeded the 8% maximum area limitation, which can compromise concrete quality. Additionally, the upper layer of reinforcements in the pool deck slab were sometimes placed lower or with greater spacing than specified in the structural drawings. Some basement column lap splices were shorter than the minimum required by code.

Most of the 24 failure hypotheses NIST has explored include multiple initiation points. However, not all of them are being given equal weight. The main two that NIST highlighted on March 7 were the failure of the column-to-slab connection at the pool deck, and the failure of columns along the south edge of the tower. Evidence suggests that the pool deck collapsed 4-7 minutes before the tower partially collapsed.

NIST has studied corrosion of the rebar in the pool deck slab and the degradation of the concrete in the pool deck due to infiltration of seawater, and the negative effect that could have had on the pool deck strength. Corrosion of the top rebar in the slab around the columns might have led to a reduction in the punching shear capacity at the pool deck-to-column connections. Additionally, deviations in the landscaping planters and field paving may have added loads to the pool deck structure, increasing the likelihood of collapse.

However, while NIST believes there is robust evidence suggesting the collapse initiated in the pool deck, investigators have not yet ruled out a failure initiation in some part of the of the tower that precipitated a collapse in the pool deck. Videos show severe structural movements in the tower between grid lines K and M and grid lines 4 and 9.1 prior to the precipitous drop of the tower along grid line 9.1.

NIST’s hypotheses are similar to those posed by Dr. Abi Aghayere, a professor of structural engineering at Drexel University. Aghayere’s own separate investigation has suggested column K-4 was under-designed. Aghayere has also been vocal about what he feels needs to change at bureaucratic levels. Earlier this year, in an article for The Conversation, he noted that like the partial collapses in Davenport, Iowa, and New York City, the Champlain Towers South building showed “visible signs of the defects that eventually led to the building’s demise.” Perhaps more concerningly, he notes all three tragedies can be traced back to decisions to ignore “a history of documents submitted to city building departments clearly showing deteriorating conditions.” 

“All in all, to me, the NIST presentation … reveals that the cause of this collapse should have been obvious,” Aghayere wrote in an email to SmartBrief. “And had the necessary proactive action been taken, starting from when the structure was originally designed and constructed, to the maintenance, or lack thereof, of the structure over the years, to the time about three weeks before the collapse when the planter boxes on the pool deck were showing obvious signs of structural distress … 98 innocent people would not have died.” 

NIST has noted that all hypotheses presented are still preliminary, and no definitive conclusion has been made about the pool deck or the columns of the tower. However, there are some hypotheses that NIST has categorized as having a lower probability. For example, geotechnical evaluations show no evidence of large karstic voids that affected the tower’s foundations. NIST also believes that differential settlement in the pool deck foundations were very small and had minimal impact on the pool deck foundations. 

Scope of testing and modeling

During its investigation of the collapse of the World Trade Center buildings, NIST conducted more than 1,000 interviews and gathered as much physical evidence as possible, including hundreds of structural steel components and thousands of pieces of media. The investigation of the Champlain Towers South tragedy has been similarly thorough. According to Chris Segura, co-leader for the evidence preservation project in the investigation, as of March 7, NIST has extracted nearly 500 concrete cores and 369 reinforcing bar samples. Of the samples that have been extracted, 417 concrete cores and 156 reinforcing bar samples have been designated for mechanical testing. The team has completed 368 mechanical tests on concrete cores, 274 of which are for compressive strength, 22 are for modulus of elasticity measurement, and 72 are splitting tensile strength measurements. NIST has also completed testing on 40 reinforcing bars samples. 

However, when it comes to applying these material samples to computational models, engineers have encountered a number of challenges. NIST is aiming to replicate the mechanical properties of the concrete used at the time of the tower’s construction, but the aggregate has a fairly high water absorption capacity, which complicates efforts to simulate 40 years of corrosion. As officials noted during the hearing, the concrete is very porous. While NIST has tried to replicate that type of concrete in large-scale structural tests, matching the degree of corrosion has proven to be something that requires careful consideration. 

“When you accelerate corrosion, you are causing that corrosion product to form more rapidly than it normally would,” explained Ken Hover, co-lead of the material science team for the investigation. “Even if we were to achieve the same degree of corrosion over a short period of time that was naturally developed over a much longer period of time, the impact on the structure could be different. So we are being very very careful about what the amount of electrical current is going to be so that we don’t cross over a line between over-accelerating and therefore generating unnatural conditions.” 

Additionally, engineers can no longer get the exact same reinforcement type that was used in the building because the deformation pattern that was widely used back then is less available today. To combat this, the team collected ASTM A615-grade steel rebar with properties within the range engineers have tested in the field and then developed computer models to simulate effects of different materials and configurations on structural behavior.  

Due to the highly intricate nature of the investigation and the number of complexities of the potential failure hypotheses, “there is not a single model that would be capable of addressing all initiation and progression mechanisms of collapse,” said Fahim Sadek, co-leader of the structural engineering analyses for the investigation. Rather, the team is working on a series of models to capture the variety of failure mechanisms that may have occurred during collapse. At the structural level, there are four models. The first one evaluates multiple potential failure initiation mechanisms in the pool deck slab. The second focuses on the first floor slab. The third looks at potential failure progression mechanisms at the pool deck slab-tower interface. The fourth looks at the entire tower. 

At the geotechnical level, engineers are focused on the interaction between the structure and the soil beneath it. Work at the site included geotechnical borings with sample extraction and cone penetrometer tests, while laboratory testing evaluated grain size and mineralogy. Nonlinear behavior simulations incorporated changes in soil properties depending on load magnitude. Vibrations from neighboring construction were studied using enlarged scale computer simulations.

Interdisciplinary collaboration

A plethora of experts from fields including structural, geotechnical and materials engineering, social science, and remote sensing have been involved in evaluating all available evidence. This includes videos and photos from the day of the collapse, eyewitness interviews, documents from the building’s design and maintenance history, and materials testing of concrete and steel samples extracted from the debris pile. 

“I think this is the future, bringing all these levels of information into one platform and being able to see all the different things at once,” said Aspasia Zerva, an engineering professor at Drexel University and a member of NIST’s National Construction Safety Team Advisory Committee. 

Another member of the advisory committee, University of Utah public health professor Kimberley Shoaf, cautioned those involved in future investigations to be mindful that their language is respectful across interdisciplinary bounds and acknowledges that even so-called “soft science” that incorporates potentially biased eyewitness accounts is reinforced by thorough methodology. 

“That is a very defined methodology that takes all of that bias into consideration in terms of presenting results that are as equally valid as any evidence that comes from examining a piece of concrete,” Shoaf said.

Legal implications

While much of the discussion around the partial collapse of Champlain Towers South has centered around its own design and construction, one lawsuit targeted a nearby building. The suit alleged that the excavation and construction of an adjacent 18-story property, known as Eighty Seven Park, “badly damaged and destabilized” Champlain Towers South and allowed water to intrude and corrode its structural support. The suit also claims that work at the Eighty Seven Park site came with heavy vibratory activity, including sheet pile driving and compaction, that weakened CTS. Coincidentally, the pool area of CTS was near the property line with Eighty Seven Park. 

While that theory wasn’t ever fully substantiated, it still resulted in a monetary victory for the plaintiff. DeSimone Consulting Engineers, the structural engineer of record for Eighty Seven Park, settled its part of the case for $8.55 million, thanks to the work of law firm Zetlin & De Chiara. But while other engineering experts have written off the theory, it underscores the impact of new construction on existing buildings as “an area that needs to be more appreciated,” says partner Michael De Chiara.  


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