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The National Institute of Standards and Technology’s (NIST’s) National Construction Safety Team (NCST) has released an extensive video update on its investigation into the June 2021 partial collapse of the Champlain Towers South building in Surfside, Florida. The update reviews the investigation’s history and progress, shares preliminary findings, and highlights potential impacts that this complex investigation could have on building codes and standards.
In the video, investigative lead Judith Mitrani-Reiser and co-lead Glenn Bell explain how the team has determined that some of the hypotheses they are considering for how the failure occurred have a higher likelihood than others. The team has reviewed two dozen hypotheses, relying on extensive physical evidence, imagery, historical records, witness interviews, remote sensing data, laboratory testing, computer modeling and more.
“As we have shared in previous updates, there were many design and construction problems that weakened the building from the start,” said Mitrani-Reiser. “These deficiencies posed many potential failure initiation possibilities both in the pool deck and the tower, and each is being carefully considered so that we can narrow our focus to the most likely ones and seek to rule out others.”
The two experts describe the extensive planning and coordination that helped the team systematically work through analyses, testing and modeling to arrive at its preliminary findings. They note that from NIST’s initial deployment of a preliminary reconnaissance team in the first 48 hours after the collapse, this investigation has relied on collaboration with local authorities and expertise from across the federal government, private industry and academia.
Higher-Likelihood Collapse Hypotheses
Bell walks viewers through three hypotheses with higher likelihood, beginning with the failure of one of the typical slab-column connections in the pool deck. He describes factors that contributed to low margins of safety in the pool deck, including understrength of the building’s original structural design relative to the requirements of the building code. Additionally, he notes that steel reinforcement was not placed where it should have been, leading to significantly diminished strength of the pool deck slab and slab-column connections. He also points to heavy planters that were not in the original design, as well as a rehabilitation of the pool deck decades earlier that added sand and pavers, increasing the load on a system that was already functionally and structurally inadequate. The team also found corrosion of the steel reinforcement in the pool deck concrete, which can weaken the slabs and slab-column connections.
“While there is strong evidence that the collapse initiated in the pool deck, we have not ruled out a failure initiation in the tower,” said Bell. “The fact that the pool deck collapsed before the tower does not preclude the possibility that there was some initiating event in the tower that set off the collapse of the very vulnerable pool deck.”
Some of the design, construction and degradation issues found in the pool deck are also evident in the building tower and present other plausible hypotheses that the team continues to pursue. In addition to the misplacement of steel reinforcement within slabs and columns, some basement columns had prolonged exposure to water due to ponding and flooding in the garage. This can cause corrosion of the steel reinforcement and deteriorate the concrete. The team therefore also considers it a higher likelihood that the collapse was initiated by either the diminished strength of the columns in the tower or the failure of a slab-beam-column joint in the southernmost column line of the east part of the tower, close to where the tower joined the pool deck.
Lower-Likelihood Collapse Hypotheses
The investigation team determined that there is a lower likelihood that the partial collapse was initiated by two potential problems beneath the building: voids known as “karst” or pile failure. Mitrani-Reiser explains how satellite data was used to look for gradual settling or sinking of the ground in the general area of Champlain Towers South. None was seen in the area in the five years before the partial collapse, nor was localized sinking observed near the building in the days leading up to the tragedy.
The team found no evidence of karst in the limestone on which the foundation sits, and careful studies of the limestone showed it has features that actually inhibit the formation of karst. Team members calculated that the foundation pile capacity shown on the design drawings was sufficient to carry the building loads and laboratory and nondestructive testing of pile concrete showed adequate material strength. Finally, the basement slab did not show any distress or trauma that would indicate karst formation or pile failure, such as cracking or sinking.
Bell also notes as a lower likelihood scenario the separation of the pool deck/street-level slab from the south basement wall.
Preliminary Findings Rely on Broad Range of Evidence
In the past few months, the team has updated the collapse timeline based on interviews and records, modeling results, and new analyses of audio and digital evidence.
Although there is very little video from the night of the collapse, every image was meticulously analyzed to determine its precise perspective and identify clues that could inform the timeline, such as changes to reflections of light on building surfaces, such as a wall.
Mitrani-Reiser describes how team members made a breakthrough by using a novel approach to analyzing videos. They compared the soundwaves of the audio recorded by two videos from different parts of the building to find and correlate patterns of sounds in each video. This helped pinpoint when the videos overlapped in time and provided insight into what was happening in the building by comparing the building’s movement at the same time on two different floors. All audiovisual evidence in NIST’s possession has now been timestamped.
Mitrani-Reiser also notes the importance of social science research to develop carefully crafted interviews that have helped to elicit important memories not reported elsewhere. Information gained in these interviews has helped confirm the collapse timeline, in tandem with the video evidence.
Implications for the Future
“Two clear questions coming out of this investigation are why the design and construction problems were not discovered when Champlain Towers South was built, and how do we evaluate the structural safety of existing buildings?” said Bell.
While the video presentation does not offer recommendations for changes to codes or practice, it does highlight some areas that industry experts could consider. These include how special inspections that are mandated for safety might impact construction quality control by giving builders a false sense of security that someone else will catch their errors later.
Mitrani-Reiser also shares that the investigation found no records from the original construction of the building, and few from its early life, and notes the importance of records retention going beyond initial drawings to include “quality assurance records and, particularly, peer review reports where they exist.”
Finally, Mitrani-Reiser calls on the engineering and construction professions to take seriously the apparent lack of quality control and quality assurance found in the case of Champlain Towers South. She noted that, “this tragic event has revealed flaws in our systems, and quality is at the heart of it.”
The team is finalizing its analysis and has begun drafting its investigation report, which is expected to be completed in 2026.