The Champlain Towers South Collapse: A Forensic Engineering Analysis

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The tragic collapse of Champlain Towers South (CTS), which claimed 98 lives on June 24, 2021, in Surfside, Florida, stands as one of the most devastating structural failures in U.S. history. Champlain Towers South was a 12-story reinforced concrete flat plate structure completed in 1981. The building featured 136 units, a lobby-level and basement garage, and a pool deck terrace on the south side. The South Florida Building Code (1979) and ACI 318-77 applied during the design and construction phases.

Witnesses reported hearing loud noises from the building in the hours leading to the collapse. Just after 1:00 AM on June 24, 2021, the collapse initiated. Surveillance footage and early investigations indicated that the initial failure originated in the pool deck slab, which exhibited punching shear failures. These failures triggered a disproportionate and progressive collapse of the eastern portion of the building, which the central core shear walls arrested. The sequence of events underscores how interconnected vulnerabilities in structural systems can rapidly escalate into widespread failures.

Working on behalf of insurers and the court-appointed receivership for CTS’s Condominium Association, Wiss, Janney, Elstner Associates, Inc. (WJE) conducted a comprehensive investigation into the collapse as part of the civil litigation that settled in June of 2022. Our findings shed light on structural design and construction flaws that led to the failure and provide valuable lessons for improving engineering practice.

Our investigation involved multiple stages to identify the causes of the collapse and determine the contributing factors.

Document Review and Analysis. We reviewed numerous documents, including the original drawings, maintenance records, meeting minutes/videos, documentation from other engineers/architects (including the 2018 recertification report and subsequent repair design), and social media data. This information was used to understand CTS’s original design and as-built construction, changes to the structure, and, ultimately, the failure sequence.

Field Investigation. At the collapse site (Fig. 1) and the National Institute of Standards and Technology (NIST) National Construction Safety Team Evidence Facility (Fig. 2 & Fig. 3), we conducted detailed surveys, assessed the condition of structural elements, and investigated the pool deck waterproofing. Concrete, reinforcing steel, and soils were sampled for laboratory testing with the parties involved in the civil litigation.

Laboratory Testing. Concrete and reinforcement samples were tested to evaluate material properties such as steel tensile strength and ductility and concrete compressive strength, carbonation depth, chloride content, and other material characteristics via concrete petrography.
Structural Analysis. Based on the understood loading, finite element models of CTS’s tower and the pool deck slabs were developed and used to understand the original structural design and behavior at the time of collapse. These models pinpointed vulnerabilities in the pool deck design, particularly at the slab-column connections. Ultimately, these models offered a clearer picture of how specific deficiencies contributed to the collapse.

We evaluated numerous potential failure theories. Ultimately, what is presented here highlights what we found most relevant.

Almost immediately after gaining access to the collapse site, we commissioned a detailed survey of the basement structural slab (structurally connected to the columns to resist hydraulic groundwater uplift pressures), revealing that the slab elevation varied less than 5.5 inches across the entire structure. No unexpected or large gradients of slab elevation were observed. Geophysical test methods confirmed that voids or karstification were not present. Further, numerous soil borings revealed that the soil was typical to the area with layers of sand and limestone. Based on this, we do not believe that the geotechnical conditions contributed to the collapse of CTS.

We identified significant design and construction flaws and changes to the structure that critically undermined the building’s structural integrity.

The Original Design. The pool deck slab included a topping slab of varying thickness sloped to drain that was not shown on the original structural or architectural drawings but was determined to be original to the structure (Fig. 4). The negative moment region reinforcement (i.e., bars over columns) was specified to have a cover of 0.75 inches. Concrete was specified to have a compressive strength of 4,000 psi. The design live load was 100 psf. No shear reinforcement or drop panels were used. Considering this information and assuming that the design engineer must have known about the topping slab installation, the as-designed punching shear strength was insufficient at nearly all columns supporting the pool deck slab, with demand-to-capacity ratios (DCR) for design up to 1.7 (at column K/13.1) (Table 1). Our structural models of the pool deck slab show that DCRs for flexure in positive and negative moments regions were also insufficient, with DCRs up to 2.0. Simply, the deck design was inappropriate.

As-Built Conditions. Our investigation also found numerous incidences of reinforcement at the top of the slab with excessive cover, reducing the effective depth (d) from 8.125 inches to 7 inches, reducing the critical perimeter for punching (b0), and lowering the shear load-carrying capacity based on 4√ƒc'b0d. Further, the low top flexural reinforcement ratio (less than 1%) has resulted in a mechanism known as flexural-induced punching shear. As a result, the stress allowed by ACI 318-77 of 4√ƒc', can be unconservative. Experimental testing has shown this stress to be as low as 2√ƒc'. Reviewing the original drawings and site conditions showed that more planters were on the pool deck than originally designed (it is unclear if the engineer considered them). A 1996 pool deck renovation added further unanticipated loads to the deck, including concrete pavers on a sand bed (4+ inches thick in some locations). However, concrete testing found that the pool deck concrete’s average compressive strength was approximately 4,500 psi. These unanticipated loads significantly increased the load on the slab and contributed to its eventual failure. Considering this information and ignoring any ponding water in planters or the pool deck due to poor drainage, the DCR for punching at collapse was approximately 0.85 (no load factors or reductions) (Table 1). The calculated DCR of 0.85 did not consider three important factors: (1) the flexural reinforcement ratio is exceedingly low, which reduces punching shear strength relative to the ACI 318 equation; (2) sustained long-term load reduces punching shear strength compared to normal loading, and (3) the effects of the transfer of unbalanced moment were not included. Quantitative consideration of these effects would result in a DCR substantially exceeding 1.0. In other words, the pool deck has been near its collapse capacity since at least 1996 (when sand and pavers were added).

The Warnings. During our document review, we discovered two sets of photos showing punching shear and other slab distress present on the pool deck before the collapse of the structure (Fig. 5). Photos from November 12, 2020, 7 months and 11 days before the collapse, show evidence of efflorescence deposits due to water flowing down the face of column L/13.1 (adjacent to K/13.1), indicating that a punching shear-related crack had formed. Photos from June 2, 2021, 22 days before the collapse, show evidence of significant vertical slab movement above column K/13.1 (the most heavily loaded column in the pool deck) and manifesting in the masonry planter walls. An April 13, 2020, photo shows the wall absent this distress. As a result, punching shear-related distress manifested and spread around the slab before the collapse, and the failure mechanism likely took years to develop fully. Ultimately, both L/13.1 and K/13.1 punched through the pool deck slab during the collapse.

WJE Failure Theory. Our failure theory is based on our understanding of the as-built conditions, structural analysis, videos taken during the collapse and posted on social media, and eyewitness accounts as reported to news outlets. This sequence highlights how localized failures can rapidly escalate into full-scale structural collapses without sufficient redundancy. Our failure theory is outlined as follows:

1. The initial failure occurred when the pool deck to the south of the building collapsed between 1:10 and 1:15 AM. This failure was reported by residents in Unit 111 and others, as well as the security guard. A vacationer at the resort north of CTS captured a video documenting the pool deck collapse on TikTok, noting the noise and the rush of wind from the garage entrance, with visible debris on the garage floor (Fig. 6). Additionally, post-collapse observations of punching shear failures further support this sequence of events.

2. The failure of the pool deck exerted horizontal tension forces on the building’s columns, particularly those on the south face, leading to column failures and significant structural movement (Fig. 7). Horizontal forces, which are typically ignored in beam and slab design, became critical as large displacements occurred.

3. A step in the slab created reinforcement detailing at the building edge that reduced its strength, further compromising the structure.

4. Structural displacements were observed through eyewitness accounts, including reports of cracking walls in Unit 611 and security camera footage from Unit 711 (Fig. 8). These observed displacements ultimately indicate the failure of the columns along the south exterior wall of the east wing, which triggered the progressive collapse of the eastern half of CTS approximately 7 to 12 minutes later, at 1:22 AM.

5. The final collapse was captured by security camera footage from the property south of CTS (Fig 9). Meanwhile, the remaining western portion of the building remained standing due to the presence of the shear wall at the elevator core.

Critical Factors. As shown by our investigation, the failure of the pool deck can be attributed to several critical factors.

1. One of the primary issues was the inadequate design of the pool deck slab, particularly in terms of punching shear capacity. The landscape feature loads on the deck only worsened the issues. It is unclear if the original designers accounted for the topping slab and the additional planters. Furthermore, adding pavers in 1996 placed additional load on the already deficient slab, further exacerbating its structural weaknesses.

2. Additionally, the shallow top reinforcement within the slab reduced the structural section’s effective depth, weakening its overall capacity to resist applied loads. This deficiency made the slab more susceptible to cracking and eventual failure. At the time of design, no integrity reinforcement was required, so the slab’s ability to redistribute load is further reduced (integrity reinforcement was introduced in ACI 318-89).

3. Finally, the visible signs of slab distress documented in columns L/13.1 and K/13.1 were missed and/or never diagnosed. This lack of intervention played a significant role in the ultimate collapse of the structure.

Contributing Factors. Additional factors exacerbated the vulnerability of the building and should be noted.

1. Long-term load effects (i.e., creep) play a significant role in concrete behavior at loads near the failure load.

2. Water buildup on the deck and in the planters was likely present due to the repeated repairs to the planters and the inadequate drainage at the pool deck. However, we do not know the amount of water present at the time of collapse.

3. Finally, while corrosion was present in some areas, its significance remains unclear. We saw only limited corrosion during our investigation.

Regardless, the structural design, construction, and added load were the primary causes of the collapse.

The tragic collapse of CTS underscores important lessons for the structural engineering community. A thorough review of the failure highlights key areas where improvements in design, construction, and inspection protocols could prevent similar disasters in the future.

Importance of Structural Redundancy and Appropriate Design. The collapse demonstrated the consequences of inadequate punching shear capacity in the pool deck slab. The failure of a structural component triggered a progressive collapse, emphasizing the need for structural redundancy to prevent disproportionate failures. Thankfully, nearly all buildings are structurally safe, and those constructed after ACI 318-89 adoption include integrity reinforcement. More recently, ACI 318-19 adopted changes to attempt to prevent the formation of flexural-induced punching failures. Appropriately designed and constructed flat plate slabs are both safe and economical. Furthermore, plan reviews and special inspections, which are increasingly common, can potentially catch design and construction errors.

Recognizing the Impact of Unanticipated Loads. The failure of the pool deck slab was exacerbated by excessive loading beyond its original design capacity. The best practice for engineers is to carefully evaluate any modifications or renovations that could impose additional loads on existing structures. Proper documentation and thorough structural assessments should happen before significant changes occur.

Effective Inspection Protocols. Visible signs of distress, including efflorescence deposits, cracking, and vertical slab displacement, were documented in the months leading up to the collapse. These warnings were either missed, misdiagnosed, or not acted upon in time at CTS. Recent examples of severe punching shear strength deficiencies discovered after analyses and testing include the Riverview Condominium in Cambridge, MA, and the Dockside Condominium in Charleston, SC. In both cases, the buildings were evacuated. Regardless, there are countless other examples of punching shear strengthening retrofits after identification, analysis, and testing. If significant visible signs of distress are noted in flat plate construction, structural review of punching shear strength based on non-destructive assessment of as-built conditions is prudent.

Engineers and building owners should be vigilant concerning newly forming distress. In Florida, a new milestone inspection law has been passed mandating the inspection of many condominiums (SB-4 and SB-154). Nevertheless, engineers and the general public should understand that no inspection can guarantee the structural performance of a building.

Continued Research. The tragedy caused by the CTS collapse highlights the need for additional structural engineering research. Punching shear-related research should consider the effects of time and environmental factors on punching shear capacity. The CTS failure propagated rapidly, demonstrating the disproportionate impact of localized failures. Research should explore improved progressive collapse models and structural retrofitting methods to improve redundancy and failure resistance. Finally, the structural engineering community should develop strategies to better communicate with the public regarding structural integrity and the potential risks for their existing infrastructure.

This tragedy is a stark reminder of the need for effective design standards (including plan review and special inspections) and timely intervention to address structural concerns. By applying the lessons learned from this disaster, the structural engineering community can build safer, more resilient structures—ensuring that past failures drive future innovation rather than repeat mistakes. ■

About the Authors

Matthew Fadden, Ph.D, PE, is Associate Principal, Wiss, Janney, Elstner Associates, Inc., Ft. Lauderdale, FL, specializing in failure investigations, structural analysis, and repair design. His work includes high-profile failure investigations, such as the Champlain Towers South collapse, advanced finite element modeling and structural testing for evaluating reinforced concrete and steel structures. Fadden serves on AISC’s Committee on Specifications Task Committee 4 Member Design.

Sedona Iodice, EI, is an Associate III at Wiss, Janney, Elstner Associates, Inc. (WJE). She holds a BS and ME degree in Civil Engineering from the University of Florida. Ms. Iodice joined WJE in 2022 and has contributed to numerous structural investigations and repairs.

Gary J. Klein, PE, SE, is Executive Vice President and Senior Principal at Wiss, Janney, Elstner Associates (WJE). His work includes high-profile collapse investigations (Hyatt Regency Walkway, Koror-Babeldaob Bridge) and extensive research in concrete structures and volume change movements. Klein is a member of the National Academy of Engineering and the ACI 318 main committee, has served on the NIST National Construction Safety Team Advisory Committee, and received the University of Illinois Distinguished Alumni Award for his contributions to structural engineering.