Though the trigger may remain a mystery for some time, by the end of the week, the structural engineer probing the partial progressive collapse of a 40-year-old Surfside, Fla., residential condominium expects to complete a computer model of the unstable, 12-story remains of the building. The computer model of the still-standing wing of Champlain Towers South will initially be used to alert the search and rescue team to suspend operations if a hurricane is coming. 

“We are collecting data and building structural models," with the help of the original drawings, to predict the wind forces that the unstable tower can withstand, so that "we can warn people” to vacate the site if necessary, says Allyn Kilsheimer, founder of KCE Structural Engineers. Kilsheimer, who arrived at the site a day after the June 24 debacle, is working for the city of Surfside, north of Miami Beach.

With a cyclone category 5 in the Caribbean that is expected to become Tropical Storm Elsa, the completion of the model could not be better timed. At a June 30 press conference in Surfside, Florida officials announced the storm, if it hit at all, would not have any impact on South Florida through Saturday. Officials also said they were tracking the storm and making appropriate preparations, including developing a plan to keep the debris of the tower from becoming projectiles.

Concurrently, KCE, with the geotechnical consultant Mueser Rutledge Consulting Engineers, is piecing together the complex puzzle of the collapse scenario, based on observations, measurements, a 2018 field survey, photos, a surveillance video and the original 1979 documents for the reinforced concrete structure. The investigators are also trying to sort out the locations of the two foundation types on the drawings: pressure-injected footings and 14-ft-square precast concrete piles.

“There may have been three collapses,” in quick succession, Kilsheimer speculates. Determining “why this happened is a long process of collecting data and creating computer models,” he adds.

Surface Parking Zone of Interest

An area of interest is the surface parking zone near the lobby entry drive that goes partly under the remaining wing. That slab for the parking, which is next to the grade-level pool deck, is the roof of a one-level basement garage that fills the site's entire footprint.

According to Kilsheimer, it appears, from the position of the cars that dropped a level—one car’s nose ended up pointing to the pool area—that the lobby slab columns moved sideways toward the pool deck, causing the pool deck to drop, which then pulled on the tower columns, causing them to fail. “This will not prove where it started,” he says. "That is not something we can determine in a hurry."

However, Kilsheimer has all but ruled out an overload from the parked cars as the trigger. “It could be that a drunk driver hit a column,” he says. “We don’t know yet.”

KCE and Mueser Rutledge will soon be joined at the site by another investigating team. Also at the press conference, the National Institute of Standards and Technology of the U.S. Dept. of Commerce announced it will soon be launching a technical investigation into the causes of the failure under the authority it was given by the 2002 National Construction Safety Team Act.

Fact-Finding Not Fault-Finding Investigation

"This is a fact-finding not a fault-finding technical investigation" that could take a couple years,  said James Olthoff, NIST's director. The goal is to determine the cause of the collapse and make recommendations for changes in codes and standards, he said.

Judith Mitrani-Resider, NIST's associate chief of the materials and structural systems divison, said the NIST team, which will be composed of NIST staff and at least one outside expert, will go into the probe "with an open mind," to collect evidence and study the design, construction, modifications and maintenance of the building.

There was no announcement of when the team would be deployed. There were assurances, however, that there would be no interference with ongoing search and rescue operations. At the time of the press conference, there were 18 dead and 145 people still missing.

No one has been allowed into the remains of the evacuated building. That includes the basement level that fills the entire site. The only shoring posts installed to date were to create a tunnel of sorts through the debris pile to help rescue workers search for survivors.

Miami-Dade County Mayor Daniella Levine Cava also said at the press conference that the county is reviewing its rules regarding buildings, including recertification when a building is 40 years old.

It does seem clear that the tragic failure started on the grade-level slab, “but there are lots of questions,” Kilsheimer says. "We are learning more things every second," he adds.

The video taken from a nearby building, which has been widely studied, is helpful, but it starts a few seconds into the collapse. Kilsheimer, at the site since June 25, says he hopes other videos will surface, including one that shows the very beginning of the collapse. “The one video [we have] doesn’t give you a 3D view of the building,” he says.

deck area

The collapse may have started in the at-grade parking area alongside and under the elevated first occupied level of the remaining wing (at right, in photo) and brought down the adjacent pool deck. The deck likely sent a shock wave toward the east-to-west tower, which then collapsed into a pile of debris. 
Photo by Chandan Khanna/AFP Via Getty Images 

Sequence of the Debacle

In terms of the sequence of the debacle, there are reports of eyewitnesses hearing noises around the grade-level elevated pool deck and surface parking areas. Photos show part of the pool deck dropped onto the basement garage slab underneath it. The collapse would have sent a shock wave through the floor system at grade level, which is the garage roof, says S.K. Ghosh, president of S.K. Ghosh Associates LLC.

In this scenario, the shock wave was large enough to take down one or two critical columns, he adds. “From there, it was a progressive collapse.”

Engineers are concerned about a design that would then result in a progressive collapse of so much of the tower.

The 136-unit Champlain Towers South was completed in 1981, built by the defunct Nattel Construction, a Miami Beach contractor owned by the late project co-developer Nathan Reiber. The reinforced concrete building, engineered by the defunct Breiterman Jurado & Associates, has flat-plate floor slabs and lightly braced shear walls. The relatively thin slabs, lightly reinforced near the top of the slab, are supported directly on columns. The one-level basement fills the entire site, including and beyond the building’s footprint.

Champlain Condo

Champlain Towers South stood for 40 years. Area of pool deck nearest the building is under scrutiny. Grade-level site plan is inset.

The building met all building codes in effect in 1979. But Florida codes have been greatly improved since then, agree sources. Still, the collapse raises questions about the condition of the rest of the older building stock in Florida.

Worry About Newer Buildings

“Everyone is worried, even about newer buildings,” says one structural engineer in the region, who declines to be identified. He emphasizes that 30 to 40 years ago, construction in Florida was like “the Wild West,” but that the process for at least 15 years has been “as rigorous as anywhere else” in the U.S., with mandated peer review and more for certain structures.

The Champlain Towers condominium had just begun a 40-year building recertification program required by Miami-Dade County. The process mandates that a registered engineer or architect submit a field survey report on the “general structural condition” of the building’s systems.

The 40-year recertification process is “not a significant evaluation,” says the engineer who requested anonymity. “It’s like window dressing.”

On June 25, Surfside officials released public records related to Champlain Towers, including the 2018 survey report by structural engineer Morabito Consultants Inc. and the original 1979 plans by William M. Friedman and Associates Architects Inc. “The goal of our study was to understand and document the extent of the structural issues that require repair and/or remediation in the immediate or near future,” wrote Frank Morabito, the firm’s president, in the report.

Morabito indicated “that many of the previous garage repairs are failing resulting in additional concrete cracking, spalling and leaching of calcium carbonate deposits. At the underside of the entrance/pool deck where the slab had been epoxy-injected, new cracks were radiating from the originally repaired cracks.”

Morabito recommended the entrance/pool deck concrete slabs that were showing distress be removed and replaced in their entirety.

The field survey was based on visual observations only. “The report doesn’t outline any [structural deterioration] we haven’t seen a thousand times elsewhere,” says the anonymous engineer, who studied the report. “There were no red flags,” he says.

A letter recently sent from Champlain Towers condo association board president Jean Wodnicki to building residents and released to the media by family members, shed further light on the structure’s condition prior to collapse. A revised estimate dated Oct. 15, 2020, from Morabito—who was overseeing the recertification repairs—showed that costs had ballooned to nearly $15 million from the original $9.1 million.

In the memo to residents, Wodnicki “indicated that the concrete damage observed would begin to multiply exponentially over the years, and indeed the observable damage such as in the garage has gotten significantly worse since the initial inspection. When you can visually see the concrete spalling (cracking), that means that the rebar holding it together is rusting and deteriorating beneath the surface.”

“The concrete deterioration is accelerating,” wrote Wodnicki. 

In the 2018 estimate, a single line item of “garage, entrance and pool-deck remediation” was $3.8 million. The 2020 estimate was $4.1 million. That included five line items covering work such as “waterproofing,” “structural repairs” and “miscellaneous repairs.” 

Major issues will arise about responsibility and liability, as a result of the recertification report and more. “I am mindful of Florida’s statutes of limitations and repose,” says Eric Ruzicka, a partner at the law firm Dorsey & Whitney.

According to Ruzicka, under Florida Stat. 95.11(3)(c), an action founded on the design, planning or construction of a building must be commenced within four years from the time the defect is discovered or should have been discovered.  No claim can be brought 10 years after completion of the work regardless of when it was discovered.

The statute will likely eliminate the liability of those involved in the original development, design and construction, he says. Rather, victims and their families’ recovery will be limited to those involved in the building’s maintenance and those assessing the condition of the building over the past four years, Ruzicka adds. 

Champlain condo collapse

The progressive collapse of the east-west wing stopped at the shear wall elevator core, leaving much of the north-to-south wing standing but unstable.
Photo by Jeff Greenberg/Universal Images Group via Getty Images

The surveillance video suggests the collapse of the tower itself began lower down in the central south-facing perimeter of the building, which is a fat L in plan. The failed long leg of the L extended east to west on the north side of the footprint. 

The video also shows a “very rapid vertical collapse of the interior portion of the building, with relatively little apparent side sway,” says Jack Moehle, a professor of structural engineering at the University of California, Berkeley.

The north side of the long leg followed the south side collapse, almost immediately. Next, the east end of the building, nearest the Atlantic Ocean, got pulled toward the midsection to its west.

The video shows the east end standing for a short while after the midsection collapsed, gradually leaning toward the midsection. Likely there was an imbalanced load due to loss of support from the portion of the building that collapsed, which dragged the remaining portion sideways until its gravity load-carrying capacity was exhausted and it also collapsed, says Glenn Bell, a forensic structural engineer and director of Collaborative Reporting for Safer Structures (CROSS-US), a division of the American Society of Civil Engineers’ Structural Engineering Institute (ASCE/SEI). 

Possible Disproportionate Collapse

In addition to the distress in the pool deck slab, engineers speculate that column failure, slab failure due to punching shear or failure of the pile foundation system may have contributed to the progressive collapse. If the failure did start at the grade-level slab, and spread to the tower, the disaster could be considered a disproportionate collapse, which is a more specific type of progressive collapse. In a disproportionate collapse, the failure is out of proportion to the trigger because it spreads way beyond the trigger zone, in this case perhaps to the tower itself. By coincidence, ASCE/SEI expects to publish a Disproportionate Collapse Mitigation Standard in January.

Both progressive or disproportionate failure is caused by columns that fail axially due to axial over-stress or shear damage that progresses to axial failure or punching shear failure, says Moehle.

In punching shear, one or more of the building’s flat slabs develops a shear failure around the column and drops relative to the column. If there is no continuous bottom reinforcement in the slab passing over the columns—a detail not required in 1981—then the slab can move downward relative to the column, redistributing load to adjacent slab-column connections, which in turn can fail. A slab that lands on the floor below it overloads that floor system, and there is more punching shear, says Moehle.

Some photos of the debris suggest that the tower collapse involved—at least as a secondary mechanism—punching shear failures. “This could have been a secondary aspect of the collapse, with the primary cause of the collapse being something else,” says Moehle, who adds that his observations are “very preliminary speculation.”  

Other possible contributing factors: It is “plausible” that sea spray over 40 years increased the chloride content of the concrete, which, after reaching a critical point, starts corroding the rebar, adds Moehle. There could have been a more recent acceleration of corrosion, which accelerated a loss of column strength. Another theory is that differential settlement across the site can cause the redistribution of internal forces and can over-stress some members.

“It will be a long time before this has been thoroughly studied and thoughtfully considered” to give engineers the confidence to draw lessons about what happened, says Moehle.

ENR Southeast Editor Scott Judy contributed to this article