A Washington Post examination of video and images from the deadly collapse of a high-rise apartment building outside Miami — along with interviews with structural engineers, a key witness and an investigator — deepens questions about whether existing damage to a deck in the pool area contributed to the disaster.

A resident told The Post that minutes before Champlain Towers South in Surfside came down, she noticed that a section of the pool deck and a street-level parking area had collapsed into the parking garage below. The husband of another resident has said that his wife, who has not been seen since the disaster, made a similar observation in a telephone call shortly before the collapse.

An engineer in 2018 found “major structural damage” in the pool deck area caused by what he said was a flaw that limited water drainage. At least 16 people were killed and more than 140 remain unaccounted for following a type of disaster that was unheard of in the United States.

The investigation into the collapse is likely to take months and may find no single definitive cause. Experts urged caution and some structural engineers said they doubted that a collapse in the deck slab would have jeopardized the beachside building’s overall integrity. But Allyn Kilsheimer, a veteran engineer hired by Surfside to investigate the collapse, told The Post that such a failure could have set off a wider catastrophe.

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“There is a possibility that part of the pool [area] came down first and then dragged the middle of the building with it, and that made that collapse,” said Kilsheimer, who had previously voiced skepticism about the significance of the damage noted in the engineer’s report. “And then once the middle of the building collapsed, number two, then the rest of the building didn’t know how to stand up and it fell down also, number three.”

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Of more than a dozen experts interviewed for this article, including nine structural engineers, most agreed that the collapse appeared to involve a failure at the lowest levels of the building or in the parking garage beneath it. In images of the rubble, four experts saw indications of “punching shear failure,” in which concrete slabs that make up the floors of a building detach from the structure’s vertical support columns.

Such a catastrophe is extraordinary for a 40-year-old building even if a range of imaginable failures had occurred, the experts said. The building collapse is as shocking “to the public in general … as it is to the engineering community,” said Joe DiPompeo, the president of the Structural Engineering Institute of the American Society of Civil Engineers. “There’s got to be a very specific sequence of events that somehow evaded all the fail-safes in the code and everything else.”

Sara Nir, a resident, told The Post that shortly before 1 a.m., she noticed loud “knocking” noises that she assumed were caused by construction work. Around 1:14 a.m., she heard a noise that she thought sounded like a wall crashing down, and she left her ground-level apartment to complain to a security guard in the lobby.

She estimated that about a minute later, while she was in the lobby, she heard a very large boom and saw that part of the surface-level parking area — and part of the pool deck — had collapsed into the underground parking garage. She and the two of her children who were home at the time then ran from the building.

Nir’s son called 911 at 1:19 a.m., he said, a time that he said he confirmed by checking the time stamp on his phone. About a minute later, a dispatcher with Miami-Dade County Fire and Rescue called for an engine to respond to an alarm at the building, audio shows.

According to additional EMS audio, the building collapsed between 1:24 and 1:25 a.m. while Engine 76 was en route.

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Nir was previously interviewed by COLlive, an independently run Orthodox Jewish news service.

The Champlain complex was a steel-reinforced concrete structure with 136 apartments spread across 12 stories plus a penthouse. The only public video footage of the building collapse appears to be a recording of surveillance footage captured by a camera on a neighboring property. It has no time stamp. Some experts who reviewed the footage found it significant that the exterior of the upper floors appeared intact as the building began to fall.

“You can see the failure came from the bottom,” said Kit Miyamoto, a veteran structural engineer and the chairman of the California seismic safety commission.

Miyamoto described the middle section as “vertically imploding,” indicating that columns toward the bottom must have been “compromised.” Several other experts said that the video pointed to columns in this area having given way, setting off a cascade of damage.

The standing columns highlighted suggest the columns “punched through” the deck, according to two experts. (Photo by Saul Martinez for The Washington Post)

The experts said that the columns may either have suffered “axial failure,” meaning that they suffered too much stress from compression, or punching shear failure, when a concrete slab fails under pressure at the point that it connects to the column and falls. The column effectively “punches” through the slab. This can cause a succession of collapses as weight accumulates from above, the experts said.

Scott Homrich, the president of the National Demolition Association, said that while there was no sign that the Surfside collapse was caused deliberately, the sequence of events resembled that seen in many controlled demolitions.

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“You see basically the bottom of the full building go away and you just see the whole building drop all at one time. And that is very reminiscent of what an implosion would do,” said Homrich. Officials in Florida have stressed that there is no indication of foul play in Surfside.

These observations amount to a significant early lead for the investigators, according to Glenn Bell, a consulting structural engineer and the director of Collaborative Reporting for Safer Structures-U.S. “It will help them begin to focus on that particular area of the structure,” said Bell.

The video footage shows that about seven seconds after the initial collapse, a second section of the building starts to fall.

According to Jack Moehle, a professor of structural engineering at the University of California at Berkeley, the first section probably “dragged the remaining portion sideways until its gravity-load carrying capacity was exhausted and it, too, collapsed.”

Experts noted that, unlike the first section, the second section visibly twisted and sloped to one side in the moments before it fell.

“The reason for that is that part on the right side — its columns were still OK,” said Albert Bleakley, a professor of mechanical and civil engineering at the Florida Institute of Technology. “The structure of that building was still good. It’s still holding it up. But because it’s dragging, being dragged down by the rest of the building to the left of it, it starts leaning to the left, which also causes extra stresses on it because it’s not designed to be leaning to the left. But then also it’s just being pulled down.”

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Homrich, the demolition expert, agreed that the second collapse “wasn’t catastrophic failure,” echoing the observations of several others. “What that was [was] the debris underneath — probably a combination of half pulling it over and then piling up against the columns — and it finally failed. And then it comes down,” said Homrich.

The end result can be observed in photographs of the wreckage, the experts said, several noting that the section of the building that fell second ended up in a distinct formation probably caused by the “pancaking” of floors collapsing from the top down.

Speculation about the collapse has centered on the pool deck area because the 2018 engineer’s report found that the slab under the deck was not sloped to drain properly and had suffered damage as a result. The engineer, Frank Morabito, was hired by the condominium association to inspect the building as part of requirement that it be recertified 40 years after construction. He said the building required timely and expensive repairs, which had not begun by the time of the disaster.

Jean Wodnicki, the condominium board’s president, told residents in an April letter that visible damage in the garage had grown “significantly worse” since the 2018 inspection, adding: “When you can visually see the concrete spalling (cracking), that means that the rebar holding it together is rusting and deteriorating beneath the surface.” Wodnicki said it was “impossible to know the extent of the damage to the underlying rebar until the concrete is opened up.”

Nir’s description of the collapsed deck area adds to the account provided to The Miami Herald by Mike Stratton, who said he was speaking by phone that night with his wife, Cassie. Stratton said his wife told him that she could see a crater in the pool area from her fourth-floor balcony. Then the line went dead, he said.

Some experts told The Post that a worsening of the damage could have led to the collapse in the deck area noticed by the two residents.

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“If these two accounts are indeed indications that the plaza area near the pool collapsed first, then deterioration from leaking and consequent slab damage could be consistent with this,” said Bell, the director of Collaborative Reporting for Safer Structures-U.S., who cautioned against jumping to conclusions and said the investigation would take time.

Kilsheimer, the engineer investigating the collapse, said that corrosion caused by water that got inside the slab could have eventually caused its collapse. “If enough of the reinforcing steel was 100 percent rusted through, that could be something that made it happen,” Kilsheimer said.

Images of the site following the disaster show that the deck and surface parking area were left partially sunken, though experts said falling debris may have contributed to this.

Jason Borden, a Fort Lauderdale-based structural engineer who examined the Champlain building last year while his firm prepared to bid for work there, said that sustained corrosion of the slab “could have impacted the overall integrity of the building” because it joined with a main external wall. “It may be one of the factors that contributed to the collapse,” Borden said.

Some experts said that, even in light of the witness accounts, they doubted that a collapse of the pool deck slab could have been enough to bring down most of the building. “I don’t immediately see how that particular area would influence the stability of the building itself to the north,” said Troy Morgan, an adjunct professor of engineering at New York University.

“From what I see, it didn’t look like something that I would say, ‘Get the people out,'” said Norma Jean Mattei, an engineering professor at the University of New Orleans, referring to the 2018 report. “The deterioration played a part, but it wasn’t what caused this failure. Something else had to push this building over the top.”

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Investigators will examine the ground beneath the building to check whether any kind of collapse or failure around the building’s foundations could have caused the disaster. An academic study published last year found that the building appeared to have been sinking during the 1990s.

Experts said investigators would likely carry out geotechnical and geophysical surveys, taking samples of the earth and using sensors to collect data on the different layers beneath the surface.

But plans indicate that the building rested on deep concrete foundations, known as piles, which experts said were appropriate for the area’s sandy soil. “Generally, the piles are not just going to start to move or let go unless the ground, independent of the building, moves,” said DiPompeo, of the Structural Engineering Institute of the American Society of Civil Engineers.

Mattei said the fact that sinkholes were not a common problem in the area caused her to focus on the building’s core structure. “That leads me to, ‘What wiped out some columns?'” said Mattei. “It’s all conjecture, but what wiped out some major columns at the lowest levels, where they’re actually holding up the most load?”

Although two major sections of the Champlain tower collapsed, a significant part of the building did not fall and remains intact. Experts who reviewed plans for the building said reinforcements in the surviving part of the building may have helped to protect it.

They noted that plans show the surviving structure featured rectangular columns and horizontal concrete beams at the second-story level. It also contained the shear walls around the elevator shaft, the strongest part of the building. The collapsed portion of the building had square columns and lacked the same concrete beams for reinforcement.

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The design may have been intended to help the building withstand powerful gales in an area that is frequently battered by hurricanes. “My guess is those columns on that left side [that] were shaped rectangularly and had those big beams going that way [were] probably designed that way for wind resistance,” said Bleakley of the Florida Institute of Technology.

All the experts interviewed by The Post urged caution at this early stage of the investigation. Substandard materials or shoddy workmanship during the building’s construction from 1979 to 1981 may be discovered. Some said that the cause of the collapse may be something that has not yet even been considered. At present, “there is no smoking gun,” said Morgan.

While some experts see building-specific circumstances, including deferred maintenance, as leading suspects in the collapse, the role of more systemic factors such as sea-level rise remains unclear. Experts said every possibility needs to be considered.

“We need to find that out: whether this building had something wrong with it, or if this is a problem with high-rises on the coast,” said Harold Wanless, a professor of geology at the University of Miami. “That has huge implications.”

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The Washington Post’s Daniela Santamariña, Maria Aguilar, Lori Rozsa, Karly Domb Sadof, Julie Tate and Tik Root contributed to this report.

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About this report

Methodology: The Washington Post built a 3-D model of Champlain Towers South based on the original 1979 plans and the preliminary review plans for its 40-year-recertification process.