From cracked slabs to offshore blast: What probe into Florida condo collapse might look at

A catastrophic structural failure can take place in a flash. Determining what went wrong and why is a slow and agonizing process.

Experts interviewed by the Miami Herald said the investigation of the Champlain Towers South, likely to be undertaken by the National Institute of Standards and Technology, will have many possible causes to explore, including:

• A failure in the condo tower’s foundation;

• Steel corrosion in the concrete structure;

• An explosion;

• Construction damage;

• Improper design or construction.

Six experts discussed with the Miami Herald the potential causes of Thursday morning’s calamity and which they thought most plausible based on the information currently available in photos, video and records released by the city.

While none felt certain of their answers, most seemed to agree that the way the building collapsed seemed to be triggered by a failure near the base of the structure.

“It looks similar to something that was intentionally demolished,” said Kevin DuBrey, the director of project management at Hillman Engineering, a firm that inspects high-rise buildings in South Florida for potential damage. “Usually they would do that underneath. They would set the charges on the columns and beams down below and let it collapse on itself.”

Gene Santiago, a structural engineer and retired building inspector, said he “still can’t get over the way it fell down — like an implosion,” he said. “Like a demolition job.”

While experts suggested the following hypotheses were most likely to have caused the collapse, they also stressed that a combination of various factors was possible. Atorod Azizinamini, chair of the Department of Civil and Environmental Engineering at Florida International University, told the Herald that this type of collapse is usually the result of “a perfect storm of several factors coming together at the same time.”

Hypothesis 1: STEEL CORROSION IN STRUCTURE

Structural plans from 1979 show that the Champlain Towers South Condo was built using a “flat slab” style of construction in which concrete slabs reinforced by steel rebar sit directly on top of support columns — a common method of construction for everything from parking garages to apartment buildings, experts said.

Over time, the rebar in the concrete can become corroded and crack the slab and joints, potentially weakening the structure. If one part of the structure collapses due to corrosion, it could cause a larger catastrophe.

“When you have one structural failure in one column or beam, it’s a domino effect,” said Charles Danger, retired building chief for Miami-Dade County.

A concrete slab that’s damaged by corrosion can be prone to a type of failure called “punching,” in which the column punctures the slab and weakens the overall structure, Abieyuwa Aghayere, a professor of forensic engineering at Drexel University. He said that could be one explanation for the building’s sudden collapse.

“Punching is usually sudden,” he said. “It doesn’t give much warning.”

A 2018 report by Frank Morabito, an engineer who inspected the condo, found that poor drainage in the pool deck caused “major structural damage” to the concrete slab below.

[Inspectors raised concerns in 2018 about ‘major error’ in design of Florida condo building that collapsed]

“If the pool is leaking and the pool slab is gradually getting destroyed, that could be a problem if it is not maintained,” Santiago said. “Again, it seems like something the residents and condo association would have noticed and corrected.”

Normally, experts said cracks indicating corrosion would be visible, but Morabito said he had limited access to assess the extent of the damage that may have been partially obscured by tiles placed on top of the slab. It’s unclear from the limited records made available by Surfside whether the damage was repaired.

Still, experts cautioned not to jump to conclusions. Investigators will go through the debris to determine the likelihood of failure due to corrosion.

“Contaminated concrete should not have been an extreme problem that would cause catastrophic failure in a 1981 building,” said Miami engineer John Pistorino.

When asked about the likelihood of this kind of damage, experts agreed context is important.

“It’s not inconsequential that this building was on the beach,” said Peter Dyga, president of a Florida chapter of the Associated Builders and Contractors. He called the salt water and humid air “some of the harshest conditions on earth.”

Hypothesis 2: FAILURE IN THE FOUNDATION

The condo was built on a “concrete pile foundation” that appears to sit directly on top of the limestone bedrock, according to the 1979 plans.

Danger, the retired building chief for Miami-Dade County, described the Champlain towers as “conventional concrete buildings for that era, nothing fancy, nothing out of the ordinary.” Experts agreed that the type of foundation should have withstood shifts in the soil and other changes in the ground.

“There should be no issues with the foundation, period, because we design here for harsh and corrosive conditions, high winds and storm surge,” said Pistorino, an engineer with a 50-year career in Miami who has been instrumental in writing state and county building safety laws. “The only time I’ve seen whole buildings collapse because the foundation failed was when Hurricane Gilbert hit Mexico — but it’s totally different construction down there.”

To investigate foundation issues, inspectors will generally take “concrete cores to look at the actual foundation and additional borings to see how the pilings held up,” he said.

“Nothing strikes me [as a red flag],” Aghayere said after reviewing the foundation design. Still, he said, investigators will look at more than just the plan.

“Again, as part of the failure hypothesis, is to say OK, how stable is the rock at the bottom? At least to rule that in or rule that out,” he said.

Hypothesis 3: CONSTRUCTION DAMAGE

Champlain Towers South was in the early stages of three years of expected construction on various aspects of the building, starting with the roof, said Jeff Rose, a contractor who has done renovations on units there and whose parents lived there — and were out of town when it collapsed.

Rose said construction on the roof had begun about six weeks ago, which involved replacing the roof, replacing the stands on which air conditioning condensers rested, and replacing electrical disconnects for air conditioning units. Rose said his parents could hear jackhammers or other demolition tools being used to remove the old roof.

“If they were storing materials and equipment on the roof in an incorrect location in the middle of the slab instead of on top of the columns, that could cause stress on the roof,” said Santiago, explaining that roofs are designed to hold 30 pounds per square foot to meet code. “If you’ve got an overloaded roof slab and it punches through, it goes down, down, down, one level pulling down the next — pom, pom, pom — and it never ends until it hits the bottom.”

Roof failure could have triggered a “perfect storm” collapse, theorized John Colby, a retired architect from Boca Raton.

“Causes could include tendon corrosion due to water exposure or inadvertent severing or weakening of tendons, leading to progressive failure,” he said in an email. “This would of course be exacerbated if, at the same time, unusual dead and live loads were being imposed by re-roofing equipment and materials. And even further if any destructive testing was done on the roof during the ongoing 40-year structural evaluation. Such a roof failure would also be consistent with reports and the video of the ‘pancake’ sequence.”

But the concerns were contradicted by Surfside’s building official, James McGuinness, at an emergency commission meeting on Friday. McGinness said he had been on the roof just 14 hours before the building partially collapsed. He said the roof work had been paused because of rain, but that he was there to inspect newly installed anchors that support so-called “swing stages” used by workers who scale the side of the building to clean its windows.

McGuinness threw cold water on the idea that heavy machinery on the roof could have contributed to the collapse.

“There was no extraordinary mass materials on the roof that would do this,” he said.

But experts said any previous roof repair work could have caused any number of other problems that would worsen over time.

“If they were doing roof work, it could be one of the drains was plugged and caused water to collect. But it doesn’t look that way based on the way it collapsed,” said DuBrey, from Hillman Engineering.

Hypothesis 4: SHAKING AND TREMORS FROM EXTERNAL CAUSES

South tower tenants had complained about shaking and vibration in their units last year during construction of the neighboring Eighty Seven Park building. But the builder would have taken mitigating measures to prevent any damage at Champlain, such as digging a trench to buffer the reverberations, Santiago said.

“Yes, you might notice your dishes rattling but the chances of that nearby construction leading to catastrophic instability are very low,” he said.

In addition to the nearby construction, some in South Florida raised concerns that a Navy test explosion off the coast of Florida that caused tremors like a small earthquake in South Florida last week could have also been a potential cause of the collapse.

The Navy put out a statement saying that on June 18 it conducted the first of several planned so-called Full Ship Shock Trials (FSSTs) in the coming months off the coast. The USS Gerald R. Ford was about 100 miles from Ponce Inlet in Volusia County when a 40,000 pound explosive was set off right next to it. Video provided from the Navy on Twitter shows the ship, and then and a cloud balloon in the water.

Ponce Inlet is about 250 miles north of Surfside.

“[Investigators] are going to check it out,” Aghayere said, noting most buildings in Florida are not designed to withstand earthquakes.

What comes next?

Even with the inspection records and building plans released by the city, experts said it’s still too early to draw any conclusions.

“What started it? Who knows?” said Danger, who feared speculation is hurting the people still waiting for news about family and friends who were in the building at the time of the collapse. “I hope when the investigation is finished, there will be some answers and some relief for the survivors. But it’s going to take time to find out if this was preventable or if it was going to happen no matter what as a once-in-a-lifetime catastrophe. Certainly in my lifetime, I’ve never seen anything like it.”