By Ian Thomsen
News at Northeastern
As first responders frantically search the rubble for survivors of a collapsed residential tower in Surfside, Fla., the investigation has already begun—and it is likely to focus on multiple causes, says Mehrdad Sasani, a professor of civil and environmental engineering at Northeastern. An engineering report conducted on the building in 2018 and released Friday pinpointed structural damage issues that could have contributed to the catastrophic collapse.
Other clues have emerged as well about Champlain Towers South, says Sasani. “Usually, when there is a collapse, there are several factors involved,” said Sasani, who has researched the progressive collapse of structures, earthquake engineering, and structural integrity and reliability.
Two sections of the 12-story tower gave way without warning, one after the other, at 2 a.m. Thursday.
As of Sunday night, at least nine people were dead and 152 people were unaccounted for, according to Miami-Dade County officials.
Sasani notes that U.S. building requirements changed in 1989—eight years after the construction of the Surfside building.
“There were [new] structural integrity requirements that would help the structure to be better integrated,” he says.
In the meantime, says Sasani, this should be viewed as a rare event. People should not be anticipating other buildings to crumble in the United States, he says.
“The general concept of design is such that if failure is going to happen, it is going to happen gracefully with enough warnings,” Sasani says. “The taller the building, the more checks and balances are there, the better design is there, the better construction is there—in most cases.”
How has new information about the Surfside building that has come to light in recent days shaped your thinking about possible causes of the collapse?
Perhaps the most important piece of new information, as of Sunday morning, comes from an eyewitness. It is reported that a resident “called her husband moments before the collapse to tell him she could see a crater in the pool area from the fourth-floor balcony of their ocean-front apartment. Then the line went dead.”
This along with a 2018 inspection that reported a major structural damage below the pool deck may suggest that a foundation failure was an important contributor to the building collapse.
What other structural clues jump out at you?
There have been reports that based on the surveillance video, “the top several stories appear to fall first.” While that is a possibility, if you closely look at the bottom (particularly the right side of the building portion that collapses first) there are early motions visible as well and I am a bit more inclined at this time, that the actual motion/failure is initiated from close to the bottom and not the top of the building.
The structural drawings of the Champlain Towers South Condo show that the structural floor system above the second floor is made of 8-inch-thick flat plates, which span up to about 24 feet. Having said that, the possibility of slab-punching shear failure, which is often brittle (as opposed to being graceful/ductile) should be considered.
How should we sort out other potential structural issues?
A building goes through several phases. The first phase is obviously design.
Then the building is constructed.
Then, over time, the building could have been affected by negative impacts—environmental or other factors—that could affect the deterioration of the building. In this case the proximity to the ocean could have affected the concrete; and then, more importantly, it would lead to the corrosion of the reinforcing bars. And that could be a cause.
And then, finally, the collapse happens when the demand on the structure overcomes the capacity.
I heard there was some work on the roof. Maybe there was some slight increase in the load on the building again, which could push the building over the limit of collapse. Additional load could be one factor, which would increase the demand over capacity.
Are there examples of buildings collapsing because of design errors?
There was a building that collapsed in Singapore in 1986. It was constructed in 1971, and the designer had forgotten about the so-called dead load: The calculation did not include the weight of the building. Why did it stand up for 15 years and then collapse? Apparently they put a heavy air conditioner on the roof, a few months before the collapse. The building was on the verge of collapse all the time, but that little trigger pushed the building over the limit.
There was a collapse of a building in the Boston area in 1971. That building was under construction. The combination of bad design, poor construction, and overload led to the collapse of that building.
How will investigators proceed?
The first thing is collecting data about that building that exists from the past—the design of the building, construction, any studies about buildings in that area that are sinking. Any of those reports are very helpful.
The immediate action is the search and rescue. But after that, they will be looking into the debris on the ground, trying to put pieces of the puzzle together. That is what forensic engineers do. A careful reconstruction of those pieces can provide some clues.
They will look at the portions of the building that are standing now, how they behaved, how the separation occurred. Those things can potentially help to identify the cause.
There has been a lot of talk about how we don’t see these kinds of collapses in the U.S. very often. Is it true that buildings in the U.S. tend to be safer than in other countries?
You would expect so. There are [in the U.S.] better ways of designing and verifying the designs, and in general better construction and inspection.
I can talk about earthquakes: When an earthquake of magnitude and intensity occurs, in some countries you have tens of thousands of casualties; in the U.S. you may have hundreds of casualties. That can be attributed to the way the design, construction, and inspections are done, which altogether would lead to better buildings.