Some engineers looking at the failure of a 13-story condo tower in Florida said the collapse appeared to have begun somewhere near the bottom of the building.
The investigation into what may be the deadliest accidental building collapse in American history has just begun, but experts who have examined video footage of the disaster outside Miami are focusing on a spot in the lowest part of the condominium complex — possibly in or below the underground parking garage — where an initial failure could have set off a structural avalanche. Called “progressive collapse,” the gradual spread of failures could have occurred for a variety of reasons, including design flaws or the less robust construction allowed under the building codes of four decades ago, when the complex was built. But that progression could not have occurred without some critical first failure, and close inspections of a grainy surveillance video that emerged in the initial hours after the disaster have given the first hints of where that might have been. “It does appear to start either at or very near the bottom of the structure,” said Donald O. Dusenberry, a consulting engineer who has investigated many structural collapses. “It’s not like there’s a failure high and it pancaked down.” The early examinations came as rescuers on Sunday spent a fourth day pushing through the enormous heap of debris created when half the 13-story building, Champlain Towers South, fell away early on Thursday. The death toll climbed to nine as additional remains were found, and more than 150 people remained unaccounted for. While a number of bridges, overpasses and buildings under construction fail each year, the catastrophic collapse of an occupied building — absent a bomb or an earthquake — is rare, and investigators are struggling to understand how it could have come with so little urgent warning. “It would be like a lightning strike happening,” said Charles W. Burkett, the mayor of Surfside, Fla., where the collapse occurred. “It’s not at all a common occurrence to have a building fall down in America,” he said. “There was something very, very wrong with this situation.” The National Institute of Standards and Technology, a federal agency, was sending scientists and engineers to do a preliminary review, hoping to identify and preserve materials that might help understand the collapse. Officials said they expected a number of local, state and federal agencies also to be involved in the inquiry, though it was not clear which agency would lead the effort. The search for an explanation comes with a sense of urgency not only for sister buildings near the complex but also for a broad part of South Florida, where a necklace of high-rise condos, many of them decades old, sits on the edge of the Atlantic Ocean, enduring an ever-worsening barrage of hurricane winds, storm surge and sea salt. Structural engineers were shocked that a building that had stood for decades would abruptly crumble on an otherwise unremarkable summer night. But three years before the deadly collapse, a consultant found alarming evidence of “major structural damage ” to the concrete slab below the pool deck and “abundant” cracking and crumbling of the columns, beams and walls of the parking garage under the building. While no definitive conclusions could be drawn from the surveillance video, which was shot from a distance and reveals only one perspective of the disaster, some of the engineers reviewing it last week said it seemed to suggest that the failure began at a specific point near the bottom of the structure — perhaps as far down as the parking garage beneath the building, or on the first few floors. From what can be seen in the video, part of the structure first slumped, seemingly falling vertically in one giant piece, as if the columns had failed beneath the southern edge of the center of the building, not far from the pool. Like a nightmarish avalanche, the failure quickly spread and brought down the entire center of the building.
