re: Real bad condo collapse in Surfside, FL

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The equipment room runs along the southern wall of the building — an area that did not collapse.

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Mohammad Ehsani, an engineer and concrete restoration expert who invented QuakeWrap technology, a way to reinforce old concrete columns,

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Thanks to a surveillance video, photos, a 2018 building-system survey and the original 1979 plans, structural engineers have pieced together the probable sequence of the fatal progressive collapse of part of the 12.5-story Champlain Towers South residential condominium, north of Miami Beach in Surfside, Fla. But the initiating event on June 24 that brought down a wing, about 150 ft long, of the 40-year-old reinforced concrete structure remains a mystery. “I’m trying to find the trigger,” says Allyn Kilsheimer, founder of KCE Structural Engineers, who is investigating the collapse for the city.

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The reinforced concrete building has flat-plate floor slabs. Flat plates are relatively thin concrete slabs supported directly on columns. The building is lightly braced by shear walls.

The video shows that the failure occurred 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. “It is possible the failure started higher up in the building on the interior and cascaded downward but it is not visible in the video,” says Jack Moehle, a professor of structural engineering at the University of California, Berkeley.

The video also shows a “very rapid vertical collapse of the interior portion of the building, with relatively little apparent side sway,” says Moehle.

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The north side of the long leg followed the south side collapse, almost immediately. “That is apparent from the video,” 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.

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 Bell. In the second part of the collapse, the floors are a little offset from one another because of this effect, he adds.

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There are at least three other theories about the trigger of the progressive collapse, beyond the pool deck slab: column failure, slab failure due to punching shear or failure of the pile foundation system. Kilsheimer, Bell and others speculate there may have been more than one contributing factor, as in a perfect storm.

Progressive failure is caused either 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 eventually lands on the floor below it overloads that floor system, and there is more punching shear, says Moehle, who has done extensive testing of concrete structures.

If the columns were the trigger, then it is likely but not certain that the collapse started at the bottom of the building where the axial loads are largest, adds Moehle. If it was the slab-column connection, then the failure could have started just about anywhere in the building and progressed downward in a vertical collapse that extended across the floor and down to the base.

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Some photos of the debris suggest that the 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 corrosion of the rebar, adds Moehle. There could have been a more recent acceleration of corrosion, which accelerated the loss of column strength.

Differential settlement across the footprint of the building 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.

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She said crews worked nonstop under the rubble to stop it, using infrared red technology, foam, water and other tactics to contain the fire and minimize the smoke, which had spread through the pile.

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So perhaps this wasn't a construction failure, but rather, a maintenance/upkeep one?

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They were about to spend 15 million on repairs... and could have spent 9 million a few years ago, but put it off.

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Owners were given until July 1 to decide how to pay

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