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Originally Posted by CurtEgerer
What are the 2 bright flashes at the beginning of the collapse video? Maybe power cables being severed but looks too bright for that. Lightning strike? I've seen unbelievable structural damage from lightning. If there was roofing work being done, there are three common collapse occurrences: plugged roof drains causing water ponding (extreme weight), improper storage of new roofing materials/equipment causing localized stress, and fire from torches. Deterioration of concrete may in fact be the cause but a total collapse from that would be somewhat surprising to me. Roof (or other construction) work would be the first thing I'd need to rule in or out. -- retired forensic structural engineer.
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I'd imagine they were electrical cables. There would have been at least a couple good size panels up there, to feed the rooftop A/C units and (possibly) the elevators.
The roof replacement permit was for 5800 sq. ft., which is a little less than a quarter of the total roof area. I don't know what section was being redone but they did a moisture survey and the worst areas of that were in the area where the collapse seems to have started. Although, an inspector was up there 14 hours before the collapse and didn't see enough equipment and supplies to warrant any concern.
The lady that was on the phone and actually in the building during the collapse stated that the first thing she saw was the collapse of the upper parking slab between the building and the pool. That seems to have failed at the column-to-slab connections. I wonder if that collapse could have damaged some columns enough to trigger their failure? I wouldn't think so but there may have been some differential settlement over the last 40 years and that may have changed to loads on individual columns. An engineer was quoted as having measured settlement of 2mm/year there, in the 1990's.