Saw a video today of a simulated collapse that suggests a possible failure mode given the likely parking garage collapse that occurred before primary collapse:
That's an interesting theory that I hadn't seen before. It would require the horizontal rebar in the pool deck to be strong enough that it could pull over the columns, which contrasts with that rebar also being weak enough to collapse. Likewise, in this scenario, would there still be punch-through columns in the parking deck area, instead of those columns falling over too?
The other question that obviously follows from this is, were those columns to fail, why did the failure of the columns on one edge of the building cause the entire section of building to collapse catastrophically? At 1:08 of the simulation, you can see the rest of the columns are still standing under the building.
Steel is strongest in extension, and a horizontal pulling moment is going through the cross section of the rebar rather then perpendicular to it.
The rebar would have different load bearing capacities in different directions, particularly if it was rusting (i.e. most likely the corrosion was principally on the top face).
Fwiw: there's a video circulating social media showing what appears to be rubble in the parking garage, claimed to be taken a few minutes before the tower collapse.
This is an interesting theory... one would wonder, how come it collapsed at the place, where it only held one slab, and not somewhere under more weight... but i'm not a civil engineer, so yeah... time (and analysis) will tell.
That area, the pool deck, was exposed to the elements. Rainwater was allowed to pool on top and seep into the concrete because it was not constructed correctly. It should have been slanted to allow water to drain off.
The slabs and pillars under the actual building may have been in better condition since they were mostly protected from the weather.
In salty environments, exposure to the weather tends to allow rain to wash away salt. Semi- exposed areas tend to fair worse for corrosion due to the absence of scouring by rain.
A bit counter intuitive.
But that’s building by the sea. Salt can be deposited by mist and chloride ions are the nemesis of reinforced concrete.
This would also be consistent with less steel being included than originally designed, as was rumored during construction of numerous hi-rises during the boom.
Looks almost like punch-through of all floors simultaneously due to shock, almost as if it was a planned demolition where stuctural supports are compromised with precision to assure orderly collapse.
Only this part of the building fell precisely without any carefuly placed dynamite.
But when buildings are stronger, sometimes intentional demolition projects do not complete the job so thoroughly on the first attempt.
The other question that obviously follows from this is, were those columns to fail, why did the failure of the columns on one edge of the building cause the entire section of building to collapse catastrophically? At 1:08 of the simulation, you can see the rest of the columns are still standing under the building.