STability. There doesn't appear to be any sort of lateral load resisting system. That's why all the columns on the main floor move in unison. A strength failure likely would've been just a single column going down.
First guess would appear to be stability. Hard to tell what type of structural system it is. I would guess steel beams/decking and columns but the video clarity isn't the best. You can't can't see what is happening along the right side of the building either.
All of the columns appear to move along the first floor, in the same direction at the same time. I would like to think the members were not undersized as they are not really seeing their full design load at all.
The workers are pouring concrete at the far end, and one operates a leveling/troweling machine all around the slab, do you see any form works at floors below?
Wow.... I'd say that this is primarily stability No significant lateral load resisting system. So, the second you get a little bit of lateral displacement or lateral loading, the lateral displacement increases until 2nd order / stability effects cause strength failures and the whole thing goes down.
The camera has a lot of distortion, which is why the third level shows a significant bow, compared to the second level.
Seems to me to be a very ad hoc "design" since there is nothing resisting lateral forces, so even if it had successfully completed construction, any wind likely would have toppled the building, with possibly a lot more people in it.
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Usually the floor should be fully supported during concreting, and the full shoring shall at least stays through the end of curing period, then reduce the shoring only when the concrete has achieved specified strength. In this case, the contractor are in a hurry to complete the project by removing the form works too soon, while the concrete is very young with very little strength. The concreting activity near the edge produces an eccentric loading effect, causing sway mechanism that pushed the structure to fail.
It's not clear whether there ever would have been shear walls to be built, though. Seems to me that if there were shear walls in the design, the engineer should have either designed them to be present during the floor pours, or had required shoring, but it seems dubious to me that sufficient shoring could have been installed that wouldn't have interfered with the pour, vs. building the shear walls in the first place.
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Yeah, I can't imagine a situation where you would pour the columns but not the shear walls. Generally speaking for cast-in-place type construction you do a vertical pour (columns and walls) then a horizontal pour (slabs and/or beams). So I don't see why the shear walls would have been left out.
If, however, it was supposed to be braced with steel bracing, then sure I can see why that would interfere with construction. But that means then you have to provide an alternate temporary bracing until all lateral load resisting elements are installed.
I don't believe there was any formwork on either level at any time. Looks like the floor system was intended to carry a concrete topping, so no formwork was necessary. Looks like a stability failure; the columns behaved as though they were hinged at the second level.
It is a possibility, but depending on the age of the concrete, I've difficulty to accept "no formwork" is necessary. See how many workers gathered in a small area of the slab, a highly unbalanced live load pattern.
The floor system is the 'formwork' here. Cannot tell what type of floor it is though. But it isn't uncommon in precast (double tees, flat slabs) and composite steel deck floors to not require any falsework or propping.
