If it's flat plate construction, that means a concrete plate with no beams. You have to proportion the slab thickness and column size so that the slab doesn't punch through the column. Picture a wet paper towel oriented horizontal and you push it down on a finger that's oriented like a column. It punches through. If your finger is bigger, you have to push harder for it to punch through. Now take a hole punch and put a hole right by where you're going to poke the paper towel. Now try to push the paper towel down on your finger. Does it punch through easier with the hole really close to the column/your finger? If they added penetrations for HVAC or piping, etc, and put it close to a column, then that COULD cause what we witnessed.
It could also be that they used something called shear rails to increase that punching shear capacity, but they were corroded because of proximity to the beach. The openings were checked, everythign is fine, then you get a failure because you didn't have the shear capacity you thought you had.
It could also be that equipment was being stored on the roof for whatever work they're doing and they stored it where they shouldn't have. I've had a job where they stacked material 4'-5' high in one section of the roof and then bad things happened. Something like that could have happened. Those type of things are still punching shear failures, it just means they overloaded the roof capacity and not the result of cutting holes anywhere.
I should add, cutting holes in floors/roofs after the fact is pretty common. If that was a PT slab, then that's different. You try to avoid cutting a PT slab whenever possible.
abc3340.com
