While visiting Vancouver, Canada recently, I noticed there is a lot of construction for apartments and condos. Attached is a photo of one of many of those sites, and at these sites, I noticed something unusual to me: temporary columns on lower floors appeared to support floors above, and all floors are made of concrete. In my photo, three of the uppermost floors were supported by floors beneath them. This seems fairly consistent across several construction sites. So my question presumes that this practice is structurally sound, Canada is not a country that is devoid of competent architecture design. I just don't know why it is sound.
I understand that concrete has very little tensile strength, and so perhaps the concrete flooring used in these buildings are reinforced - presumably with rebar.
My question is, why only 3 floors? If the concrete is not reinforced, wouldn't all floors from the ground up need to support the floors above it? And if the concrete is reinforced, why is there a need at all to support the floors above?
It would seem to me that the 4th floor in the photo is bearing the brunt of all of the support above it, as it is not being supported by anything underneath it.
I'm further curious about what happens next: will each of the floors be given a permanent support? If so, why hasn't it been done already? If not, what will happen when those temporary supports are removed?