Why is this structurally sound?

Question

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?

Answer 1

What you see is temporary support, only till the time the floors attain their design strength. Then all these will be removed.

The rate of progress of the project must be faster than the time required for the concrete to be strong enough.

As far as using rebars, just looking at the spans and proportion of cantilevered area's it seems they are using some of the most advanced and strongest technologies. I read in some engineering magazines the are using welded/bolted rebars as opposed to overlapping, extensively.

The thin slabs indicate they are using pre and post-tensioned high tensile strength strands or pre-tensioned cables, this as well as using the very high strength concrete (in the range of 4-8 k psi).

Combination of these high strength reinforcing and thin lightweight slab and shear walls lead to a leaner, more ductile structure, better prepared to withstand potential earthquakes.

Answer 2

Why is this structurally sound?

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, ...

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?

Generally it's approximately one floor per week, setup, laying rebar, in-floor conduit and pipes, then the pour. The supports stay in while it's hardening. As soon as it's hard they get the supports out and return them for use at the next site - everything costs money.

That you saw the same thing at a few places and nothing collapsed says: "It works".

Since you didn't say where that is (exact address) I can't Google more photos so I had to supply my own pick. You need side, inside and downview to get a glimpse of the technique.

These photos are from this webpage: "HRG Projects", for the apartment titled: "Cielo (2007) - 1205 West Hastings Street, Vancouver, BC"

Supports pulled from lower floors.

Supports left on most recently poured floors.

Inside view - look at ceiling above worker's head - notice the marks from the supports.

Finished - No supports - Still standing.