Quote:
Originally Posted by Nowhereman1280
Just look at the building again for a second and ask that question once more. Do you really think those floorplates are just holding themselves up?
Yes those are load bearing arches and the striation patterns do not mean they aren't. In fact, all concrete in all buildings has that pattern because it is nearly impossible to pour concrete on an angle without horizontal striation patterns since it would just flow out of the forms if they were on an angle. I believe the concrete on Prentice is only half the story structurally. I believe there is a large curving I-Beam inside the bottoms of those arches just like the bottoms of the balconies at Marina City that ties the whole thing together. I'm not certain though and this could easily be verified as there are construction pictures around the internet somewhere.
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You yourself doubted that concrete cantilevered arches could hold up the building, because you assumed there were I-beams, and then you just discovered there aren't.
Those cantilevered arches are at such shallow angles -- and have rarely been repeated in the half-century since -- that it's reasonable to think that (and ask, not assume, whether) they have a limited contribution to supporting 7+ occupied floors above them, so the floorplates' own cantilevering (rigidity) would seem it might play a significant role.
Also, the striations point you're not understanding is that the lateral forces of the arch (with the 7+ story building above it) would have to be borne by rebar (or whatever is being used to fuse together the horizontal layers of the concrete). Normally concrete's best structural use is to take a compression force (or whatever the proper engineering term is), and in this case rather than using concrete to take on the horizontal forces of the arch, it's something else, such as rebar. I have barely any idea what rebar or concretework technology was like a half-century ago compared to today. So, this is why it's reasonable to ask (not assume, just ask) about the striations.
What might be helpful is say a comment from ardecila about this - it's a question about trying to connect together 2 distinct structural members, in the same vein as his comments above about the problems of bonding I-beams with concrete.