Quote:
Originally Posted by Philly Fan
Well, the concrete slab on sub-grade is generally NOT a structural part of the foundation in many buildings, and isn't carrying the load of the structure and floors above (with some exceptions such as mat foundations). As long as the rebar in the floor above is tied into the rebar in the foundation walls and columns that ARE carrying the load, I don't think that the scaffolding holding the forms is really carrying enough of the load during the initial curing of the concrete above it, for a slab below to be required. Again, not my field, but I'm pretty sure I've seen sub-grade slabs poured in other commercial buildings AFTER the floor and structure above has been poured.
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There is a method that was used in San Fran on it's new tallest building---the name leaves me right now---that after all the piers were dug and poured, as in very deep, vertical construction was started and shortly thereafter digging down around the piers was also started. I think that was done in an attempt to save time.
But that's not the case here. The above grade part of the building isn't that tall and theres no need for a forest of columns and posts and piers. I'm surprised that apparently from what is being said the floor slabs aren't being poured on a steel deck as most commercial buildings are built. Residential is often built from a poured in place, all reinforced concrete construction, as the tower part of this project is being built. While commercial buildings, even fairly small ones are steel frames, with concrete decks/floors. This type of construction doesn't need to have the concrete floor supported in any additional method other then the regular steel frame, as its this steel frame that transfers the load, the weight to the vertical columns, while the concrete floor supplies very, very little actual structural load . While the poured in place all reinforced concrete buildings have a dense grit of temporary posts holding up the horizontal floor forms which slowly come down as the slab cures and hardens. The load is still transferred to the vertical columns through the combined structure of the concrete and the rebar working together. But obliviously the concrete is poured in a mostly liquid state and has to go through a chemical change as it cures and hardens. I don't know the actual schedule but in a week half the posts come down and are hoisted up to be reused, then in another week half of what's left comes down and so on. What your left with is 6" to 10" thick concrete slab with steel reinforcing bars buried in the slab, one can see concrete both on the top and bottom of this slab/floor. The W hotel site was a good example of this type of construction and that thread has many photos.
If all the parking is below grade I wonder if that means all the floor slabs are slopped to form ramps and if this would more easily be done after the above grade construction creates a roof of sorts over these ramps. But again the W site had sub grade parking and the whole hole was dug, the sides and matt was poured, then the work started going vertical, and that included ramps.
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