Elevated Slab - Concrete Strength?
Elevated Slab - Concrete Strength?
(OP)
Prior to setting interior load bearing walls on an elevated slab constructed with composite steel floor joists, what is an accepted minimum compressive strength for the concrete?
TIA
TIA






RE: Elevated Slab - Concrete Strength?
However, for your case with load-bearing walls, you, as the engineer, should check on the adequacy of the whole system (slab and beams) to receive load-bearing wall loads at various levels of f'c.
So the answer to your question is not a general one, but is specific to your project and the loads applied. You are the only one who knows this so you are tasked with checking the flexural and shear capacity of all affected members and deciding on what f'c level is appropriate.
I would guess that you would want at least 90% of the f'c (if not all 100%) in this case.
RE: Elevated Slab - Concrete Strength?
Because the slab supports load bearing walls, I would DEFINITELY recommend performing a structural analysis if loads are going to be applied before the 28 day strength is achieved. Also, since the joists are composite you should check the design for whether it is based on a shored or unshored condition. Even if your calculations indicate otherwise, I would still specify that 75% of the strength be achieved before loading.
Good luck.
RE: Elevated Slab - Concrete Strength?
As minimum you should have 70-75% of f'c before any work is being preformed on the slab. But you need to check the entire system to determine what you really need.
You might find that at the reduced concrete strength, the deflection of the steel joists might exceed that maximum allowed by MSJC of L/600 or 0.3".
RE: Elevated Slab - Concrete Strength?
At this point I could use some advice on defending a 75% strength requirement. Any codes or publications which specify this minimum number?
A joist company (supplier of the floor system for my project) has indicated that construction may continue the day after the concrete has been poured. They state that a 1000 psi concrete strength is sufficient for construction loading with a recommendation to exercise care when working (i.e. storage of materials, bundled mesh, etc.) Is this practical, responsible and/or safe?
Your thoughts are appreciated.
RE: Elevated Slab - Concrete Strength?
Take a look at the following thread for a similar discussion: Thread507-90025
RE: Elevated Slab - Concrete Strength?
I took a look at the link and the "rule of thumb" is evident in everyone's posts on this forum.
The problem I seem to have is that the joist supplier is also the slab designer. The overall system is a composite system. Many of their design concepts, including construction loading, is based on testing. Because this slab is a proprietary system, I cannot really run numbers on their system and tell them it does, or does not, work.
The criteria specified on their slab system is to strip the forms when concrete reaches 500 psi, and construction may continue at 1000 psi.
My issue is that the entire engineering world feels differently.
RE: Elevated Slab - Concrete Strength?
In this case, the weight of the joist itself, and the wet concrete, can be successfully carried by the joist itself, with no composite action required. Once the concrete has hardened, then the joist/concrete begin to act compositely and the system is much stiffer. The ultimate strength of the system is based on a full f'c and this limit state is based on a fully loaded floor (100% live load, with factors).
During construction you do not really approach this load level and the floor system engineers may be correctly stating that the dead load of the floor, and subsequent "construction live load" of perhaps 10 to 20 psf will not hurt the concrete as the stresses are low. So the 1000 psi concrete limit may actually work just fine.
The other consideration is the ability of the concrete slab to span between joists - this also should be looked at in addition to the joist/slab beam system.
RE: Elevated Slab - Concrete Strength?
RE: Elevated Slab - Concrete Strength?