Minimum areas of steel for large foundation slabs
Minimum areas of steel for large foundation slabs
(OP)
Hi all,
I was wondering if you could advise me. I have designed a reinforced concrete foundation slab (with greatly appreciated help from fellow eng-tippers) and i'm nearly there, however i'm pondering over the minimum areas of steel required by Eurocodes. This is given at around 0.18% of the cross sectional area as i'm using C35/45 concrete which is rather large compared to my actual requirements for flexural resistance. I have done some research and found 2 claims that may mean the minimum amount is not required.
Firstly that slabs on ground are not as sensitive to temperature changes as suspended slabs and therefore need much less reinforcement.
Secondly that the redistributing soil pressure beneath would guarantee ductile failure rather than crushing.
This all seems legitimate to me however I am a bit dubious about "breaking the rules" as it were without fully understanding why, or without finding a clause in the Eurocode stating that this is permissible. Is it just generally waived for massive foundation slabs? (This is an octagonal wind turbine slab by the way)
I have already checked serviceability requirements and all my steel stresses/max bar diameters have been correlated to ensure cracks widths are minimised. The only problem I am left with is shrinkage and I am wondering if problems would arise with thise as a result of not using the minimum area of steel.
I was wondering if you could advise me. I have designed a reinforced concrete foundation slab (with greatly appreciated help from fellow eng-tippers) and i'm nearly there, however i'm pondering over the minimum areas of steel required by Eurocodes. This is given at around 0.18% of the cross sectional area as i'm using C35/45 concrete which is rather large compared to my actual requirements for flexural resistance. I have done some research and found 2 claims that may mean the minimum amount is not required.
Firstly that slabs on ground are not as sensitive to temperature changes as suspended slabs and therefore need much less reinforcement.
Secondly that the redistributing soil pressure beneath would guarantee ductile failure rather than crushing.
This all seems legitimate to me however I am a bit dubious about "breaking the rules" as it were without fully understanding why, or without finding a clause in the Eurocode stating that this is permissible. Is it just generally waived for massive foundation slabs? (This is an octagonal wind turbine slab by the way)
I have already checked serviceability requirements and all my steel stresses/max bar diameters have been correlated to ensure cracks widths are minimised. The only problem I am left with is shrinkage and I am wondering if problems would arise with thise as a result of not using the minimum area of steel.





RE: Minimum areas of steel for large foundation slabs
RE: Minimum areas of steel for large foundation slabs
RE: Minimum areas of steel for large foundation slabs
RE: Minimum areas of steel for large foundation slabs
I'm trying to create an economical design so don't want to put in excess steel where there's no need for it and as such I don't want to have to meet minimum steel requirements if there is no practical requirement. I've found a few bits and pieces here and there stating that minimum reinforcement areas are unrealistic for massive foundation slabs since they are lying on soil and do not really require extra reinforcement.
RE: Minimum areas of steel for large foundation slabs
Codes may allow design of something as unreinforced, and you can put a little reinforcement in just for the fun of it, but you say it is designed as reinforced. To be considered reinforced, you have to meet all of the requirements, including minimum reinforcement ratios. I don't know your codes specifically, but ACI is fairly clear about this. We heavily reinforce wind turbine slabs here, because they do get large loads from various directions, and without proper reinforcement for continuity the foundations could fatigue and fall apart quickly. (Also, with the expense involved in this kind of project and the cost of failure, I'm not sure cutting corners on reinforcement is a good way to save money.)
RE: Minimum areas of steel for large foundation slabs
As this is open to a judgement call in my company as all calls have a check by another engineer I would ask say two of the potential checkers if they agree and discuss with them. If they do then I know there will not be a disagreement later.
I disagree with tx you always have to have min steel. In a suspended beam where you do not have any redundant load paths yes. In a foundation slab no. Basically by putting 25 percent more than the uls loads you are providing an extra 25 redundancy over a ULS load that should never be realised anyway.
RE: Minimum areas of steel for large foundation slabs
RE: Minimum areas of steel for large foundation slabs
Thanks very much for your help everybody.
RE: Minimum areas of steel for large foundation slabs
RE: Minimum areas of steel for large foundation slabs