Minimum areas of steel for large foundation slabs

[CopperGalaxy]

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.

 

[ukengineer58]

the min area is based on effective depth not cross sectional area. You can also limit area to 1.25 x ULS requirements for 'secondary' elements. AS the foundation would only ever fial in a 'ductile' manner i.e. there will be warning I would and do go to this 1.25 x required. If you are subject to third party checking this may be a point of contention however.

 

[JedClampett]

I'm not a Eurocode guy, but if this is a wind turbine slab, isn't it subject to bending stresses? And wouldn't that throw you into the minimum bending steel requirement?

 

[CopperGalaxy]

Sorry UK engineer you're quite right I meant 0.0018bd rather than cross sectional area. Thank you very much for taking the time to help me out. I've found the clause you're referring to in EC2 section 9.2.1.1 (1) which states 1.2 ULS is accepted for secondary members where some risk of brittle failure is present.

 

[CopperGalaxy]

Hi Jed yes you're quite right. I've worked out the area of steel required to provide flexural resistance however compared to the calculated minimum steel requirement this does not match it and would take a considerable increase in steel to meet.

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.

 

[TXStructural]

The reasoning behind minimum steel in flexural members is to avoid sudden, catastrophic failure when you exceed the capacity of the concrete itself. Once you have exceeded the tensile capacity of the concrete, or another cracking mechanism has resulted in a critical crack, without meeting minimums, it is felt that the concrete may act as just a pile of big, tightly-fitted rocks.

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.)

 

[ukengineer58]

It comes down to an engineering v cost judgement. The code is quite clear (ish) you can limit to 1.2 times uls requirement. in some circumstances. It's not going to list the hundreds of potential scenarios. My feeling is a wind turbine is principle variable load would be wind, which is transient and so you can use the lower Value as, probably as you found the principle requirement is dead weight to keep it down. now if you have two founds maybe I would say just put in min steel to save the hassle, if you have 100 the cost benefit would be great and maybe you would go with the reduced figure.
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.

 

[ukengineer58]

Unless maybe it's piled then the thought process may be more conservative.

 

[CopperGalaxy]

Ukengineer - your assumption was correct this is a simple spread footing gravity based foundation so what you say rings true about the loadpaths. I truly don't believe the minimum reinforcement is required but I have plumped for it anyways to control cracking which can be a nasty business with wind turbine foundation due to the cyclic loading patterns, and i'll seek a second opinion as you suggest.

Thanks very much for your help everybody.

 

[connect2]

Minimum Steel is also a requirement to deal with plastic shrinkage and temperature requirements, as it cures and sets and over time.

 

[ukengineer58]

Connect. not the section referred to no.