huangyhg

aci minimum area of steel
i have 3 feet deep pile cap. due to lateral loads, i am placing rebars e.w. t&b. the ast from analysis is less than minimum ast = .0018 x b x h as per aci 318-05, section 7.12, 10.5.4.
i am placing rebars each way both at top & bottom. now i need this minimum ast each way at top and bottom of pile cap.
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you need the minimum in each direction, but not at the top and bottom. you can combine your top and bottom (in each direction) to meet the minimum.
can you please refer where in code or commentary it is clearly specified
see 7.12.1. if you used as,min for top and bottom you would get twice the required area for t & s.
just as a side note. i would be sure that you are increasing your actual required as (from analysis) by 1/3 before using using the minimum for t & s. if your required as is less than 200/fy, you can get out of the 200/fy min by increasing the required by 1/3. if that is still less than t&s steel, you can use the t&s steel, but check the 1/3 increase first. see 10.5.3 in aci for the 1/3 increase provision. all my references are to aci 318-05.
nashe, you do need the minimum 0.0018bh both top and bottom. note that 10.5.4 requires the minimum area of tensile reinforcement to be the same as that required by 7.12. so if you split the 0.0018bh between the top and bottom, half of it will be in compression and will not satisfy the tensile reinforcement requirement. this is the interpretation that every publication i have seen (pca notes, crsi etc.) uses.
it can be combined. but don't do nothing crazy like space bars at like 18" c.c., lol.
nashe
i agree with you, aci is not clear at all on this point. i believe structueral eit is correct, based on the following;
aci 350 for environmental structures has a similar provision, i.e., 10.5.4 refers you to article 7.12. article 7.12 has minimums for t/s like aci318, but based on length between wall joints. pca publishes a design guide based on aci 350. example 1 for a single tank bases the minimum steel on half the wall thickness.
i think the confusion arises because most design examples for pile caps do not have two laysers of reinforcement, so all the t/s reinforcement must go to the single layer.
structuraleit is correct.....and i think the actual guidlines are also specified in the crsi manual as well.
if you use half of 0.0018bh in the tension zone for slabs and footings, you may up with an unsafe design!
run the numbers to prove it to yourself. 0.0018bh in one layer will provide a strength that is fairly close to the cracking strength of the plain concrete section. but using half of this amount of reinforcing will provide a strength that is much less than the plain concrete. so after the concrete cracks, it can collapse suddenly.
taro is correct on this.
there is a minimum reionforcement requirement for strength and this is required at the tension face. it is based on the requirement that the ultimate strength of the
i thought that tension face minimum requirement was the 200/fy*b*d minimum for flexure. if memory serves me correctly, .0018*b*h is something completely different (temp and shrinkage). i'll check tommorow.

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