Maximum Allowed Steel Ratios In Slabs And Footings

[Waqas Haider]

Assalam o alaikum,

Can any one tell me what is maximum limit of positive and negative steel ratios in slabs and footings? ACI code is giving me minimum limit in terms of percentage of gross area i.e. 0.002.b.h or 0.0018.b.h. but it reports maximum allowed reinforcement in terms of net tensile strain in tension reinforcement. Kindly explain what value it means in terms of gross area for maximum allowed steel ratio to avoid over-stressing of slabs and footings? 

[zenith.international]

All  references are to ACI 318-05.

Minimum flexural reinforcement is not controlled by the provisions of ACI 10.5.1 and 10.5.2 as they do not apply to structural slabs and footings of uniform thickness, rather the minimum flexural reinforcement is determined by 10.5.4 as 0.0018bh. For a structural slab having top and bottom reinforcement both of which exihibit flexural tension, this minimum flexural reinforcement of 0.0018bh for each layer would control over the temperature and strinkage reinforcement of 0.0009bh for each layer (0.0018bh).

 

The questio is whether the provision of 10.5.3 applicable also?

The If not, As minimum would always be 0.0018bh for each layer.

If so, and in the situation where the required As is small enough, the minimum flexural reinforcement determined by 10.5.3 as 4/3 * As, reg could be less than that determined by 7.12 as 0.0009bh for each layer, thus As minimum being 0.0009bh for each layer.

Another way to write it (applies to slabs and footings having top and bottom steel):

If As > 0.0018bh, use As
If As < 0.0018bh < 4/3As, use 0.0018bh
If 0.0009bh < 4/3As < 0.0018bh, use 4/3As
If 4/3As < 0.0009bh, use 0.0009bh

[Waqas Haider]

Thank You so much For such a detailed Answer. Kindly also explain me about Maximum Allowed reinforcement in slabs/footings.

[mhdhamood]

Dear Eng. Waqas for your question see the attachment
Regards 
Mohammad

[Waqas Haider]

Thank you very much sir for detailed answer. The above attached document presents calculations for a beam but since minimum steel limit differs for slab and beams, i think maximum steel ratio limit must also differ for beam and slab.

[mhdhamood]

No dear, the maximum area of steel aims to limit the area of steel put in the section so as to control the strain in steel to be equal to or larger than 0.005.
This will not differ between slabs and beams.
Regards

[UmarMakhzumi]

A maximum reinforcement ratio for beams and slabs is not directly given in ACI 318-05. Instead, Section 10.3.5 requires that non-prestressed flexural members must be designed such that the net tensile strain in the extreme layer of longitudinal tension steel at nominal strength t is greater than or equal to 0.004. In essence, this requirement limits the amount of flexural reinforcement that can be provided at a section. Using a strain compatibility analysis for 4 ksi concrete and Grade 60 reinforcement, the maximum reinforcement ratio is 0.0206.

Thanks.

[WR1]

A maximum reinforcement ratio for beams and slabs is not directly given in ACI 318-05. Instead, Section 10.3.5 requires that non-prestressed flexural members must be designed such that the net tensile strain in the extreme layer of longitudinal tension steel at nominal strength t is greater than or equal to 0.004. In essence, this requirement limits the amount of flexural reinforcement that can be provided at a section. Using a strain compatibility analysis for 4 ksi concrete and Grade 60 reinforcement, the maximum reinforcement ratio is 0.0206.

Thanks.

 

yes, you have to calculate the net tensile strain in extreme layer. Approx it will be somewhere near to 0.02