Plain Concrete Walls Allowable Flexural Stress 1

[cap4000]

The formula I have used for flexural stresses in plain concrete walls is 5 x Square Root f'c x Section Modulus. I understand this value has increased due to empirical results obtained from many homes built without any foundation wall stress cracks. Does anyone have the ACI new formula. Is it now 7.5 instead of 5??. Any info will be appreciated. Thanks.

 

[Lion06]

not only is that a nominal stress (not an allowable stress), but it gets a phi factor applied to it. Check chapter 22 of ACI 318. The nominal moment for plain concrete is [5*sqrt(f'c)]*. This nominal moment gets a phi factor applied to it. I guess the phi factor depends on your interpretation of a tensile failure, but I would use the 0.65.
The "allowable" stress that you mention is for cracking in prestressed concrete, and you can find that in the PCI manual. It is not meant to be used as an allowable stress for unreinforced concrete.
The idea for using that value in precast is that quite often the precast will be architecturally exposed (columns), or you want to check cracking in the section, but you still have the reinforcement in the prestressing.

 

[UcfSE]

The modulus of rupture for strength of plain concrete is 5[√]f[´]_c, implied in 22.5. It is 7.5[√]f[´]_c for deflection only, in 9.5.2.3. Prestressed concrete is checked a little differently, as noted above.

 

[cap4000]

UcfSE

I think the new ACI332-04 Code for plain concrete foundation walls allows a new 50% increase for flexure. I don't have that code and I am just looking for verification.
Thanks.

 

[UcfSE]

If you want to work with the ACI 332, then you should buy it. Problem solved. I didn't mention it but my post above references 318.

 

[cap4000]

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