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# Plain (UnReinforced) Concrete Allowable Stress

## Plain (UnReinforced) Concrete Allowable Stress

(OP)
Hello,

Does anyone know where in the ACI (or any governing concrete code) gives the allowable tension strength of PLAIN (UN-reinforced) concrete? I've seen a small section in one of the codes a few years a go but I can't seem to remember where it was.

The main general rule of thumb is the tension strength is 10% of the compression strength F'c but I need to reference the ACI or other code for calculations.

Jeff

### RE: Plain (UnReinforced) Concrete Allowable Stress

ACI 318-02 has chapter 22, "Structural Plain Concrete" which would govern in areas that have adopted a code that uses/reference ACI 318.

### RE: Plain (UnReinforced) Concrete Allowable Stress

How about "Notes on ACI 318" by PCA (Portland Cement Association)?

Section 22.5 (page 32-2 of my 1999 version) states:  "The permissible stresses of ACI 318.1-89 (probably the one you want - comment added) were replaced with formulas for calculating nominal strengths for flexure, compression, shear, and bearing.  The nominal moment strength, Mn, is given by Mn=5*(sqrt(f'c))*S since it is controlled by flexural tension..."

### RE: Plain (UnReinforced) Concrete Allowable Stress

(OP)
Crossframe,

I do not have that publication.

You mentioned "The permissible stresses of ACI318.1-89" in your last posting. The word "permissible" suggests that this is an ALLOWABLE (working strength)formula. Am I correct? The Moment capacity formulas in the ACI code is all Ultimate strength which is why I am conerned with your statment. Could you verify this?

If its an ULTIMATE formula then I need to apply a factor of 0.9 to that formula and multiply a factor of 1.7 to my Live Load and a factor of 1.4 to my Dead load (1.7L+1.4D).

If its an ALLOWABLE formula then there are no factors that I need to apply to my Live and Dead Loads (D+L) and I could use that formula straight with out apply any addional factors to it.

Thanks,

Jeff

### RE: Plain (UnReinforced) Concrete Allowable Stress

I believe the phi factor for flexure for unreinforced concrete is 0.65.

### RE: Plain (UnReinforced) Concrete Allowable Stress

krus1972,

That publication should be available from PCA.  I think one is published for each edition of ACI 318 (Mine is for ACI 318-99).

Section 22 of ACI 318 seems to indicate that there was once an ACI publication for structural plain concrete, ACI 318.1-89.  It doesn't sound like it's been published since 1989, and it may have been incorporated into the body of ACI 318 - perhaps as Section 22?  Regardless, it sounds like it may be the one you want for allowable stresses.

I think the nominal moment formula I quoted is indeed for Load Factor (Ultimate Strength) Design.  That would be consistent with the remainder of the main body of ACI 318 (Since ASD is an alternate design method in the appendix).  In that case you would apply the load factors you mentioned.  It makes sense to me that ACI would "convert" something like this to Load Factor to be in-line with the rest of the manual, and "nominal" is typically used with LFD.  Be sure to check the commentary and the rest of the section for additional requirements/restrictions for plain concrete.

### RE: Plain (UnReinforced) Concrete Allowable Stress

One of my engineering professors said flatly, "The tensile strength of concrete is zero, don't use any other value because it always cracks sooner or later and then it really is zero."

DPA

### RE: Plain (UnReinforced) Concrete Allowable Stress

I also was warned that tension capacity of concrete should be neglected, not one but many times and by many instructors.

### RE: Plain (UnReinforced) Concrete Allowable Stress

Plain concrete should only be used where cracking will not lead to collapse eg. SOG, footings, bored piers.

The warning to ignore tension capacity must be heeded for reinforced concrete, where the concrete will crack before any significant tension is transferred to the steel.

### RE: Plain (UnReinforced) Concrete Allowable Stress

I've been down this road too, refer to ACI 318, I know in the '99 code it is at Section 10.2.5, see the commentary.

### RE: Plain (UnReinforced) Concrete Allowable Stress

I deal with concrete tensile strength for mass concrete dams all the time.  A good reference on this subject is by Jerome Raphael, PhD Berkeley.  But mass concrete, footings and slabs are different than flexural members.

I agree with the ZERO Tolerence crowd here. I can't imagine a case where it would be acceptable to design an unreinforced flexural member.

Concrete Cracks.  Period.  The energy required to propagate a crack is generally much less than it takes to initiated a crack.  So once it starts, it will eventually continue through the entire tensile region.  Then what have you got?

I don't know what you're designing, but you should check minimum temperature steel requirements as an absolute minimum, in my opinion.

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