AASHTO Crack Control Equations

[OSUCivlEng]

Does anyone know why AASHTO limits the reinforcing steel stress to 36 ksi (for 60 ksi steel) in the crack control equations of Section 5.7.3.4? I thought we had moved past allowable stress design.

 

[dik]

It might be related to the strain produced with that level of steel stress.

Dik

 

[rapt]

Probably because they want at least some form of crack control. You will not get much with service stresses higher than this.

We have moved to Limit State design, and the controlling limit state on the area of steel needed is normally Strength, but the controlling limit state for Serviceability (Crack Control) will often limit the bar spacing and size and it is dependant on the steel stress at service.

 

[bridgebuster]

Perhaps they should have written phiF_y rather than 0.6F_Y and assigned a value of .6 to phi.

 

[IDS]

Perhaps they should have written phiFy rather than 0.6FY and assigned a value of .6 to phi.

Why would they do that?

Stress limits for crack control have nothing to do with the yield stress of the steel; that is controlled with a separate calculation which will ensure that the stress under working loads is always less than the yield stress.

Crack spacing and width is affected by concrete tensile strength, bar diameter and spacing, cover, and creep and shrinkage, amongst other things. I don't know how AASHTO chose their limit, but if you are going to have a single limiting stress it's probably a reasonable number, if combined with maximum bar spacing and diameter limits.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[dik]

IDS:
"Stress limits for crack control have nothing to do with the yield stress of the steel; that is controlled with a separate calculation which will ensure that the stress under working loads is always less than the yield stress.
"
Agreed, but if you limit the stress to a given value, then the strain to achieve that is also restricted. For a given moment/shear, if you used 36Ksi steel in lieu of 60Ksi steel, you will have a much greater area of steel; this would reduce the value of the strain.

Dik

 

[rapt]

dik,

This limit is being applied at service isn't it? It is not going to have much effect except for members with high ratio of DL to LL.

IDS
And to benefit crack control, it needs to be an overall limit, not a percentage of yield. If someone uses 80ksior higher steel, it is no use limiting the service stress to .6 * 80. Crack control would be awful at that level of stress. You still need to limit it to 36ksi, or lower depending on the level of crack control you want.

 

[dik]

rapt: I assume it is.

Dik

 

[IDS]

rapt - Yes, that was my point! Perhaps not very clearly stated.

dik - I agree with your original comment that the intention of the stress limit is to limit the strain. For low strength reinforcement it would be unnecessary to do a separate check, but for reinforcement higher than 60 ksi the same 36 ksi limit would apply, and would often/always control.

General comment - the Eurocode and Australian codes have Serviceability Limit State stress limits based on bar spacing or diameter (whichever gives the higher stress), which allow very high stresses for small bars or close spacing. Based on experience, I apply an upper limit of 250 MPa regardless of bar size or spacing, which just happens to be very close to 36 ksi. That limit doesn't often control the design, but it does sometimes.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

I think rapt and IDS have explained it very well. I would just like to add that serviceability crack control always just depends on the amount of reinforcement, nothing to do with the grade of steel. I don't know why some codes tend to include grade of steel in requirements like the ones for minimum "T&S" reinforcement. As all steel pretty much has the same modulus of elasticity, it all stretches the same when subjected to the same stress.