As Required / As Provided

[bpiermat]

Hi,

Question: What Phi is required for calculating As Required?
If the bar is in pure tension, and is being pulled out out of a wall, should the phi be 0.75 per ACI Chapter 17. Or can I use 0.9 per AASHTO. The design is per AASHTO.

Thanks,

Ben

 

[Agent666]

Are you talking about the reduction to development lengths? If so there is no phi factor included/required for calculation of development lengths.

Edit - misunderstood the question. I'd use whatever factor is required for your governing code for tension or flexure.

 

[bpiermat]

Yes development lengths, but do you have to use the phi associated for Anchors (i.e. Appendix D) or just 0.9 for tension. Keep in mind, I am using AASHTO 8th Ed.

 

[Agent666]

Think of it this way for a minute, forget about the phi factors. One alternative way of stating the ratio is simply the demand/capacity, i.e. what reinforcement would you require to meet the demand, vs what reinforcement you provide to give the capacity. You can compare kN vs kN, kNm vs kNm, or mm^2 vs mm^2, the ratio is the same.

Therefore in a way the selection of phi is wrapped up in the type of actions and design check your member is undergoing (a phi is in the capacity equations for a given check, axial/moment/shear, etc). Therefore based on the type of action, the phi is accounted for, so use the appropriate phi based on this.

I'm unfamiliar with AASHTO, my local code simply has the same phi for bending, axial tension and compression, so its kind of something I don't have to worry about. Thinking in terms of demnnd vs capacity would in my mind help to explain what phi to select if you have different values to use for bending with compression, bending with tension, etc.

 

[Agent666]

For development of reinforcement, I wouldn't be using the ACI318 appendix D reduction factors, use what is required for member design with reinforcement.

 

[bpiermat]

Thank you for your insight. I tend to agree with you. But in the this application, the dowel bar is anchoring the wall to another (much larger) wall. Across a joint, the joint will have filler, so there will no aggregate interlock. At first glance the failure mode would appear to be like those shown in Appendix D. However, because reinforcing crosses the failure plane on both sides of the dowel, then Appendix D may not be applicable. In which case the Phi would revert back to 0.9 (at least this is how ACI would deal with this) This is my thoughts so far. (But the question is really how would AASHTO deal with this)

Thanks again

 

[BridgeSmith]

AASHTO doesn't have a phi factor applied to the reduction for As required / As provided.