Interaction Diagram Confusion (RC Section)

[nglclrk]

I'm a British engineer who has mostly used British Standards.
This is my first time using ACI 318 to create an interaction diagram. I'm having a lot of trouble and I hope some of you can set me straight. Any help would be much appreciated. I have a lot of questions but I'll start off with the one that's bugging me the most.

My main problem is knowing which steel strain to use. It seems that when the section is tension controlled (c/dt=0.375 (and less)) I should use steel yield strain = 0.005 (extreme tensile fibre) and when (c/dt=0.600 (and greater)) I should use steel yield strain = 0.002 (extreme tensile fibre). And in between, I should interpolate between 0.005 and 0.002. Should I be using these values of strain in order to find the force in the rebar? I realise that these steel strain values have got something to do with (phi) and finding the design interaction envelope but if we could ignore that for the moment until I get this figured out.

The reason I'm confused is because if I create the envelope assuming the above paragraph to be correct, my balanced condition doesn't seem to be at the value of maximum moment capacity.

Thanks in advance

 

[Lion06]

Your max moment should be at the balanced point (before phi is applied). Regardless of the strain in the rebar, the stress in the rebar can never be greater than 60ksi (assumed yielding is max for design purposes). If you are multiplying a strain of 0.005 times E, you are getting a much higher stress (and consequently force) in the rebar.

 

[frv]

StrEIT is correct-

The strain in the extreme fiber is what determines your phi factor; stress, however cannot be more than 65ksi.

 

[nglclrk]

StrEIT and frv,
Thanks to both of you guys for your replies. It's great to have people who are willing to share their knowledge. That makes sense and I was hoping that it would be the case because now it's similar to the british standards approach.

Thanks again

 

[frv]

I just noticed a typo.. that's supposed to say 60 ksi