Increasing maximum steel 3

[UcfSE]

With the 318-99, you could increase the maximum reinforcement ratio [ρ]_max by calculating a new balanced reinforcement ratio [ρ]_bal using compression steel.

With 318-02 ACI has dropped the limits on [ρ] and instead turned to checking steel strain. Can you still use compression steel to increase the calculated tensile strain and therefore permit more steel to be used in the section than in a corresponding singly-reinforced section? Strain can be increased artifically if the compression steel "cancels" some of the tensile steel, similar to what it does in the nominal moment strength calculation. I know there's plenty of math to work out, but is the concept reasonable?

 

[Taro]

Yes. The same principles apply. If the beam is over-reinforced, you can add compression reinforcing to offset the extra tension steel. There's nothing really "artificial" about it, it's just how the strain diagram works out.

 

[JAE]

Yes...what Taro said.

 

[UcfSE]

Thanks. I'm working on a Mathcad sheet for RC beams and came across this. Now I need to put it into writing.

 

[miecz]

I'm working on a mathcad sheet for this as well, using PCA Notes on ACI 318-02...It has a decent step-by-step procedure and an example (7.3) of a doubly reinforced beam.

 

[ash060]

Yes compression steel does allow you to get more capacity out of your section. It forces the N.A. of the beam towards the top which in turn increases the steel strain in the tension reinforcing. Make sure to keep your steel strains above 0.005 or the phi-factor will be reduced below 0.9