Corbel -reinforcement Af to resist factored moment 1

[ENGR_2321]

So I am calculating corbel reinforcement and I've gone through all the required ACI reinforcement equations ( Avf, An ) and am about to calculate As which is the primary reinforcement. In order to compute As, I need Af which is another reinforcement requirement established to resist factored moment per section 11.8.3.3 ACI-11. this moment is equal to Vu*av + Nuc*(h-d) according to this section. Since the ACI does not have a set formula for Af I had to resort to another book - The PCI Handbook to see if they had one and thank god they did.

They say Af = [ Vu*av + Nuc*(h-d)] ÷ [ Φ fy d ]

BUT I was using a design book as well while I was researching this topic on corbels and they use this equation :

Af = [ Vu*av + Nuc*(h-d)] ÷ [ Φ fy 0.9 d ]


SO now I don't know which book has the right formula. The only reason I can think that they might put 0.9 is because of the flexure theory but even then am I really supposed to put the 0.9 there because according to section 11.8.3.1 the Φ=0.75 for all design calculations in 11.8 of ACI. or maybe I'm interpreting it wrong and it is implied that I'm supposed to use the 0.9 I am unsure. I would love some clarification if anyone knows more about this than me. Thanks!

 

[briancpotter]

Without knowing what design book you're reading, the 0.9 factor is likely an attempt to calculate the depth of the compression block (the same reason that the moment arm in for typical beam reinforcing is d - a/2). I'm not sure this is really necessary since your concrete will extend past the bottom of the corbel, but it shouldn't have much of an impact on your steel values.

If this is a precast concrete project, your notes/specs likely stipulate that the design will be per the PCI handbook.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[ENGR_2321]

Okay yes I was thinking that too. I'll go with the PCI handbook. Thank you