"d" in punching shear of two way PT slabs

[Emot]

Section 11.5.3. of ACI 318 specifies the valiable "d" in prestressed members to be a minimum 0.8*h. In punching checks, if you elect NOT to design per 11.12.2.2 (which includes of the tendon component in the shear capacity), and instead use the typical 4rt(f'c)bd, would it be appropriate to take "d" as the average depth to the mild reinforcement? It would seem to me that 11.5.3. may not have even had slabs in mind, but moreso beams where there may not be top steel at midspan to provide confinement.

 

[hokie66]

Which version of ACI318 are you using? In ACI318-08, the 0.8h provision is in 11.4.3, and there is no 11.12. I must be out of date...not surprising.

 

[slickdeals]

For slabs, the "d" would be = slab thickness - (Top cover + bar diameter)

 

[asixth]

I think top reinforcement should still be installed over the column for punching shear and the d of the mild steel to be used.

 

[Emot]

I'm using the 318-05 (I'll refer to the 08 from here on out). I ask because I know several engineers who take d as 0.8h per 11.4.3 (318-08). It does say "Where provisions of 11.4 (section on strength of members with shear reinforcement) are applied to prestressed members..." How this is interpreted could mean it should be applied when designing post-tensioned slabs designed per section 11.11.3 (318-11).

 

[slickdeals]

I think the reason for the 0.8h is because if you have a beam with parabolic tendons, there may be cases when the effective depth (PT + rebar) is actually much lower than 0.8h. I haven't ever used it on PT slabs. If you look at documentations and examples, they do what I said before.