Punching Shear Stress - Interior Column in PT Slab

[CURVEB]

When designing for punching shear using stud rails at an interior column in a PT flat plate slab (ACI318-11), the stud rail section limits the maximum shear stress to 8sqrt(f'c). However, the shear provisions for pre-stressed members limit the maximum shear stress to 70 psi based on available testing data. My question is if you are designing a post-tensioned slab with shear reinforcement do you limit your maximum shear stress to 70 psi, or allow it to go to 8sqrt(f'c) as long as you are providing the code-required stud rails? This would apply where you are using high-strenth concrete (over 5 ksi) in the slab.

The other way to look at is that the section for prestressed concrete indicates that if you do not meet the required provisions (or presumably limitations) you use the non-prestressed provisions which do not place an upper-bound limit on shear stress (except for the limitation earlier in the shear chapter of sqrt(f'c)<100 psi).

I would tend to look at the "reinforcement" section of the chapter as a stand-alone design procedure for the purposes of determining maximum stresses.

 

[CURVEB]

Bumping this question - any comments?

 

[EDB9]

I would take the conservative route and use the lower value of 70 psi. You have test data that confirms this is your strength? What basis do you have for increasing it? Remember when code sections provide conflicting guidances, then the lowest value should be chosen because using the higher value does not satisfy both sections of the code. You want to have some margin too so you have a better SF and wiggle room in case you need to make any changes.

 

[hokie66]

70 psi is one limit of sqrt(fc'), not of shear stress.

 

[EDB9]

This is a little out of date but it is a good design guide for punching shear checks. Look at page 35 in this design guide.

Vc is the shear strength of the concrete, not the shear stress.

Erica
Structural and Geotechnical Engineer (yes I know this isn’t a typical combo)

 

[hokie66]

Careful. If V is capitalized it represents force or capacity, if v is lower case, it is stress.

 

[MOJOJOHN]

You are stuck with about 5000 psi maximum for use in the concrete part of shear stress. 70 x 70=4900 psi

You may go up to 8 sqrt f'c with the addition of steel such as stud rails, steel sections or stirrup type reinforcing.

Though you may use concrete greater than 5000 psi for say transmission of vertical load through a slab, it won't help in shear.

In cases of very high shear in beams, we use steel plates or steel sections with studs and use the AISC rather than ACI even though it a primarily concrete structure. (Such as coupling beams between shear walls in a 60 story structure) This beats the 8 sqrt f'c restriction.

 

[CURVEB]

Thanks. Now that I've seen some responses I should clarify:
- The question is specific to a PT slab reinforced with headed shear studs per section 11.11.5 of the ACI 318.
- Code states that Vn (the maximum force that can be developed at the first critical section) is limited to 8sqrt(f'c)x b0 x d
- Some have interpreted this limitation to be further limited by section 11.11.2.2 which limits f'c to 70 psi for post-tensioned slabs. What that means is that even if you are using headed shear stud reinforcement the maximum shear FORCE you can resist at the first critical section is 0.75x(70 psi)x b0 x d.

My understanding is that when using headed shear stud reinforcement, at the first critical section at d/2 from the face of the column, section 11.11.5 applies and your maximum shear force is 0.75x8sqrt(f'c) x b0 x d. Beyond the outer layer of shear stud reinforcement, the maximum shear stress (unfortunately ACI mixes and matches force and stress in this section) is 0.75x2sqrt(f'c).

My point is that it would appear you have to apply the provisions for punching shear reinforced with headed studs independently from those that apply to PT slabs. For a PT slab, I could see the logic in limiting the out perimeter stress capacity (f'c<70 psi) to still conform to the PT slab punching provisions, but not at the first critical section around the column, where the slab is clearly a reinforced section. It would seem if this is the intent of the ACI they should provide a separate section for post-tensioned slabs reinforced with headed shear studs.

 

[KootK]

My take:

1) I believe the 8sqrt(f'c) business to be about avoiding a diagonal strut compression failure.

2) I suspect that the 70 psi limitation is little more than keeping things within the range of that which as been tested.

3) Because I feel that #1 and #2 are fundamentally different mechanisms capping various things, I don't feel that the 70 psi needs to be hard cap on the total capacity as 8sqrt(f'c) is.

4) Presented as you have, I agree, there is some room for interpretation here. That said, Prior to this thread, I never would have guessed ACI's intent to be anything other than:

a) Go get your Vc value from whatever section you deem most advantageous and/or appropriate, applying any limits within those provisions to only those provisions.

b) Take your Vc and plug it into the reinforced punching shear provisions applying any additional limits found there.

 

[CURVEB]

@KootK Right on - thanks.

Thanks all, esp. @EDB9 for the valuable feedback and references.

 

[MOJOJOHN]

I have designed 100's of Post tensioned slabs and have always considered the 5000 psi limit (70 psi sqr f') to be the maximum Vc before using steel for Vs part.

I believe commentary discusses the reason is testing, but I don't have my ACI here.

 

[CURVEB]

@MOJOJOHN: I think I would agree to limit the concrete strength to 70 psi for the concrete portion in prestressed slabs as you stated. The argument on the opposite side is to limit Vn by using 70 psi as the upper limit on f'c, which doesn't seem correct based on how I'm reading the code.

Coincidentally, there is a limitation on Vc in the headed stud reinforcement section for punching shear that says Vc can't exceed 3sqrt(f'c)b0d, which probably results in concrete capacities even lower than the provisions in the prestressed concrete section. I would think it is prudent to use the more conservative portion of the code for the concrete capacity.