Punching Shear Capacity of High Strength Concrete Slabs

[Luceid]

Hello all!

Looking for some thoughts on a bit of a code grey area if anyone has any prior experience. I'm working on a two-way PT flat slab with a fairly high concrete strength 60 MPa (8700 psi for the US folk) and I'm currently working through the punching shear calculations. Here are the code equations that are relevant to the design (Based on ACI 318-19, leaving out the extra column dimension equations for simplicity in comparison):

Table 22.6.5.2: For two-way members without shear reinforcement,
vc = 4*sqrt(f'c)
where sqrt(f'c) is limited to 100 psi (f'c = 10,000 psi).

Section 22.6.5.5: For prestressed, two-way members conforming to 22.6.5.4, vc shall be permitted to be:
vc = 3.5*sqrt(f'c) + 0.3*fpc + Vp/(bo*d)
where f'c is limited to 70 psi (f'c = 4900 psi).

The concrete strength being used is in between the sqrt(f'c) limits of 70 and 100 psi, which brings up our lovely code grey area.

The wording of the prestressed equation says "shall be permitted" which leads me to believe it is intended to be an additional equation to increase punching shear capacity due to precompression (makes some sense). However, since the prestressed equation limits the maximum concrete stress to a lower value, at low levels of precompression, it actually reduces the shear capacity of the slab, compared to the non-PT equation (!!!).

So I've got a couple of options at this point, and I'm not sure which makes the most sense:
1. Use the PT equation, limiting the concrete strength to 4900 psi
2. Use the Non-PT equation, limiting the concrete strength to 10000 psi
3. Use the Non-PT equation, limiting the concrete strength to 4900 psi

Any thoughts? Appreciate it!

 

[HTURKAK]

Clause 22.6.5.1

22.6.5.1 For nonprestressed two-way members, vc shall be calculated in accordance with 22.6.5.2.
For prestressed two-way members, vc shall be calculated in accordance with (a) or (b):

(a) 22.6.5.2

(b) 22.6.5.5 if conditions of 22.6.5.4 are satisfied .


So , you can ignore the PT effect and Use the Non-PT equation, limiting the concrete strength to 10000 psi










Not to know is bad;
not to wish to know is worse.

NIGERIAN PROVERB

 

[slickdeals]

Agreed. Use the equations for non-PT when the concrete strength is greater than 4900 psi. I believe that's what computer programs do as well.

 

[Luceid]

Look at me, closing my eyes as I read the code, that doesn't help much!

Appreciate it, thanks to both of you.

 

[hokie66]

Why would you use 60MPa concrete in a slab?

 

[slickdeals]

In my region, 8750 psi concrete is used commonly for PT slabs for two reasons. It gains strength early even during the cold winter months and it allows vertical elements strength to go up to 12,000 psi without needing to puddle concrete around the columns. The mix performs reliably and allows for an efficient slab design utilizing the higher concrete strength.

 

[hokie66]

Thanks, Slick. Makes sense.

 

[structSU10]

vertical elements strength to go up to 12,000 psi without needing to puddle concrete around the columns.

curious about this comment. Do you mean you would puddle to column mix to improve punching shear? I was doing some preliminary work on a 14 story building and noted I wanted higher column strengths. I imagine you carry the column through and use keyways / shear friction bars to maintain the higher strength? Or do you deal with that some other way?

 

[Tomfh]

When pouring the slab, you first pour higher strength concrete at the column zone, then you place the rest of the lower strength concrete elsewhere. It’s harder for the builder, and requires extra coordination.

 

[Luceid]

structSU10, ACI requires the column strength to be at most 1.4x the slab strength, so you either have to bump up the slab strength or puddle around the column as Tom and slick have indicated. This is a fairly early stage, so the slab concrete strength is definitely a variable we are working with as the columns are being designed, it is definitely on the higher end right now.