Punching shear % steel

[nitin36537]

Dear All

We are designing a foundation having two RCC slab as per BS 8110.
Three layer of rebars provided. Refer attached PDF file

As per BS 8110 , for calculating punching shear capacity we have to consider % tensile reinforcement.

Critical section for punching shear is at a distance of 1.5 d from column face (section A-A). As per slab dimensions , only bottom slab having depth d2 is effective for resisting punching shear.

Steel Ast1 & Ast2 both are in tension zone.( top & bottom bars of bottom slab )
effective Depth = d2 - cover
% tensile steel = 100* ( Ast1 + Ast2) / ( B *effective Depth)

Considering Ast1 + Ast2 for calculating % tensile steel is correct approach ?

Thanks
Nitin Patel

 

[KootK]

I recommend using just Ast1. I disagree with you that both Ast1 and Ast2 will be in tension. Once you're outside of the plinth, as your critical section seems to be, I believe that d2 is the flexural depth of the member.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[nitin36537]

Hi

Thanks Kootk

On second thought , I too agree with your suggestion.

Few more doubts.
Why compression steel is not contributing to punching shear strength ? For my case , Ast1 & Ast2 are almost same area.
Why BS 8110 is considering critical section for punching at a distance of 1.5 * d ? Other code ACI & India considering at a distance 0.5 *d only.

Thanks
Nitin Patel

 

[rapt]

At section AA, Ast2 is not in tension!

BS8110 requires checks on punching at multiple sections until reinforcement is no longer needed.

One section would be at 1.5d2 from the face of the pedestal and its depth would be d2.

The one at AA would have a deeper depth. It would make life easier if the pedestal extended to 1.5d1!

BS8110 uses beam shear logic to determine the shear capacity and uses an angle of about 30 degrees, hence the 1.5d. The other codes you mentioned use a completely different logic and are independent of the area of tension reinforcement.

No code uses compression reinforcement in shear calculations. Though it will provide some dowel action and provide an extra degree of safety against collapse, it is ignored in punching shear strength calculations

 

[hardbutmild]

I'd disagree with the others, I think Ast2 should also be considered. Why? Well, because the crack forming from punching failure is shown by a red dotted line on your drawing. The important place is the one where the crack and the bar intersect. If the crack is open, dowel action contributes significantly. Reinforcement Ast1 is in the "compression ring" and it doesn't really contribute that much. Now, if the red dotted line crossed the vertical edge inside d1 (i hope you understand what i mean) then another compression ring would be formed at the top of d2 and in that case Ast2 would be used for the first check and Ast1 for the second check.

Britain uses check on 1,5d distance because it's based on eurocode. In eurocode 2d is used because the perimeter is like a rectangle with filet-ed edges. Since in britain you use rectangle that distance is smaller.
I think it differs from american standard because (I think it's like this, correct me if I'm wrong) they wanted to use "the real" perimeter, but because of that allowable stress is different than for beams, while eurocode decided to use the same exact allowable stresses no matter what caused them so they use different perimeter to get the same resistance as in experiments.

 

[KootK]

Why compression steel is not contributing to punching shear strength ? For my case , Ast1 & Ast2 are almost same area.

I am not nearly as familiar with eurocodes as I am with north american codes so, please, weigh what follows accordingly.

My understanding is that the primary benefit accrued by having more reinforcing is an indirect benefit as follows:

a) More reinforcing implies a slab designed to carry more moment at the connection.

b) A slab carrying more moment at the connection will have more compression in the compression zone of the joint when peak shear is being resisted.

c) Higher compression stress in the slab at the joint will improve aggregate interlock and other aspects of shear resistance.

d) a + b + c means that higher tension steel quantities can generally be expected to improve punching shear capacity.

I agree that the compression bars will effect shear capacity. However:

e) I don't believe that this as aspect of shear capacity was considered in the development of the punching shear provisions.

f) The compression dowel action won't really kick in until after conventional punching shear failure takes place (concrete diagonal tension cracking). So, while the compression dowels perform a nice function as a backup punching shear mechanism, they likely don't do much for the primary punching shear failure that we conventionally check.

g) Having rebar in the compression zone of the connection will actually reduce the concrete compression there because the rebar will absorb some of the compreesion. In that respect, the presence of compression dowels may actually reduce punching shear capacity.

Why BS 8110 is considering critical section for punching at a distance of 1.5 * d ? Other code ACI & India considering at a distance 0.5 *d only.

As hardbutmild suggested, I suspect that this is just a matter of choice. Punching shear capacity equations are really empirical equations indexed statistically to the capacity, especially when significant moment transfer is involved. So the choice of where to use d/2 vs 1.5d is pretty arbitrary other than, perhaps, trying to create something that speaks to the intuitions of those using the provisions.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[rapt]

BS8110 (the original, not BS-EN) has used 1.5 d since the 1960's. Eurocode did not come into existence until the 1990's.

While Ast1 and 2 may be included at the 1st perimeter (I would tend to leave out the Ast2 contribution if the stress in this steel at ultimate is well below yield), it is the second perimeter that will control the design, with the perimeter starting at the join of the pedestal and using the smaller d and smaller Ast.

I am not sure where "dowel action" comes into the BS8110 shear logic!

 

[hardbutmild]

My understanding is that the primary benefit accrued by having more reinforcing is an indirect benefit as follows:
a)... b)... c)... d)...

I think tension reinforcement influences shear capacity directly because of dowel action. So if you provide reinforcement twice as large as the required one (for bending) you'll have larger shear capacity. Also, I think tension reinforcement will control the crack width and that should influence aggregate interlock in tension part of the section (if the crack is present, but very narrow interlock occurs. If it's wide, there's no interlock. More reinforcement = narrower crack, right?).

f) The compression dowel action won't really kick in until after conventional punching shear failure takes place (concrete diagonal tension cracking). So, while the compression dowels perform a nice function as a backup punching shear mechanism, they likely don't do much for the primary punching shear failure that we conventionally check.

I agree and I just wanted to comment because it's usually overlooked. I think it's important to put some compression reinforcement exactly because it gives that "backup punching mechanism" as you called it. I called it collapse reinforcement because it avoids total collapse even if punching occurs. That gives structure robustness (I hope I used the right term).

I just wanted to add that there are some articles by prof. Muttoni (et al.) that discuss mechanisms of punching shear failure so maybe more details can be found in those articles. I just skimmed them but they seem really good.

 

[KootK]

More reinforcement = narrower crack, right?

Yup, I agree. And I don't doubt that, to some degree, additional tension reinforcement helps to improve capacity by simply doing a better job of holding stuff tightly together.

I called it collapse reinforcement because it avoids total collapse even if punching occurs.

The jargon for this in north america is "integrity steel". I favor less jargony descriptors when I know that I'm dealing with folks from other parts of the world.

just wanted to add that there are some articles by prof. Muttoni (et al.) that discuss mechanisms of punching shear failure so maybe more details can be found in those articles.

I believe that I have that if you're interested in discussing any part of it in detail.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235