punching shear capacity of piled slabs

[oneintheeye]

checking punching shear of piles, does anyone use the two layers of steel as the area of rebar? I have always used the one layer (in tension zone). seems to be a bit of disagreement in speaking to people on whether 2 layers can be used. i.e. have same rc in top and bottom of slab continuing over pile head.

 

[hokie66]

I assume you are using the ACI Code. I think the concrete codes in most countries do not allow an increase in punching shear stress based on slab reinforcing unless there is prestressing present. That used to be the case with the ACI also, but I am not current with that code.

 

[par060]

this is a good question I have often wondered if ...say shear friction could be included to resist shear....in both punching and beam shear....if its not allowed by ACI where is that called out?

It seems to me that the steel is there and would have to fail if the concrete were to fail.

 

[oneintheeye]

yes that would be my view on it and is certainly the view of other engineers I have spoke to. I am UK based so use British standards, i can't find anything concrete (excuse the pun). In beams in gives various conditions in using top or bottom but nothing combining then. I would think by the way it would work structurally that both layers would contribute? Almost wish I was still in college and destructive testing beams to find out.

 

[miecz]

Reading ACI 11.7.1 and the Commentary R11.7.1, shear friction is intended for applications where there is an interface between materials cast at different times, between concrete and steel, or for precast joints.

Research is always done on the type of applications for which a concept is intended. Extrapolating research to applications not expected is not appropriate, in my opinion.

If ACI intended the Shear Friction concept to be applied to beams and slabs, then I don't think they would have carefully worded the applications in 11.7.1. They would have simply said that it applies everywhere.

 

[csd72]

The pile will usually project 1' into the cap, past the bottom layer. It could therefore fail in a cone that does not include the bottom reinforcement.

Dont use the bottom reinforcement.

 

[oneintheeye]

yes but we layer the steel to continue over the pile head, i.e. increased cover to the bottom.

 

[apsix]

I always use tension reinforcement only.
I'll check the CIRIA flat slabs design guide when I get to work tomorow.

 

[vincentpa]

If you are looking for a design procedure, ACI references the CRSI Design Handbook for the precedure to use in designing pile caps. If you don't have a copy, you should buy one. It is very usefull even though the 2002 (latest edition) is based on the '99 ACI code. The latest ACI code still refers you to the CRSI book. I would scan you the procedure but I am afraid of copyright infringement. The procedure treats the pile cap as a deep beam. Shear is taken at the face of pile if I can remember correctly. You also have to check punching shear, beam shear, moment and shear at the corner pile.

I agree with miecz. You cannot extrapolate shear friction to this method. Shear friction assumes the surface is cracked or is going to be cracked and shear friction alone resists the forces. I believe I would not like to rely on shear friction to resist foundation loads. The design methodology for shear in concrete for punching shear assumes the concrete resists the shear alone. There is no help from reinforcement. You can reinforce for punching shear in slabs using some kind of shear head reinforcement as per the ACI 318-05 code. There is an example in PCA Notes on ACI 318. I don't see why you would want to use shear heads to reinforce for punching shear. Just make your cap thicker. We never use reinforcment for punching shear in footing of any kinds and I do commercial and industrial work.

 

[oneintheeye]

i am not looking at shear friction, however the amount of reinforcent has an effect on the ability of the section to resist shear, indeed i don't see how it could be suggested it wouldn't! In the British standard there is a tables/equation for determination and it is quite clear except whether you can combine layers. States somewhere average of tension reinforcement in both directions. I would agree that it is not the steel itself providing the strength but it is the effect of the area of steel present that enhances the ability of the concrete section to resist shear. On the shear at perimeter yes this is based on ability of concrete alone and values are limited to set figues in code. In short at perimeter must be < value specified (concrete only) while at punching shear perimeter shear stress < shear resisitance which is a function of section thickness and area of stee/concrete strength.

 

[vincentpa]

I meant to write the design methodology for pile cap design relies only on the concrete. For concrete slab design, we can use shear head reinforcement. I imagine averaging the layers of tension steel is a good enough approach if your code allows it, but I would still just make the cap thicker. It is much easier to construct. For supporting columns, we only use bottom steel for pile cap design and make the cap thick enough to resist punching shear. For supporting heavy machinery, we use bottom and top steel because the caps are usually massive but we still rely only on the thickness of the concrete to resist punching shear.

 

[oneintheeye]

yes, think there is obviously a differance in code between countries here, at least in how the theory is applied. If I was designing flat base I would check the shear stress at perimeter which is required to be < 5n/mm2 or .8*square root Fcu. Then at perimeters from pile head 1.5d again check shear stress this must < Vc which is the shear resistance calculated using a formula which incorporates steel area, not shear link steel but main steel. I would not provide links in a piled slab to resist the shear no, I agree I would simply make it deeper. What has never been clear to me or anyone by the look of it is whether I can use the main steel provided in both the top and bottom of the slab i.e. I may have say 20mm diameter bars at 200mm spacing in both top and bottom. Is my steel area 2 * 20mm's at 200c/c or only the one layer? I can't imagine the US adopts a completly differant approach to UK (or eurocode for that matter). Maybe its the way the equations etc. that are set up in your codes?

 

[vincentpa]

Our code calls for vertical elements and does not account for the confining effect of the horizontal elements. Our shear reinforment must be stirrups, steel shapes, or proprietary products.

 

[oneintheeye]

well to an extent ours is same. Shear reinforement is in form of stirrups. These are only supplied if other clauses above are not met. Seems a bit strange no account is made for horizontal main steel in your code and is in ours. Does this make your designs more conservative? Could be worth us carrying out exercise of min. depth for a load and comparing on here?????

 

[theonlynamenottaken]

I read a bunch of the old ACI papers on shear strength and they agree that the "dowel strength" only contributes about 10% to the overall shear strength. "Dowel strengh" is defined as the reinforcement's resistance against "tearing out" from the face past the edge of the failure cone. Based on this low contribution it was neglected in ACI 318

 

[vincentpa]

It's not worth carrying it out for us because we will never be permitted to use it. I am not familiar with your code and I don't have the experimental/logic backround to know your deign methodology and parameters. Sorry, I can't help. It seems to me that it might be a waste of the engineering budget to run these exercizes. I would just make the cap thicker.

 

[oneintheeye]

I was only suggesting a quick paper exercise you do it your way i'll do it mine, see if yours ends up thicker than mine. Just out of professional curiosity of different codes.

 

[vincentpa]

I really don't have the time. I'm sorry. I have too much going on.

 

[apsix]

Designing to BS8110; Vc for punching shear is based on Table 3.8, the same as for beam shear.
Clause 3.4.5.4 references Table 3.8 and clearly states that As is the area of longitudinal tension reinforcement which is adequately anchored.
Bottom 'compression' steel can only be used in a specific case where the beam has been designed as simply supported but is cast monolithically with the column.
I can't see justification anywhere for combining the top and bottom steel.
Shear cracking initially develops at or near the tension steel and it is the cross-section area of this steel which directly affects when cracking will start.

 

[Zambo]

It is correct that in accordance with BS8110 you should only use the tension steel to calculate for punching shear i.e the top reinforcement.

Logically the top and bottom rebar must fail for punching to occur this suggests that from an engineering point of view there is justification to use the full area of available steel. One way to look at is to check whether the bottom steel adjacent to the pile is required in your design for flexure and whether it can be defined as compression rebar.

There is a handbook to BS8110 which describes this in detail, but unfortunately I don't have all the details of the publication with me now.