punching shear capacity of piled slabs

huangyhg

punching shear capacity of piled slabs
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.  

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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.
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.
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.  
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.
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.
yes but we layer the steel to continue over the pile head, i.e. increased cover to the bottom.
i always use tension reinforcement only.
i'll check the ciria flat slabs design guide when i get to work tomorow.
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 re  
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.
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.