huangyhg

shear reinforcing pile cap
folks,
i am required to design a pile cap where i am limited in depth and controlled by one way shear at "d" beyond the face of the column. i am using aci prescribed value for vc = 3.5 sqrt (fc').
can anyone provide pointers to how shear reinforcing could be used to make up the difference needed to carry the shear through reinforcing?
i have looked in vain online for suggestions and i am sure someone here has done something similar.
add j-hooks between top and bottom reinforcing mat to use as shear reinforcing.
pile caps are use thickness and concrete strength to resist shear. most steel additions for shear strength are not used. (this is a war that i have fought with dots for 25 years).
civilperson has valid point here. do not take shear in pile cap lightly. in the case with thickness restricted by whatever reasons (really want to know why can't make it thicker), suggest to look into aci code for two way flat slab shear reinforcement around column penetrations.
the reason i can't make it deeper is because my boss does not want to . we hit water at 1'-0" below grade and he wants to have as less a depth as possible, so i was exploring the option of using shear reinforcing. any pointers for some research or design examples for shear reinforcing in pile caps?
how does your pile layout look like? single row, or multiple piles arrangement?
need more info, but what is commonly done in australia for wide 'band beams' is that they us the typical u shear stirrups and over lap them horizontally (e.g first u goes to first and fourth top bar, second u to second and fifth, or whatever works)
also have you considered flat slab punching shear reinforcement if you can get any deep enough?
we need more info to give you better advice.
it is a modified 16 pc, 18" dia. 250 ton piles. piles are spaced at 4'6" o.c., edge distance of 2'6".
it is picking up 2 columns centered on the pile cap.
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how about design embedded beam grids, and reinforced accordingly to take on shear as regular narrow beams, bridging the 12 center piles (in both directions). or you may try to use embedded structural shapes, watch out for concrete-steel shape interface though.
okay, good you posted that one up.
strength of the pile cap is not you major concern with this flexibility is!
the shallower you make it, the more load goes on the nearer piles and the less load goes on the farther piles. this always happens but it is small enough with the stiff pilecaps that it evens out with the soil. in your case you will make it much more flexible.
you have not said what the piles are bearing on, if it is rock then this is definately an issue, if soil it may be depending on the flexibility of the pile cap relative to the soil. any deflection more than about 1/4" will have significant effect on pile loads.
csd72 brought up the crucial concern - pile cap rigidity. make sure your pile cap can be assumed rigid, and linear distribution of loads apply. otherwise you are having hands full of problems.

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