one way shear check for a pile cap

[rfd23]

Hello,

I have a circular bin supported by 8 columns. Bin dia is 12m. we are using a octagonal pilecap with 56 piles. I am using 1.5m deep pilecap. otagone has 17m dia. To check the one way shear at d from the face of column, I am having 30 piles contribute to that section. shear capacity of 1.5m mat with 30Mpa concrete is far less with 17m width. Even I am reducing shear by column loads in that section. I cannot increase the depth otherwise my piles will be overstrsed.
I can not put more piles due 3d spacing requirement.

1. Should i check one way shear based on pile capacity or actual reactions from various load combinations.
2. Is it normal to provide shear ties in a pile cap with scattered column on the top.
any reference material related to large pilecap is greatly appreciated.
All useful thougths are welcome.
Thanks in advance.

 

[civilperson]

Use pile capacity for shear calculation, one pile may max out prior to sharing load by settling or deflecting. 3 x dia spacing is recommended, not required. Try lesser spacing, i.e. 2.5 x dia, or higher strength concrete or thicker pile cap. I was told never to use steel for additional shear strength in pile caps but was never told why this ban was needed.

 

[rfd23]

thanks for reply.
We can not use more than 30Mpa. if I use 2.5d, my piles capacity will be reduced by 25%, which i can not afford.
However to increase the depth, i need more piles which means greater footprint. How do i justify this against shear capacity. However I can increase shear capacity by using shear ties and lesser depth.

 

[hokie66]

Your pile cap does not sound like a flexural element. It is more like a deep beam, and I sugggest it is best designed by strut and tie methods.

 

[rfd23]

thanks.
Even I designed it as strut and tie method, i still need to check oneway shear as per code.According to this all piles outside the d face of column will contribute to shear. My question is why we do not recomend to use shear ties in footing. is it due to cost of labour or material.
I mean if we have option to use shear ties to overcome this problem. I discuss with many colleagues, but all of them in opinion of increase depth to overcome shear. but my problem is that my piles would be overstressed.

 

[hokie66]

Perhaps if you could explain the setout of the piles it would be easier to understand your problem. And the column loading and the pile capacity.

But there is nothing inherently wrong with using shear reinforcement in a big footing like this. Shear reinforcement is used all the time in mat foundation slabs supported on piles for high rise buildings.

 

[rfd23]

thanks for every one.