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# Buttress design help

## Buttress design help

(OP)
Hey,
I am at present designing an underground water storage tank, howeer due to the tanks size i  have had to include buttress's at 12ft intervals (5m), i have a moment coming from the surcharge and the soil to the buttress. to check for punching etc i need to calculate the force acting through the buttress to the tank's base from this moment. How is that done? thanks

### RE: Buttress design help

If you designed the wall to take the fixed end moment from butress to butress, I dont understand how you could worry about punching shear?  If you have pca tank design book you can find shear at common points.  Is this a precast tank?  You dont want to use counterfort instead?

### RE: Buttress design help

(OP)
Well i suppose i could, in effect have it as a counterfort support as it is. Basically it to reduce to lengths of wall to reduce reinforcement. Lets say there is a force of 50kN acting on wall (1/3 x L) up from base due to soil. To assess the force acting through the counterfort should I fisrt get the moment which would be (50 x L/3), then as the counterfort is triangular in shape of width 1 metres. To get the force acting down through the support (counterfort) am I right to multiply the (50 x L/3) by 1/3 (the width of counterfort/3), this would result in an increase in the force acting downwards. Is this correct??? Thanks

### RE: Buttress design help

Oh I see, you are trying to analyze punching shear at the base of the tank.  This is out of my league.  I usually dont have my buttress connected to the slab because my designs are not to for holding water.  I do know that you dont have to worry about this if you had counterfort instead ron.  You are using the weight of soil to counteract the overturning moment instead of pusing on the base of the tank.  I guess if I were to design it the way you have it I would:

Use fixed,fixed, fixed base, free top design chart and find the average moment at the base and multiply by the lenght of span from butress to butress.  This will give you a good number for the moment at the counterfort.

If you do 50 x L/3, you are not assuming you have geometry of the tank helping you with the overturning but it is conservative. I hope I am making any sense.  I hope the MVPs can help you soon.

### RE: Buttress design help

(OP)
Thanks for your help, I have been in touch with one of my colleagues and the matter is sorted, it was only a small matter really as we didnt want to use shear reinforcement.
Thanks again.

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