Clarifier foundation reinforcing 1

[Struct1206]

I am in the process of designing my first clarifier (roughly 95' diam x 20' tall) and I have spent some time reading other threads on the subject but I have an issue when it comes to the design of the foundation/base slab. I am using the Circular Concrete Tanks publication from PCA but it doesn't do a very good job at describing how to deal with the hoop stress in the slab at the base of the wall. I have read comments from other users that state that the shear at the base of the walls should be resisted by circular reinforcing in a thickened portion of the slab beneath the wall. I understand this approach but I am having a hard time convincing myself that this is the most economical approach since it requires such a large amount of reinforcing. Is there any reason that you could not reinforce the slab radially and tie that reinforcing in with dowel bars that hook from the foundation up into the wall? I'm thinking something that would look similar to Fig.32 in the PCA publication. Is there any reason this wouldn't work?

 

[JAE]

The challenge with radial reinforcement is developing it past the wall shear point (i.e. having an extension of the base slab past the outer circumference of the wall).

Also - with radial bars you get a high congestion near the center of the slab. But otherwise it should work....probably more directly than the circumferential reinforcement.

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[JedClampett]

The whole economy of circular structures on slabs is based on the slab providing a support to the walls. Otherwise, you'd just have basic hoop stresses, with the maximum stress at the bottom.
I don't know why you're resisting just putting in the circumferential bars. For a 95' clarifier, 20 ft tall, what do you end up with, 20 #7's? So #7's at 6 inches for the projection and about 4'-0" in? That's not that much reinforcing! You don't need to place them in a one foot zone or any other arbitrary distance.
Do you want to explain to your boss that you tried something different if it doesn't work? And to save what, a few hundred bucks?

 

[hokie66]

Listen to Jed. He has done it.