Sewage Lift Station Minimum Wall Thickness

[Trillers]

Happy New Year All:

I am designing a sewage lift station with 28' deep walls and liquid level will be maxed out at the 22' depth level. This work is being done for a general contractor who received this task as a change order to their contract. In the drawings provided for the contractor to price the change, the wall thicknesses are shown as 10".

The utility engineers provided this drawing.

My question is: All our previous work of this type conformed with ACI 350-6 which states that the minimum thickness of any wall 10' or higher, in contact with liquid, shall be 12". The utility designer is saying that the 10" thickness is acceptable since both sides of the lift station wall (liquid side and soil side) will be thoroughly sealed and the concrete protected against the moisture.

They also state that this exception is in the code (I'm assuming ACI?) but cannot tell me where and I cannot locate it.

Has anyone out there encountered this issue?

Can anyone provide guidance?

Thanks!!

Trillers

 

[JedClampett]

Ah, the elusive ACI recommendation.
I'm not sure you're going to be able to use ACI 350 or 318 to make an ironclad case. They're more likely to hint at thicknesses than to require them. But trying to drop concrete 30 feet and vibrate it is damn near impossible. Fitting waterstops is very difficult. Even 12 inch walls are tough. It's common sense.
A lot of contractors (and owners) are really stupid about concrete thicknesses. They have to build basically the same forms and tie the same reinforcement for a 10 inch wall vs. a 12 inch wall. The only additional cost is the concrete out of the truck (about $110 a cu yd.). They'll eat up that money fixing one rock pocket, not to mention the four or five they're going to get.

 

[bimr]

The thickness will depend on the diameter. The rule of thumb is 1-Inch of thickness for each 1-foot of diameter.

http://www.concretepipe.org/wp-content/uploads/2014/09/DD20_CircularPrecastConcreteManholes.pdf

 

[JedClampett]

bimr, i don't think so. I've designed 120 foot diameter tanks and the wall thicknesses weren't 10 ft.
I'd say start with 12 inches and add an inch for every foot depth over that.

 

[bimr]

Believe the discussion has to do with a lift station, which would look more like a concrete manhole than a 120 foot diameter tank.

Regardless, the document presents the formula to calculate the thickness.

 

[hokie66]

Agree with Jed. Constructability is more important than saving a bit of concrete. The OP didn't say it was circular, but that shape makes sense structurally, if the ocntractor has the means to form it.

 

[Trillers]

BIMR you are correct. This is a lift station. We took a shot at designing this using PCA for circular tanks but that proved useless because the geometry of the lift station resulted in values for H-Squared /Dt being way off their ranges since the pit is 28'H x 7'-6" in Diameter.
So we cannot use the reduced PCA Tension force based on their tables.
We are now leaning towards designing it as a cast in place manhole but can't find any good references for analyzing the Steel stress to maintain lower concrete stresses.
We want to ensure that our tensile steel is adequate. One colleague suggested that we s imply treat the curved wall section as a series of short sections of simply supported beams.
Not sure i'm comfortable with this.
Any ideas or references to suggest?

 

[JedClampett]

Just design it as a pipe and ignore the bottom support. Design the reinforcing for the hoop stresses.
By the way, your colleagues suggestion is wacky.

 

[Trillers]

Following up on this topic - we have designed the reinforcing for the hoop stresses as suggested by Jed Clampett but now we are confused about the vertical bars and how to size them. According to all the literature (mostly referring to PreCast) that we have read, we would compute moments to be resisted by vertical reinforcement assuming an empty tank subjected only to Equivalent Fluid Pressure (yes, the famous EFP).
I am thinking we treat the vertical sections with bw being 12" as a cantilever...but then that gives some large moments, nonetheless I think this is the correct approach.
Your thoughts?

 

[JedClampett]

Technically, if you've resolved everything into hoop stresses, there's no vertical bending. But I would put in minimum bending reinforcing (#6's at 12 inches), because it can't hurt.