Circular Cast-In-Place Concrete Manhole minimum wall thickness

[oengineer]

I have a question regarding minimum wall thicknesses for Circular Cast-In-Place Concrete Manholes. I am trying to specify a minimum wall thickness on the detail sheet. The Circular Cast-In-Place Concrete Manhole for the project has an interior diameter of 6 ft and the overall height of the manhole is 13 ft tall.

In the Design Data 20: Circular Precast Concrete Manholes from the American Concrete Pipe Association, it mentions the following criteria to determine the wall thickness:

The minimum wall thickness equals one-twelfth the inside diameter of the manhole, when the thinnest wall thickness specified in ASTM C76 is used.

Would it be acceptable to apply this criteria to a Circular Cast-In-Place Concrete Manhole? If not, is there another a available minimum wall thickness criteria to specifically address Circular Cast-In-Place Concrete Manholes?

Suggestions/comments are appreciated.

 

[phamENG]

In the absence of a pre-approved detail from the local wastewater authority or DOT, I'd say analysis will be your best friend. Figure out what it needs to be, and use that. Of course, you'll need to add a few inches to the inside. Not sure how familiar you are with H₂S and what it can do to concrete. Nasty stuff.

 

[strucbells]

Generally I'd think you'd need a slightly thicker wall due to cover requirement differences between precast and CIP concrete.

 

[jayrod12]

I wouldn't be too keen on using the precast guidance for cast-in-place. One of the reasons that precast is generally allowed to do more cutting edge stuff (smaller clear covers etc.) is due to the fact it's cast in well controlled environments and typically wet cured therefore producing a higher quality product.

For casting on site, you need to account for the lower quality construction and less attention to details.

If it were me, I'd likely be starting it at 8" thick and see where that gets you with the numbers.

 

[r13]

I don't know the base of the call by ACPA, so couldn't comment on it. We usually design the manhole lining to resist at rest pressure, with construction surcharge, or traffic load, and the maximum expected hydrostatic pressure. The precast manhole usually is more economical, unless lead time consideration is involved.

 

[oengineer]

I wouldn't be too keen on using the precast guidance for cast-in-place. One of the reasons that precast is generally allowed to do more cutting edge stuff (smaller clear covers etc.) is due to the fact it's cast in well controlled environments and typically wet cured therefore producing a higher quality product.

For casting on site, you need to account for the lower quality construction and less attention to details.

If it were me, I'd likely be starting it at 8" thick and see where that gets you with the numbers.

Currently, I have stated a Minimum of 8" wall thickness on the detail sheet for the manhole.

My concern is if I should state a Minimum of 12" wall thickness for the manhole, but I am thinking that a 12" thick wall may be too conservative.

A cast-in-place baffle wall is to be constructed inside of the 6 ft interior diameter manhole. The original goal is to have the contractor build the manhole based on governing city/county specification for manholes.

 

[JedClampett]

It only costs marginally more to pour a 12 inch wall than an 8 inch wall. You've paid for the forms.
Eight inch walls are hard to pour, vibrate and waterstop.
Go with 12 inch walls.

 

[r13]

You are designing a drop/vent shaft, which may subject to water and hoop tension, so reinforcing and thicker concrete covers are required, 12" is better then. The 6' ID is rather small, watch out for form work requirement, and expecting joints (ASTM C76).

 

[jayrod12]

I'd agree with the others that 12" would likely be better. It's about 50% more concrete to go from 8" to 12" thick and as others have indicated, the bulk of the costs is in the formwork, excavation etc.

 

[oengineer]

You are designing a drop/vent shaft, which may subject to water and hoop tension, so reinforcing and thicker concrete covers are required, 12" is better then. The 6' ID is rather small, watch out for form work requirement, and expecting joints (ASTM C76).

I am designing a Baffle Wall inside of a Cast-in-place Circular Manhole.

Please see the attachment for how this structure will look:

https://files.engineering.com/getfi...2-682d1c63ce8f&file=Manhole_-_baffle_Wall.pdf

I see several post (thank you all for your comments) recommending to specify a 12" thick wall for the Cast-in-place manhole for the contractor to construct. Is anyone aware of any technical documents that can be use to help justify the wall thickness increase?

 

[r13]

Yes, it is an overflow inlet/drop shaft with gate chamber (baffle wall and sluice gate). It is a hydraulic structure, so the concrete elements should be detailed accordingly (ACI 350). Manhole usually is a shaft accessible by human for service purpose.

 

[dik]

Can you get 8" precast conc 'rings' that you can stack and seal?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[oengineer]

Can you get 8" precast conc 'rings' that you can stack and seal?

Due to the construction of the Baffle Wall inside the manhole, the contractor prefers to go with a Cast-In-Place option. This way they can cast the baffle wall & the manhole together in the field.

 

[robyengIT]

I have my own spreadsheet as per "r13" together with a calculation procedure but in italian (both for rectangular and circular pit). Tell me if you can manage it : ready to post