Deep Precast Manhole Design 1

[msquared48]

Have to look at a 25 foot deep 54" precast manhole to see if special reinforcing is needed. Seems to be just a ring analysis with uniform loading, the allowable concrete compressive force not to be exceeded, using minimum steel.

That being said, does anyone ever use allowances in their design for uneven compaction of the manhole that would yield uneven lateral pressures, possibly necessitating steel other than the minimum? Seems to me that the loading would almost never be uniform.

Modeling this in RISA would be as b)(%& to allow movement of the walls, but doable.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[cap4000]

Check out this link.

http://www.concrete-pipe.org/pdfdd/DD_20.pdf

 

[dcarr82775]

On something small like this I would think it is reasonable to assume it is a ring in compression. For larger shafts (10+ ft in diameter) where the soil conditions could be variable around the shaft I will often assume some eccentricity that is a couple of % of the shaft diameter and design it as a ring with some bending as well as compression.

 

[msquared48]

Thanks for the info.

I kinda felt that the variation might only apply to larger diameter structures, but felt that it could be based on the diameter to thickness ratio of the manhole too.

This manhole is apparently only 4" thick, which, intuitively, seems too thin for the load. I'll run the numbers though. Thanks again.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com