Concrete chamber / manhole

[EntryLevelEIT]

Hey fellas,

Quick question, maybe not a quick answer. I'm curious how to design a two-way slab, rebar T & B, with a manhole size opening. ACI 318-05 13.4 doesn't seem to be much help. The two-way slab needs to handle a truck load. Doesn't matter what type of truck I'm more interested in the procedure regardless of what the load is. Please see the attached picture. The red arrows represent a truck load transmitted through a precast concrete manhole to the top of the chamber.

My thoughts are as follows, please correct me if I'm wrong:

1.) Design the entire two-way slab of the chamber to handle the load produced by the truck (which will be worst case). I realize this is very conservative but I'm not sure how to design the slab to handle the uniform load that will be produced around the manhole opening alone.

2.) All reinforcement cut-off by the opening shall be placed around the edges of the opening along with diagonal reinforcement.

 

[JedClampett]

Almost everyone I know designs them as one-way and uses some shortcuts in AASHTO (M=900S, etc.). You can definitely go crazy with wheel locations, impact, one ot two wheels on the slab if you try to start from scratch.

 

[minorchord2000]

I agree with Jed. I use one way analysis in the longer direction and use the equations for distribution in AASHTO chapter 3. Much easier and more direct.

 

[rscassar]

One way analysis to provide some redundancy in the structure. What I normally specify for the opening is 1-#4 hoop bar around the opening (to act as a trimmer), cog the reinforcement which would otherwise run thru the opening and provided additional reinforcement to compensate for the loss of reinforcement due to the opening (with a minimum of 3 bars either side of the opening continued a bar lap past the opening).

Why do you have a manhole in the centre of your chamber? Doesn't it require an access ladder?

The urban management division of your local council should have standard detailing for these types of structures. I would follow their detailing.