Vehicular load on buried structures (vaults, manholes, culverts, etc) 1

[Astructengr]

I have a project where I have to come up with schematic drawings for some underground structures (vaults, tunnels, etc). These are for estimating/feasibility and not for construction. Some will have lids close to the surface or be deeper and have manhole access. They will be under roadways. Everything is going pretty well except for the design of the top slab. I am coming up with needing a thick top slab to avoid one-way shear and also needing heavy reinforcement for bending.

When I compare what I am coming up with to a typical DOT manhole (attached), it does not make sense. Under HL-93 loading, there will be a 16kip point load near the end of the slab or in the middle of the slab. I am assuming minimal spreading, 1' unit width/one way action. Is one-way shear typically ignored and the only shear check is 2-way/ punching shear around the entire perimeter?

 

[JedClampett]

There used to be a slick simple formula (one for predominate traffic each way) for this. It was in the 1996 AASHTO but has disappeared. You designed for moment (3.24.3.1 or 3.24.3.2) and got a pass on shear (3.24.4). I'm not sure why it's gone, but it's not like a lot of bridges and other structures were failing due to it being incorrect. I'm sure some of the more AASHTO savvy participants could vouch for or against it.

 

[WARose]

We've kicked this around before on here (I would link to those threads, but the new search engine here is lousy). In short, you come up with a lot more reasonable designs following AASHTO than ACI. But you have to use AASHTO requirements for reinforcement (in both directions) and so on. Just straight figuring a load and following normal one/two way distribution (and ACI 318) will spit out a pretty heavy design. (Compared to AASHTO.)

 

[Lomarandil]

I can't speak to underground structures specifically, but I know that AASHTO says that shear in bridge decks essentially never controls as a limit state, since membrane action kicks in first. So we don't even check it.

 

[CDLD]

WArose, I feel like you are thinking of this thread, but I could be wrong.

https://www.eng-tips.com/viewthread.cfm?qid=510455

 

[WARose]

Yeah, that's a good one. Bridge Smith is always good help with this stuff. I found another one where I pondered this question myself:

https://www.eng-tips.com/viewthread.cfm?qid=451967

 

[BridgeSmith]

Per AASHTO, for the top slabs of buried structures under 2' or more of fill, the wheel load is distributed over the 10" long by 20" wide contact area, and then spread through the soil at a 1:1 slope (45 degrees). That's for typical soil backfill; for select backfill, the slope is 1:1.15.

For top slabs under less than 2' of fill, the load is distributed, and the slab is reinforced, as if it's a bridge slab.

 

[DoubleStud]

That lid was probably designed using FEM. They put 16K load on 20"x10" area and add the impact factor per AASHTO (0 soil cover) at various locations and more importantly next to the MH cover.

 

[Astructengr]

Thank you everyone! This is extremely helpful. The difference between ACI and AASHTO on this subject is really surprising.