Punching Shear for Underground Tank 1

[Struct_Dre]

Hello!

I'm designing a manhole with about 3' of soil above it. The manhole is below a street. I do not believe that I need to check punching shear on the top slab because the traffic loads should be distributed before it reaches the top slab, but I was told to check the code to be sure. I haven't had any luck finding this in the code. Does anyone know if I need to check punching shear here?

 

[Agent666]

I'd say if the load is spread out to the point where it is more or less a uniformly distributed load on the lid that it's more of a beam shear issue. Though you may have a case where the larger patch load from wheel point loads will only be just/partially on the lid.

This is kind of like a isolated column with slightly higher load on the edge of the lid. So you could probably apply the basics of a punching shear check to this scenario. I can't imagine it will govern over and above having the full load on the lid though.

 

[TehMightyEngineer]

If you're in the US, the ASTM standard on manholes refers back to ACI 318 for structural checks like this. At 3 feet of fill I can guarantee that punching shear will not control for anything that's not aircraft loading.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL, CO) Structural Engineer (IL, HI)

 

[Struct_Dre]

Thanks so much!!

 

[TLHS]

If you need to prove it, sketch your wheel load projection through the soil as per code. Then show that the footprint of that compared to the manhole cover. Wheel loads are not actually very heavy, and the area will be significant at that size. You can do a quick proof by inspection.

Honestly, you could use the wheel area at surface and show that punching shear doesn't govern even without accounting for load spreading.

 

[JAE]

AASHTO also has information dealing with lids of box culverts:

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