bottom slab stresses in submerged structures 1

[bobbygee]

In our office there is a discussion of the need for reinforcement in the concrete slab for 4 foot diameter wastewater fiberglass manholes. The manufacturer recommends a minimum six inch concrete pad poured to anchor the manhole with additional concrete poured inside the manhole to form a channel and around the outside to seal the pipe penetrations. The manufacturer recommends no reinforcement. There is no history of our manholes failing due to lack of reinforcement. Now there is concern that the manhole slab will fail due to "unbalanced hydrostatic forces".I considered the slab and additional concrete as a monolithic concrete pad subject to bending in both directions due the "unbalanced hydrostatic forces" and the stresses that resulted were less than those allowed for plain concrete structures by ACI. I don't believe this is the most accurate analysis of the actual forces but I proceeded for the sake of the discussion. In spite of my calculations, the manufacturer's recommendations and historical lack of failures, our department is headed towards adding reinforcement to the slab.Does anyone have any thoughts on the actual forces at work or knowledge of failures due to lack of reinforcement? We have conservatively assumed saturated soil to the top of the manhole at finish grade.

 

[jdonville]

bobbygee,

Is the pad to provide a stable foundation for the MH, or is it to provide resistance against uplift? Hydrostatic pressures due to groundwater should be uniform on the base of the MH, so are the unbalanced forces due to the level of fluid in the MH?

If this is the case, then there are two scenarios:
1) GWL higher than fluid level in MH
2) GWL lower than fluid level in MH

In Scenario 1, there is a net hydrostatic buoyant force on the structure. In Scenario 2, there is a net hyrostatic load on the base of the structure. In either case, the hydrstatic load on the base of the structure (and the slab) should not vary along the bottom of the MH. Moreover, the sides of the MH should serve to "stiffen" the edges of the slab and reduce deflections of the slab.

I can't believe that a 6-inch thick unreinforced slab on a proper aggregate base or compacted subgrade will fail under these conditions supporting a 4-foot diameter MH unless the hydrostatic pressures are so large as to be unusual.

It seems to me that unless, as I said, the MH is unusually deep and the GWL and liquid levels are so disparate, that an unreinforced slab should be OK.

Please let us know if the situation is different than I have described it.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[bobbygee]

jdonville,

Thanks for your response.The "unbalanced hydrostatic forces" that we are discussing arise from the following assumptions:1.The manhole itself is empty. 2.The compacted backfill around the manhole (plus the weight of the concrete) resists the bouyancy due to saturated soil conditions. 3. The hydrostatic pressure acting at the bottom of the slab causes the slab to bend upward across the 4 foot diameter where there is no resisting load. Like I said, not the most accurate analysis, but I proceeded with the calculations and the resulting stresses were still acceptable by code.The others in the office are skeptical and seem to want to reinforce or over reinforce everything. I'm not going to argue but I don't know how to convince them we don't need the reinforcement.

 

[jdonville]

Bobbygee,

The slab should not bend upward if the MH is merely resting on it, or if the MH is bolted or otherwise simply affixed to the slab. In this case the water pressure will act on the bottom of the MH - not the slab.

If the MH and slab are integral (the base of the MH is cast into the slab), then no water (and hence no water pressure) can get between the MH and the slab and only then will the slab be subjected to the "unbalanced hydrostatic" load - but only on the portion of the bottom of the empty manhole. Any portion of the slab extending outside the diameter of the MH will not have any net hydrostatic load on it.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com