0.6D + 1H

[Alin shair]

Using ASCE7 22 when applying the service combination 7a on 2.4.1 which is 0.6D + 0.6W with having uplift pressure forces on the raft I will add H which will end the load combination in 0.6D + 0.6W + 1 Uplift.
My question is, is this load combination is logic or adding the uplift with factor 1 is over designed ?

 

[driftLimiter]

What is the source of the uplift pressure specifically?

 

[Alin shair]

Ground water table

 

[driftLimiter]

I think the final part of Section 2.4.1 should answer your question.

It actually says H loads shall be include in combos 1a-6a. This excludes the 0.6D + 0.6W LC (which is lc 7a)

 

[Celt83]

"It actually says H loads shall be include in combos 1a-6a."
I believe that is for F loads, the paragraph indentation is a bit dubious here. Ground Water is likely to be present during the design event so should be included as 1.0H in the combination.

 

[Alin shair]

I think the code meant fluid loads - Where fluid loads F are present, they shall be included in combinations 1a through 6a - but it also stated the H after and said that it should have a factor of 1 if its will add to the principal load effect, which is more critical to take its factor as 1 also the defination of H at 2.2 is Load due to lateral earth pressure including ground water pressure so it is H as far as I know.
Or you mean that the H is lateral earth pressure from ground water and the fluid load ( F ) is the uplift pressure from ground water ?

 

[driftLimiter]

No you read it correctly I read it too fast. H is for ground water pressure and we add it to the principal load effect as you indicated. I would call Wind the principal load effect for LC 7a and add it to 0.6D + 0.6W + 1.0H like you suggested.

 

[Alin shair]

So basically I have now tension on my raft due to this load combination, so I was trying to find an alternative of using tension piles,so I was hoping that there's any structural background on increasing the 0.6D to be 1D or decreasing the 1H to be 0.6H

 

[Struct123ure]

You can make your building heavier by:
1. Increasing depth of Raft slab or any other structural members
2. Use heavier denser aggregate in Raft slab.
Since this is a service case I don't think you can reduce H in any way.

 

[lexpatrie]

You could switch to LRFD that has a different "weight" on dead load, 0.9D, but it creates more issues elsewhere.

If you go the other way (as some WTP and WWTP plants could go), an appropriately done PRV and making sure the flowrate can actually happen would potentially alleviate the (buoyant) uplift from water pressure, but if that's a basement, it will be filled with untreated "ground" water. I wouldn't typically consider water as a "earth pressure" load (H), it's a F load (fluid).

If the earth is saturated, the load changes but it could be parsed between the load from the soil and the 62.4 pcf from the water. There is also the more pragmatic question of just how permeable the soil is, so even if there is water above the surface, the soil itself may not be saturated, particularly if the water event is short duration and the water isn't able to percolate downward before it dissipates. That would be an adventurous load case, however. I'm not convinced anybody has gone that far into the details on your average WTP/WWTP, (segregating H and F pressures during a design flood) all the old ones are upstream/downstream of various large cities in a flood plain (1960s style eminent domain, seize the swamp, clay soils, terrible MSR values, etc.).

ETA - maybe I should back up a moment. The definition of H includes "ground water pressure," and pressures of bulk materials (grain? soils/aggregate bins?) and F is for "well defined pressures with maximum heights" so perhaps more of a chemical tank approach. If it's not a WTP or WWTP you're in a "better" situation than the typical WTP or WWTP, on the presumption it's a concrete WTP/WWTP as those have separate load factors in addition to the standard ones (ACI 350).

https://codes.iccsafe.org/content/IBC2024P1/chapter-16-structural-design