ASCE 7 Load Combinations - 1.4D + 1.6H 1

[ProgrammingPE]

When ASCE 7-10 was released, they revised section 2.3.2 so that the basic combinations are "simplified" by removing F, T, and H. If you have a fluid, straining, or earth load, they get included manually through notes in the code after the load combinations are provided (like what was previously done for ice and flood loads).

My question is specifically regarding the 1.4D load combination. In previous editions of ASCE 7 and in the IBC, H was never combined with 1.4D, so 1.2D + 1.6H was the common load combination for designing the concrete in retaining walls. The notes for ASCE 7-10 say:

Where fluid loads F are present, they shall be included with the same load factor as dead load D in combinations 1 through 5 and 7.

Where loads H are present, they shall be included as follows:[ol 1][li]where the effect of H adds to the primary variable load effect, include H with a load factor of 1.6;[/li][/ol]

Since individual load combinations are not specified for H (unlike how F is handled), shouldn't the load combination 1.4D + 1.6H be required now? What is the purpose of the 1.4D load combination (are H loads present at that time)?

The reason this is a more significant issue now is that IBC 2021 changed so that it references ASCE 7 for the load combinations instead of providing them.

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[phamENG]

No, I don't think you do. H loads are tied to variable loads. Dead load is invariable. So you only bring H into the picture when it's 1) adding to the primary variable load or 2) is permanent and countering the primary variable load.

So you'd only add it where there's a variable load like L, Lr, S, or R.

 

[ProgrammingPE]

I didn't catch that they used "primary variable load effect" in ASCE 7-10. They actually changed the wording in ASCE 7-16 to use "principal load effect". Does that change your opinion?

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[phamENG]

Yes, yes it does. I think that otherwise semantic change is substantive here. Refer to the commentary, particularly Table C2.3-1 "Principal Loads for Strength Design Load Combinations." So for LC 1, Dead load is the "principal load." Therefore, it follows that H would be applied with either a 1.6 or 0.9 load factor in LC 1 as applicable based on the provisions in 2.3.1.

 

[LockeBT]

Slightly off topic here, I'm more worried about that 1.6H in load combinations with E. That often creates a situation where the contractors/owners wonder if I know what I'm doing. They hit me with that "why are you overdesigning your stem and your footing!? I have never seen this size for this retaining height!"

 

[HTURKAK]

Having worked with different standards , i think when the LC with F , the engineering judgement is necessary . If fluid load is well defined ( height, density etc..) F should be included to D but LC should be looked for with F and without F as necessary.

Definition of F = load due to fluids with well-defined pressures and maximum heights.

IMO C2.3.2 well explains how to use F and H ..

Copy and paste of relevant para.:

Fluid load, F, defines structural actions in structural supports, framework, or foundations of a storage tank, vessel, or similar container due to stored liquid products. The product in a storage tank shares characteristics of both dead and live loads. It is similar to a dead load in that its weight has a maximum calculated value, and the magnitude of the actual load
may have a relatively small dispersion. However, it is not permanent; emptying and filling causes fluctuating forces in the structure; the maximum load may be exceeded by overfilling; and densities of stored products in a specific tank may vary.

The fluid load is included in the load combinations where its effects are additive to the other loads (load combinations 1 through 5). Where F acts as a resistance to uplift forces, it should be included with dead load D. The mass of the fl uid is included in the inertial effect due to E (see Section 15.4.3) and the base shear calculations for tanks (Section 15.7). To make it clear that the fluid weight in a tank can be used to resist uplift, F was added to load combination 7, where it
will be treated as dead load only when F counteracts E.

The fluid mass effects on stabilization depend on the degree to which the tank is filled. F is not included in combination 6 because the wind load can be present whether the tank is full or empty, so the governing load case in combination 6 is when F is zero. Uncertainties in lateral forces from bulk materials, included in H, are higher than those in fluids, particularly when dynamic effects are introduced as the bulk material is set in motion by filling or emptying operations. Accordingly, the load factor for such loads is set equal to 1.6.

Where H acts as a resistance, a factor of 0.9 is suggested if the passive resistance is computed with a conservative bias. The intent is that soil resistance be computed for a deformation limit appropriate for the structure being designed, not at the ultimate passive resistance. Thus an at-rest lateral pressure, as defined in the technical literature, would be conservative enough. Higher resistances than at-rest lateral pressure are possible, given appropriate soil conditions. Fully
passive resistance would likely not ever be appropriate because the deformations necessary in the soil would likely be so large that the structure would be compromised. Furthermore, there is a great uncertainty in the nominal value of passive resistance, which would also argue for a lower factor on H should a conservative bias not be included.

 

[JedClampett]

ACI and ASCE are wrong on the load combinations for fluid loads (F). As HTURKAK says maybe they should be treated with smaller load factors for uplift and sliding resistance (although I contend, as a fluid, they tank might be empty at some time and should be ignored). But for design of walls or bases of tanks, you can't hardly use a big enough load factor for fluids. From ACI 350-89 (yeah pretty old), I still use 1.7 times 1.3 or 2.21 for bending and 1.7 for shear.
Fluid loads are different than other loads:

[li]They're very very large.[/li]
[li]Unlike a LL, where you might get 100 psf over a limited area, they're always over the whole plate.[/li]

Now you don't have to listen to me, but I spend a portion of my time fixing inadequately designed vessels. So 1.4F is good for business.