Foundation Self Weight for Resisting Uplift

[DTS419]

What is the current code requirement for any reduction in foundation self-weight (D) when resisting wind uplift (W)?

2021 IBC contains the following verbiage:

"For load combinations that include the counteracting effects of dead and wind loads, only two-thirds of the minimum dead load likely to be in place during a design wind event shall be used."

In spite of this, there is no load combination shown that includes 0.67D.

In ASCE 7-16, combination 7 gives 0.6D + 0.6W. This is different from the two-thirds that IBC says.

 

[HDStructural]

You are looking at alternative load combinations, not the standard ones.

Either way, I believe the 2/3rd is generally in line with the logic of a 1.5 SF for overturning and such.

 

[Aesur]

I use the basic combinations and use 0.6D for uplift combinations. As HDStructural said 2/3 is for alternative load combinations, and it can be beneficial to use those in some cases.

 

[lexpatrie]

My impression is the alternate load combinations are more work for minimal benefit. Particularly if they involve wind, which is typical.

 

[PEinc]

The 0.67DL would be a resisting force not an applied load.

http://www.PeirceEngineering.com

 

[Aesur]

The 0.67DL would be a resisting force not an applied load.

Not sure I follow what you are implying here, care to explain a bit?

 

[PEinc]

You apply the various driving forces (wind, live load, hydrostatic, seismic, etc.) to the structure with their appropriate, individual, load factors (LF > 1). When checking stability (overturning and sliding), one of the resisting forces is the dead load of the footing but, in LRFD design, you have to multiply (reduce) the dead load weight slightly by multiplying the weight by a resistance factor (RF < 1). Applying resistance factors essentially raises the overall "safety factor" above the average load factor.

http://www.PeirceEngineering.com

 

[DTS419]

I should have clarified; my question has to do with ASD loads.

 

[phamENG]

DTS419 - mentioning 0.6D makes it clear you're talking about ASD combinations. And the alternative combinations are strictly for ASD.

This is the first sentence from the code section you quoted:

In lieu of the load combinations in ASCE 7, Section 2.4, structures and portions thereof shall be permitted to be designed for the most critical effects resulting from the following combinations.

This entire code section is to be used in lieu of and not with ASCE 7. You either use 1605.1, which directs you to ASCE 7 for the load combinations, or you use 1605.2, which has its own combinations and additional rules to follow. They don't mix.

 

[DTS419]

phamENG - In the situations I am referring to, there are no other combinations to consider so it is as simple as using one or the other. Basically, wind uplift is provided by truss manufacturer and foundation is sized for resisting weight. The "portions thereof" clause in 1605.1 would cover this.

 

[phamENG]

What do you mean no other combinations to consider? In your original post, you're asking about ASCE combinations and IBC alternative combinations. That's two different sets of load combinations. Or do you mean you see now that you have to pick one of them?

 

[DTS419]

I'm saying that I'm not designing the whole structure so I'm not considering the full spectrum of load combinations.

 

[EngDM]

Regardless of if you are designing the whole structure you still need to run through the load combinations? Some of your load types may just be zero so that case doesn't govern; this doesn't mean you don't have to rationalize this out.

 

[DTS419]

I think this is being overcomplicated. It's as simple as sizing a CIP deadman to resist a provided wind uplift force.

 

[phamENG]

It's as simple as sizing a CIP deadman to resist a provided wind uplift force.

Right. And are you using ASCE 7 load combinations to do it, or are you using IBC alternative load combinations?

 

[DTS419]

That's the question. Can it be 0.67D per the IBC alternative, or does it have to be 0.6D?

 

[phamENG]

I would advise using the ASCE load combinations. The value you're getting from the truss manufacturer is not the wind uplift, but the net uplift. So they've already considered the dead load of the roof in there and they are reporting the result of the analysis considering a load combination. The only truss calculations I have ever seen use ASCE 7 load combinations. In fact, I've never once seen anyone try to use the alternative load combinations for anything - ever.

If you now use 0.67 dead for the foundation, you have mixed the load combinations, which is no good.

 

[TRAK.Structural]

Agreed with Pham here.

Also 0.6 vs 0.67 is splitting hairs imho.

 

[DTS419]

Also 0.6 vs 0.67 is splitting hairs imho

I'll let you explain that to my contractors who are trying to run competitive businesses.

 

[TRAK.Structural]

DTS - Contractor pressure is something we all have to deal with on some level, so I do understand that it is a factor. However the most competitive companies that I know are the ones who do things right and aren't looking to trim everything down to bare bones.