0.6 Dead + 1.0 Wind Load Combination

[spats]

Looking to see what others do for this situation. For the 0.6D + 1.0W ASD load combination, do you use 0.6 of full design dead load allowance, or do you use 0.6 of absolute minimum actual dead load? This isn't an overturning thing, it's a net uplift on roof thing.

 

[WillisV]

I use 0.6 of expected (more on the minimal side) dead load.

 

[jdgengineer]

I think I remember reading somewhere (I think perhaps the SEAONC IBC Seismic Books) that the intent is to use the realistic expected amount of dead load that would be in place at the time. I've read other places that the equation is calibrated to account for a 10% overestimate in dead load as there obviously is a desire in the other combinations to overestimate your dead load.

I don't think the code is particularly clear on it. Personally, I try and use as realistic estimate of dead load as I can.

The 10% overestimate argument does make sense to me though. 0.9 / 1.5 SF = 0.6. It's my understanding that the 1.5 safety factor is implied for overturning with the equation. Makes sense to me to extend this to uplift as well.

 

[Bagman2524]

AISC Commentary Chapter B2 discusses loads and load combinations.

 

[Bagman2524]

From the AISC commentary: The load factor 0.6D in load combinations (7) and (8) in Section 2.4 of SEI/ASCE 7 addresses the situation in which the effects of lateral or uplift forces counteract the effect of gravity loads. This eliminates a deficiency in the traditional treatment of counteracting loads in allowable stress design and emphasizes the importance of checking stability

 

[TDIengineer]

When designing foundations and checking soil bearing for stability I like to use the alternate load combos in the CBC which allow for a 0.9DL instead. Not sure if you have that combination at your disposal.
You don't get the reduction in overturning when checking soil bearing with this combo, but I think it yields a more realistic soil distribution if your design DL are not over done.
If you're working toward stability and using the footing weight for a resisting moment, I feel the 0.6D is too conservative for the self weight of the concrete even when thinking about vertical accelerations in combination with lateral accelerations.

 

[briancpotter]

I've always used 0.6 of the lowest possible dead load. So if, for example, the light-gauge roof is specced for a min weight of 12 psf and a max of 20 psf, for the uplift case I'll use 0.6 * 12 psf.

Brian C Potter, PE
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