Snow loads for righting moments?

[josta]

Talked to two SE's and got two different answers; do we or do we not add a portion of the roof snow load to the roof DL when calculating righting moments for a shear wall when seismic forces govern?

(It would seem logical that if seismic forces govern due to the weight of the snow on the roof, then we then be able to use the same snow to resist any uplift.)

 

[SteveGregory]

If you are using ASCE 7-05, some of the load combinations combine Dead, Snow and Seismic. However, the load combination that governs your shear wall design probably doesn't include snow. Also, the combination that governs probably includes a smaller dead load such as 0.6D if you are using ASD.

If you are talking about combining a portion of the snow mass with the dead load mass to compute the lateral seismic force, then refer to 12.7.2 for that combination. If this applies in your situation, the snow will increase the horizontal shear or overturning moment.

 

[msquared48]

Personally, I would not use the snow mass contribution to the righting, or resisting moment.

It seems to me that in a seismic event, should that occur with a snow load on the roof, there is no guarantee that the snow would remain on the roof, or remain long enough to help resist overturning. I would choose to err on the consdervative side here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteveGregory]

I agree with Mike. If anything, snow would only increase the overturning moment.

 

[JAE]

Just use the load combinations. Each combination should be checked for this.

 

[josta]

Thanks for the responses.

Mike, I understand what you're saying, but if the snow slides off (or is absent) during a quake, that would then lighten the lateral load (and reduce overturning) at that moment. So we wouldn't then "need" the weight of the snow any longer for righting anyways. Wouldn't it all be relative?

 

[msquared48]

josta:

Yea, I guess you're right. I just tend to be conservative in my thinking.

Just make sure that you do not combine the max snow with the max seismic at the same time. Now that would be too conservative.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[josta]

Mike

Thanks, Mike. If we disregard the snow for righting moments, then the holddowns would be substantially larger which I don't think is necessary.

A bit OT but I'm trying out the new LateralWorx program; it's quite amazing IMO. No, I don't work for them or anything....you import the CAD drawings into LateralWorx, and select walls in the program that will be shear walls, and the program pretty does the rest, including outputting a shear wall/holddown .dxf drawing.

However, it does not use any snow loads for righting moments; just dead loads even if seismic with snow governs.

John

 

[josta]

Correction: LateralWorx can use snow for righting moments; you just add a percentage of the snow load to the dead load bearing on the wall you're analyzing.

John