Seismic Ch12 weight 4

[phuduhudu]

Is there not an inherent contradiction within ASCE 7 in that to calculate ELFs you use a seismic weight with only 25% of LL but then when you combine load factors you use up to 0.75LL for allowable stress design. So you are combining ELFs based on 0.25LL with gravity loads of 0.75LL?

 

[StructuralCA]

The tributary area of gravity loads can be completely different from that of lateral loads on any given structural element. The LL is mainly load type not same exact load.


Regards,

M.S. Genidy, PE
Structural Soft, LLC

 

[phuduhudu]

Thanks for the response but I am not with you. You have a load case which includes 75% of LL as part of the gravity loads as well as earthquake loads. Now the Equivalent Lateral Forces you work out for the earthquake loads are dependent on the gravity loads on the structure but to calculate the earthquake loads we use only 25% of LL.

 

[WillisV]

You only use 25% of the LL in storage areas - generally you don't have to include any of the live load.

 

[DaveAtkins]

I think StructuralCA was trying to say that a lateral resisting element will be receiving lateral load from a large tributary area, which can safely be assumed to carry a reduced live load. But the gravity live load in the area immediately around the element could be the full live load.

DaveAtkins

 

[Teguci]

Couldn't this be an allowance for the potential sloshing of live loads during a quake? Boxes, books and files will shift and not necessarily impart a load in time with the fixed structure. Any of this Live Load due to people will definitely not shift in time with the structure.

A 3'-0" tall stack of paper would shift all over the place.

 

[msquared48]

The 25% factor is used whenever the live load exceeds 100 psf, whatever the type.

Also, whenever the snow load exceeds 30 psf, then a 25% factor, or whatever the local building official dictates, is added.

Mike McCann
MMC Engineering

 

[phuduhudu]

I think I have it now after a bit more research and looking around. Teguci's explanation seems the most feasible. Only things firmly attached to the structure will move with it and contribute to the seismic weight.

DaveAtkins and StructuralCA, I see what you were trying to say and that might be the case for designing individual members but for say footing design the same is true of the large tributary area for gravity loads as for earthquake loads. There are LL reduction factors for gravity loads of course to cover this but they would never get you down to 25%LL.

Msquared, thanks for that. Is that common practice or is it in the code?

 

[WillisV]

msquared48 - what code are you referring to? Per ASCE 7-05 its just 25% of storage LL...

 

[msquared48]

You can also get loads over 100 psf from computer floors and equipment, plus heavy equipment loads from manufacturing that are not associated with storage, to include overhead crane loads. Gridiron loads in stage assemblies too.

Mike McCann
MMC Engineering

 

[WillisV]

I don't disagree - I'm just wondering where you are finding the "over 100psf" designation in the code.

 

[msquared48]

I don't. I am just reading what I consider to be the intent of the code between the lines here so to say.

Mike McCann
MMC Engineering