Sunday, July 20, 2008

Re: 25% of Live Load added to Lateral

While this section does pertain to a simplified method, I will note that it is the EXACT definition for W as used in section 12.7.2...thus, your logic does not make sense to me as section 12.7.2 is NOT limited to flexible diaphragms.  The intent is that the 25% is ONLY applied where you have a designated storage area that requires a storage area live load.  Thus, if you have a whole floor that is designated as storage, then use the whole 25% for that floor...if it is just one room on a floor, then just that room gets the 25% increase.  I will note that localized 25% of storage load can effect “simple” flexible diaphragm structures as well...if that room is at one end of a flexible diaphragm, then more of that load will go to one shearwall/vertical element than the other.  And if the building actually has multiple flexible diaphragms (i.e. there are interior shearwalls which essentially break up the diaphragm into “pieces”), then if the storage load in only in one section, then some shearwalls will have to resist the additional seismic mass while others may not.

And for what it is worth, I have NEVER had a building official classify a whole level as storage just because there is one room that is storage on that level.

Regards,

Scott
Adrian, MI


On 7/20/08 3:13 PM, "Dennis Wish" <dennis.wish@verizon.net> wrote:

Scott,
This provision in Section 12.14 refers to compliance with a flexible diaphragm design (Simplified Alternative Method). In this case I would have to disagree with you unless I have additional verification. The reason goes back to my original argument – there is no way to assure compliance with the posted load in a building conforming to a Bearing Wall or Building Frame System. Therefore, the building official would classify the entire level as having to comply with the occupancy classification (light storage or heavy storage).
 
If this were a rigid analysis I might tend to agree, but even still, the center of mass would change if the occupant decides to expand the area used for storage. It is safer to assume the more conservative lateral load applied to the entire diaphragm at that level than to try and restrict it when there is not means in place (other than placard) to restrict compliance to the load restriction.
 
Any thoughts from others to help settle this. The basic assumption for the use of this section is allow for flexible diaphragm use on a 3-story or less light-frame bearing wall or building frame structure.
 
Thanks
Dennis
 

From: Scott Maxwell [mailto:smaxwell@umich.edu]
Sent: Sunday, July 20, 2008 7:02 AM
To: seaint@seaint.org
Subject: RE: 25% of Live Load added to Lateral

Yes, the 25% would only apply to the location where the storage is located...if that is one small room on the 4th floor, then that is the only area that gets the 25% of live load added to the seismic mass.  This generally will mean that it would more effect it on the full level, but when determining center of mass relative to center of rigidity (for rigid diaphragm calcs), then having it added at its real location would also matter.

Regards,

Scott

Adrian, MI
-----Original Message-----
From: Dennis Wish [mailto:dennis.wish@verizon.net]
Sent: Saturday, July 19, 2008 9:48 PM
To: seaint@seaint.org
Subject: RE: 25% of Live Load added to Lateral
Scott,
Considering this is a lateral load, then my read on it (and maybe this is the same as yours but worded differently) is that you add the 25% of the Storage load only to the 4th floor. My interpretation of what you stated is that you would isolate the area of the room and then add 25% of the storage load for this one area distributed uniformly across the 4th floor. This would greatly reduce the lateral load if the roof is a storage closet for example.
The other half of the argument is that the building department would not be able to control a portion of one level but would assume that the entire level could be used for storage if the user expands the area required.
 
I have a feeling that we are agreeing on this one. I have already made the change and have allowed for a percentage of the storage load to be added into the roof where the attic area is used for storage. I think this should be left to the discretion of the user.
 
Thanks for getting back to me. I have posted another question that is probably applicable in your area of Michigan but comes into play only in a smaller area of the California – Snow loads per ASCE 7 section 12.14.8.1. Please see this post which asks for clarification.
 
Thanks
Dennis
 

From: Scott Maxwell [mailto:smaxwell@umich.edu]
Sent: Saturday, July 19, 2008 9:55 AM
To: seaint@seaint.org
Subject: RE: 25% of Live Load added to Lateral

It is added to the overall seismic mass of the building ONLY for areas that are designated as storage per the code provisions.  Thus, if you have one ROOM on the 4th floor of a 10 story building that is storage, then only for that THAT portion of the live load would you add the 25% to teh seismic mass...all other locations would have zero percent of the live load added to the seismic mass.

At least that is how I read it.

Regards,

Scott

Adrian, MI
-----Original Message-----
From: Dennis Wish [mailto:dennis.wish@verizon.net]
Sent: Friday, July 18, 2008 8:07 PM
To: seaint@seaint.org
Subject: 25% of Live Load added to Lateral
If one floor in a multi-story building is to be used for light or heavy storage, is the 25% of the floor live load added only to the level that is used for storage or is it added to all floors?
 
Dennis S. Wish, PE
Dennis S. Wish, PE
California Professional Engineer
C-41250 Exp. 3/31/09
Structural Engineering Consultant
La Quinta, CA 92253
dennis.wish@verizon.net
http://structuralist.wordpress.com
http://www.structuralist.net
 
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