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Re: Direction of Seismic Loading

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Use 100% in the most critical direction and ignore the 100-30 rule.  The 100-30 
rule was a way to sidestep finding the most critical direction.

This was the understanding when this provision was first introduced into the 
1988 UBC.

The 100-30 rule was calibrated using a 4 column braced frame structural system, 
loaded on the diagonal, which would have given 100% and 41%.  Somebod on the 
committee developing the provision made the case for 30% instead of 41%.
What confuses the discussion is that there has been some discussion by the 
geotechs of directionality of the earthquake when  finding the magnitude of 
ground motion.  This is probably what Harold was thinking about.  This issue is 
reflected in the magnitude of the base shear equation and doe not apply to the 
100-30 rule.  This was clarified at the 2010 SEAOC Convention.

Mark Gilligan SE

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To: seaint(--nospam--at)
Subject: Direction of Seismic Loading
From: Tom.Hunt(--nospam--at)

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Section 12.5 of ASCE 7-05 provides the requirements for the direction of 
application of seismic forces and for a typical rectangular building in 
California (SDC "D") I suspect most engineers would use 100% in one 
direction and 30% in the other to satisfy ASCE 7 Section 12.5.4.  My 
understanding is that this is to capture the effects of the earthquake 
forces at some direction other than the principal axis where the loading 
would impart some level of forces in both orthogonal force resisting 
systems at the same time.

I am having a discussion with a client who thinks the 100%-30% requirement 
would also apply to a single skirt support steel vertical vessel such as 
you would see in a petrochemical plant.  Since a vertical vessel is 
essentially a single symmetrical cantilever structure with a single 
lateral force resisting system, I would assume that ASCE 7 Section 12.5 
would not apply.  That is, I would think you would just take 100% of the 
seismic load in any single direction.

Would you agree with this assumption?


Thomas Hunt, S.E.

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