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RE: Direction of Seismic Loading

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I agree.  You are essentially using the requirements stated in the first sentence of Section 12.5.1 rather than the procedures permitted by the second sentence.
Dan Devlin

From: Tom.Hunt(--nospam--at) [mailto:Tom.Hunt(--nospam--at)]
Sent: Monday, February 14, 2011 3:31 PM
To: seaint(--nospam--at)
Subject: Direction of Seismic Loading

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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