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RE: Direction of Seismic Loading

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Seismic ground motions do not just apply in one direction at the same point in time.  It is not the same as wind.  The 30% in addition to the 100% is in recognition of that observation.  The nuclear industry and critical military structures use the 100 /40/40 rule in which 100 percent of the earthquake induced ground motions are considered concurrent with 40% perpendicular of the seismic induced inertial forces in the other 2 of the 3 orthogonal directions.  This is in recognition of the vertical component. 
When you look at accelograms, you will note that there are ground motions from a single seismic event at a single point in all 3 directions.  The only reason that the vertical component is currently ignored (except in military or nuclear structures) is because the current perception is that there is sufficient margin in the vertical direction.  Three dimensional time-history analyses will often indicate that additional margin is required, even for the vertical component. 
The ASCE 7 only recognizes the 2 horizontal seismic ground motion components and not the vertical component.  The 2 horizontal ground motions will occur concurrently but not at the same magnitude.  Thus the 100/30 is appropriate.   

Regards, Harold Sprague


To: seaint(--nospam--at)
Subject: Direction of Seismic Loading
From: Tom.Hunt(--nospam--at)
Date: Mon, 14 Feb 2011 15:30:53 -0800

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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