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Re: Vertical Component of Seismiv Loads in Bridges

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Hrm.  ATC-32 "Improved Seismic Design Criteria for California 
Bridges: Provisional Recommendations" says (Commentary 3.21.4):

	The fourth effect [of near-fault conditions] is that 
	the ratio of vertical to horizontal spectra at short 
	periods is much larger than the commonly assumed 
	ratio of two-thirds.  

However, it doesn't give a "correct" ratio.  In 3.21.9.1 and 
C3.21.9.1, it adds:

	3.21.91.
	Earthquake actions shall be determined for seismic 
	input in at least two orthogonal directions (usually 
	the horizontal longitudinal and transverse axes of 
	the bridge). ... Seismic input along the vertical 
	axis is to be considered where its effects are significant.

	C3.21.9.1
	... Vertical input should be considered for unusually 
	long spans, for outriggers, for "C" bents, and for 
	cantilevers.  

There are also a series of load cases and conditions for their 
application specified.  I haven't got a copy of the new Caltrans 
(California Dept of Transportation) Bridge Design Specifications, 
so I'm not sure how much of this got adopted and is currently in 
use.  Might be worth checking out/buying the BDS and/or ATC-32.  

HTH,

Charley

-- 
Charles Hamilton, EIT                   Graduate Student Researcher     
Department of Civil and                 Phone: 949.824.8694
    Environmental Engineering           FAX:   949.824.2117 
University of California, Irvine        Email: chamilto(--nospam--at)eng.uci.edu