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TO: All Design Section Staff
FROM: J. Kapur - 47340
DATE: September 10, 2001
SUBJECT: LRFD Seismic Design

The Response Modification Factor (R) for all bridges shall be based on the Importance Category of “Other” as specified in LRFD Table, unless otherwise directed by the WSDOT Bridge Design Engineer.  The R-factors, for column design, apply only to the elastically computed moments from the seismic analysis.  The R-factor for column shear design shall be taken as 1.0.

 The live load factor for Extreme Event-I Limit State, gEQ as specified in the LRFD Table 3.4.1-1 for the majority of bridges, shall be equal to 0.0.   For bridges located on routes in urban areas susceptible to daily, heavy congestion, gEQ  shall be equal to 0.5 unless otherwise directed by the WSDOT Bridge Design Engineer.  The gEQ factor should apply to the live load force effect obtained from the bridge live load analysis.  Live load mass is ignored in the dynamic analysis.


The AASHTO LRFD Specifications Table indicates Importance Categories of Critical, Essential and Others for The Response Modification Factor (R).  Bridge columns designed and detailed in accordance with the criteria for “Other” bridges exhibit good seismic performance and it may be unnecessary and uneconomical to design bridges for Critical or Essential categories.  However, if the WSDOT Bridge Design Engineer identifies a bridge as Essential or Critical, the corresponding R-factor should be used for substructure design.

 If you have any questions regarding these issues, please contact Tim Moore at 705-7163 or Bijan Khaleghi at 705-7181.




cc:     J. A. Weigel, Bridge and Structures - 47340

         F. Higgins, Bridge and Structures - 47340