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TO: M. M. Lwin
FROM: Chuck Ruth
DATE: August 16, 1995
SUBJECT: LRFD - Section 3.10.3 Importance Categories

The Seismic Steering Committee met on August 9 to discuss the subject section in the AASHTO LRFD Design Specifications. There was a lively discussion on this subject. The committee struggled with the implementation of this section. Committee members indicated that structures designated as "essential" or "critical" could require a substantial increase in substructure cost as compared to our current seismic design criteria. Some committee members felt that most or all of our bridges should be designated as the "other" category. Other committee members felt that classification of structures was beyond the ability of the committee and should be decided in coordination with other local agencies (cities and counties), and the military. They felt that the decision to specify a bridge as "essential" or "critical" should be made at the executive level of WSDOT, with an understanding that there could be a significant cost increase.

It was also felt that our current seismic design requirements are already conservative, and an added level of conservatism was not necessarily warranted. Incorporation of the "essential" and "critical" designations in the new LRFD Design Specifications for WSDOT bridges significantly increases our design forces and moments. The ‘essential" and "critical" designations in the new LRFD Design Code are an attempt to incorporate the concept of using a maximum credible event (2500 year return period) for design of a bridge designated as "critical" (see LRFD Commentary, Page 3-45 & 3-48). In accordance with acceleration coefficient maps developed b USGS in 1988 (based on historical EQ record, only), a design EQ in Washington with a return period of 475 years would have a maximum acceleration coefficient of 0.33g, whereas a design EQ with a return period of 2500 years would have a maximum acceleration coefficient of 0.66g for the same area ( a 100% increase). The point is that incorporation of the new LRFD Design Specifications (and Maximum credible even concept) may have a major impact on our bridge design costs.

The committee recommended that sample designs be performed with the same bridge utilizing the three designations (‘other", "essential", and "critical") before the final classification criteria is determined. In this way a true comparison could be made regarding the additional substructure cost. In summary, it was felt that the LRFD criteria could result in overly conservative seismic designs, if the "essential" and "critical" designations were liberally applied.


cc:     H. L. Coffman, 47340

         Y. A. Mhatre, 47340

         H. Henley, 47340

         P. Kinderman, 47340

         T. Moore, 47340

         Dornsife, 47340

         J. Wei, 47340

         H. Zhang, 47340