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TO: To All Design Section Staff
FROM: Chuck Ruth
DATE: August 10, 1995
SUBJECT: Minimum Velocity-Related Acceleration Coefficient for Western Washington

In the last few years, there has been increased discussion regarding the geologic evidence of pervious "great quakes" in the Pacific Northwest. Evidence that "great quakes" have occurred in the past and that one will occur sometime in the future is relatively certain. The two main issue now being debated are what is the reoccurrence period and what is the likely magnitude?

Since it may be years before the scientific community reaches agreement on these two issues, an interim policy needs to be implemented. For design starts that began August 1, 1995 and thereafter, all structural designs in Western Washington shall utilize a minimum Velocity-Related Acceleration Coefficient of 0.2. This policy applies to all structures West of the Cascade Crest (West of MP 162 on SR 20, West of MP 65 on SR2, West of MP 52 on SR 90, and West of MP 63 on SR 14). If the bridge being designed is located West of the Cascade Crest, the value used for design would be the interpolated value from the coefficient map (BDM, Figure 4.1.5-1) or 0.2g, whichever is greater.

This temporary policy supersedes the seismic coefficient map (where applicable) in Section 4.1.5 of the BDM.



cc:     M. M. Lwin, 7340

         R. George, 7340

         E. H. Henley, 7340

         R. B. Stoddard, 7340

         F. Higgins, 7340

         T. Allen, 7365