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TO: All Design Section Staff
FROM: Chuck Ruth
DATE: February 22, 1999
SUBJECT: Peak Ground Accelerations Used For Bridge Design

The 1996 USGS Peak Ground Acceleration Map (475-Year Event) was incorporated into our bridge design process on August 8, 1997. Based on recent seismological discoveries of additional seismic hazards in Washington State, Section 4.1.5 of the BDM is modified as shown below:

4.1.5 Earthquake Loads

Use the USGS Peak Ground Acceleration Map (Figure 4.4-A2, 10% Probability of Exceedance in 50 Years) to obtain an acceleration coefficient for preliminary design. The project Foundation Report will contain the acceleration coefficient to use in the final design of the bridge. When using Figure 4.4-A2, interpolate between contours to find the value to use for a particular site, and round to the nearest 1% of gravity(g). In general, Figure, 4.4-A2 can also be used for bridge seismic retrofit designs. However, seismic evaluation and retrofitting of older bridges can sometimes result in excessive costs (the retrofit costs are not consistent with the benefit gained). In these cases, the Bridge Design Engineer should be consulted for direction. All structural designs in Western Washington shall utilize a minimum acceleration coefficient of 0.2g. This 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; West of MP 69 on SR 410; West of MP 151 on SR 12; and West of MP 63 on SR 14).

This new seismic design policy shall be implemented immediately for all new bridge designs scheduled to begin.


cc:     J. Weigel, 47340

        Coffman, 47341

        T. Allen, 47365