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8 <br /> SCOPE OF ENGINEERING WORK <br /> PORTAL 3 RELOCATION AT CHAPLAIN RESERVOIR <br /> PHASE 05: SEISMIC STABILITY ASSESSMENT <br /> For <br /> CITY OF EVERETT DEPARTMENT OF PUBLIC WORKS <br /> By <br /> URS CORPORATION,SEATTLE,WASHINGTON <br /> Draft September 9,Rev.0 <br /> INTRODUCTION <br /> URS will complete the seismic stability assessment for the existing Portal 3 at Chaplain <br /> Reservoir in a phased manner as follows: <br /> • Phase 05A— Seismic Stability Assessment <br /> • Phase 05B—Cracked Response Analysis(if necessary) <br /> Phase 05B will only be completed if the Phase 05A results show that the structure is marginally <br /> stable and additional analyses are needed to confirm the stability. URS has used this approach on <br /> many seismic stability projects, including three local seismic upgrades for intake/outlet structures: <br /> • Tolt Intake Tower, City of Seattle <br /> • Culmback Dam Intake Tower, Snohomish County PUD <br /> • Casad Dam Intake Tower,City of Bremerton <br /> PROJECT APPROACH <br /> URS will complete the seismic stability assessment in accordance with the International Building <br /> Code (IBC 2003)that has been adopted by City of Everett with State of Washington amendments. <br /> The IBC replaced the Uniform Building Code (UBC). The IBC determines earthquake loads or <br /> ground motion differently than the UBC, and could result in different seismic loading. However, <br /> the determination of site-specific design spectrum is mostly independent of these codes. <br /> Several factors form the basis of seismic design including ground motion, foundation materials, <br /> structure stiffness, structure mass, and structure performance if damaged by seismic forces. <br /> Ground motion is a critical factor,represented by a response spectra curve, or idealized ground <br /> acceleration based on the fundamental period of the structure.The fundamental period is the time <br /> for the natural vibration of the structure to complete one cycle.Rigid structures vibrate quickly <br /> and have smaller fundamental periods. Flexible structures have longer fundamental periods. IBC <br /> defines structures having a natural frequency larger than 1 Hz(period less than I second)as rigid. <br /> A site specific seismic hazard analysis was completed by Dames&Moore(now URS)in 1998 <br /> for the seismic upgrade of South Dam at Chaplain Reservoir.The soil conditions at the dam and <br /> Portal 3 are very different.The dam is on potentially liquefiable loose to medium dense sand over <br /> dense silty sand. Portal 3 is on dense silty sand and gravel over bedrock.The response spectra for <br /> 27 <br />