Seismic Upgrade of an Intake Tower Using Underwater Base Isolation,Preliminary Design;Vancouver Island, Canada

Abstract eng:
The Strathcona Dam located on Vancouver Island, in British Columbia, Canada, includes a 50 m high intake tower, critical to be fully functional following a severe earthquake to enable closing of the intake gates to ensure post-earthquake stability of the earth dam. The intake tower, constructed in 1956, consists of a 34 m high concrete portion (mostly underwater) and a 16m high steel superstructure. The seismic withstand of the existing tower is inadequate relative to the Maximum Design Earthquake (MDE) with PGA of 0.8g. The selected seismic upgrade scheme involves installing lead-rubber isolation bearings about 13 m below the water surface, along with local strengthening of walls, without lowering the water level of the reservoir behind the dam. This paper discusses the 3D nonlinear time-history analyses carried out, the selection of appropriate ground motion records for the analyses, the extensive full-scale testing of two isolators at the University of California, San Diego with six degrees-of-freedom characterization using motions computed from the project analyses, the preliminary design of the intake tower to accept the isolators, the design issues related to the uplift at the isolators for the upstream/downstream rocking response, and the preliminary design of other modifications to the tower to accommodate the more than 450 mm of expected lateral deformation of the portion of the structure above the isolators relative to the lower portion. It is understood that this project, if carried through to construction, would represent the first use in the world of continually submerged isolators for a seismic upgrade.

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