CS27B - Concurrent Session 27B: Time-history analysis of Pyramid Dam Outlet Works Intake Structure as a basis for the future risk assessments.

In this paper, the seismic analysis of the Pyramid Dam Outlet Works Intake Structure (OWIS) is presented. The purpose of the analysis is to evaluate various Potential Failure Modes (PFMs) for the structure as well as to estimate the threshold earthquake beyond which the structure exhibits a nonlinear behavior. Pyramid OWIS (owned and operated by the California Department of Water Resources (DWR) is a fully submerged reinforced concrete tower located near the right abutment of the Pyramid Dam in Los Angeles County, California. It is 119-feet tall and has an internal diameter of 15-feet. The intake structure is a critical appurtenant structure for the management of the reservoir levels during flood events and for reservoir drawdown in the event of damage to the dam due to a large seismic event. Given that the current codes and guidelines have more stringent requirements related to the ductility of intake tower structures, most of the 1960’s vintage intake towers like that of Pyramid Dam are lightly reinforced, i.e., they fail in a non-ductile manner. Further, considering the significant increase in the seismic forces compared to those used for the original design of the old intake towers, these structures do not meet the performance requirements of the current codes and guidelines for the seismic loading. Per the recent Level 2 Risk Assessment (L2RA) for Pyramid Dam, failure of the submerged OWIS due to an earthquake was estimated to be a “damage state”, and not lead to direct loss of life. In this work, the seismic performance of the Pyramid OWIS is checked against the requirements of US Army Corps of Engineers guidelines and American Concrete Institute code, the results of which will be used by DWR for their future risk assessments for this facility.