Structural Analysis of Shasta Dam Raise

The Bureau of Reclamation performed a static and dynamic structural stability analysis for a proposed raise of Shasta Dam, a curved gravity dam in a high seismic region in northern California. This paper discusses the structural analyses for the proposed dam modifications, which include a 20-foot raise of the non-overflow section, an eight-foot raise of the spillway, removal of existing spillway piers, construction of new spillway piers, and replacing existing drum gates with sloping fixed-wheel gates. EAGD_SLIDE, a linear-elastic finite element program is used to perform two-dimensional time-history analysis of the existing and raised dam conditions. Accelerations at the top of the dam from the EAGD_SLIDE analysis and cross-canyon accelerations for a 10,000- and 50,000-year seismic event are used as input accelerations for a three-dimensional linear-elastic time-history analysis of the spillway piers using SAP2000, which captures biaxial bending and shear forces. Time-history of axial and bending forces is compared with the resistive capacity of the reinforced piers within program PCAColumn. A sloping-fixed wheel gate is proposed as an alternative to radial spillway gates because of the large hydrodynamic forces induced on the gates and piers. A risk analysis of existing and raised conditions is used to develop a feasibility level design with appropriate risks. Post-tensioned tendons in both the non-overflow section and spillway piers are needed to meet Reclamation risk guidelines. 10 pp., 7 references, 4 figures.