Seismic Safety Assessment of Gated Spillways

This paper presents an innovative three-dimensional performance-based earthquake engineering assessment methodology for gated spillways. Several functionality and structural stability limit states are defined. The functionality of the gates is tackled by assessing seismic demand-to-capacity indices along a success path leading to postearthquake gate opening. As an application example, the proposed methodology is applied to an existing gated spillway built in 1927. Finite element incremental dynamic analyses are performed for six sets of spectrum compatible ground motions with return periods ranging from $200\text{to}\mathrm{10,000}\text{years}$ . Linear transient dynamic analyses are first performed to compute internal force resultants, displacements, and distortions in structural components. For the concrete structures, the forces from the analyses are input in a postprocessor to compute sliding safety factors using a three-dimensional extension of the gravity method accounting for concrete cracking. The assessment of the hoist supporting structure and gates is done using current steel member demand-to-capacity checks. The actual failure mode hierarchy is identified and found unsatisfactory as the concrete structures and steel lifting tower anchors are expected to fail prematurely. Specific performance criteria and related ground motion return periods are proposed (1) to maintain normal gate opening; (2) to maintain the load bearing capacity of the steel towers and bridge; (3) for life safety of personnel; and (4) to avoid an uncontrolled release of the reservoir.