Finite element seismic safety assessment of water intake structures

Abstract

This paper presents a methodology to assess the seismic safety of concrete gravity water intake structures of typical hydroelectric facilities. Water intake structures are characterized by large voids for the gates and penstock embedded in concrete. In practice, the well-known Westergaard formulation (WF), assuming a vertical rigid wall, is most often used to represent hydrodynamic pressures. However, the validity of the WF for water intake structures has not been addressed in the past. A parametric analysis is performed herein using four 40m high intake structures with upstream opening void ratio, χ, ranging from 0% to 30% of the concrete surface in contact with water. Three-dimensional finite element models (FEM) with potential-based incompressible fluid elements are used in steady-state and transient seismic fluid–structure interaction (FSI) analyses to consider water in the penstock as well as in the reservoir. Modification factors which depend on χ are derived from the 3D FEM such that a Westergaard modified formulation (WMF) is proposed to represent adequately FSI. Simplified structural models using beam-column elements with section properties accounting for the presence the penstock opening and the proposed WMF are used as an efficient alternative to complex 3D FEM. A seismic safety assessment of an intake considering ground motions of return periods ranging from 200 to 10,000 years are used to assess the safety level of the intake structure. The internal forces and residual sliding displacements are computed. It is shown that the proposed WMF and the simplified stick model formulation provide hydrodynamic thrust within approximately 10% of the reference 3D FEM. The seismic response (i.e. base shear) is also adequately predicted using the proposed simplified modeling strategy.