Seismic Analysis of Concrete Gravity Dams With Keyed Contraction Joints

Abstract

<p>Most of the design of concrete gravity dams is based on a two dimensional analysis which is suitable for monoliths with smooth, unkeyed, contraction joints. However, when keyed contraction joints are used, it is expected that the dam monoliths will interact in a manner that may affect the overall response of the system. The objective of this study is to investigate the seismic behaviour of concrete gravity dams, built with keyed contraction joints, including the effect of monoliths interaction. The scope of research work included: 1) development of a simplified procedure to investigate the effect of monoliths interaction on the seismic behaviour of the structure, 2) after this was proved to be important, a detailed and comprehensive procedure was developed and 3) investigation of the parameters which affect the response of the structure and significantly influence monoliths interaction.</p> <p>In the simplified procedure, each monolith of the dam is modeled using beam elements which has the advantages of keeping the number of degrees of freedom to a minimum and being available in most of structural engineering computer codes. The approximate added mass technique is used to simulate the hydrodynamic effects when the dam and reservoir are subjected to an earthquake ground motion. The importance of including monoliths interaction is illustrated by analyzing different cross-sections of concrete gravity dams.</p> <p>In the proposed detailed analysis of gravity dams, a substructuring technique is employed to model the structure. The dam is divided into a number of substructures equal to the number of monoliths. Each monolith is then reduced to a few degrees of freedom on the upstream face and Ritz vectors are used to represent internal degrees of freedom. The analysis is carried out in the frequency domain to include the frequency-dependent terms which appear when including reservoir-darn-foundation interactions. The results obtained are compared to typical three-dimensional analysis and a good agreement is obtained. It is noted that the importance of monoliths interaction is dependent on two factors; 1) the type of contraction joints used in construction and 2) the longitudinal profile of the dam.</p> <p>The effect of monoliths interaction is to increase the natural frequencies of the structure and as a result leads to a change in its overall response. It is concluded that in many cases the effect of monoliths interaction is important and should be included in the analysis. The geometry and material properties of contraction joints have a significant effect on the overall response of the dam. Depending on the crack width, the shear behaviour of the joints varies widely. The longitudinal profile of the dam was also found to have a substantial effect on monoliths interaction. Important variations in the response of the dam, from that calculated using typical two-dimensional analysis, is obtained for some cases. This is usually the case for short dams with rigid monoliths at the sides or for dams built in steep canyons.</p>