Torsional Coupling in Seismic Response of Reactor Systems

Abstract

<p>The design of a nuclear power station to resist the effects of a strong motion earthquake represents one of the most significant considerations confronting, the electric power generating industry today. The prevailing view in the nuclear industry is that structures are designed to remain essentially elastic and functionally-important equipment to remain fully functional during and/or after an earthquake.</p> <p>In seismic analysis of reactor buildings it is usual to consider planar models along each of the two principal axes, and to independently analyse the response of each model to the in-plane horizontal component of ground motion. Analysis on this basis is strictly valid only for structures with coincident centers of mass and rigidity. The lateral and torsional motions of the structure are coupled if the centers of mass and rigidity do not coincide.</p> <p>It is the purpose of this thesis to consider the torsional effect in the seismic analysis of Nuclear Power Plant Reactor Systems and to illustrate the effect of the lateral-torsional coupling on the equipment response. The equipment response is represented by floor response spectra. It is usually impractical to include such equipment in the dynamic model representing the building structure because of the large difference between the mass of the equipment and that of the building. Therefore, the equipment and the building are treated separately and the building response are used as inputs for the equipment analysis.</p> <p>A torsionally coupled reactor model is developed considering the effect of eccentricities between the center of mass and the center of rigidity for each floor level of the corresponding uncoupled model, and a detailed coupled analysis is investigated. To consider the effect of torsion, lateral floor spectra are developed for more than one location on each floor level. Uncoupled and coupled lateral floor spectra are presented for excitation due to several different earthquakes with the objective of evaluating the effect of torsional coupling and its influence on the equipment response.</p> <p>The second object of this study is to develop a simple procedure to compute floor response spectra of the torsionally coupled reactor building without a time-history analysis. And finally, the effect of torsional ground motion is investigated, in which, a rotational time-history ground motion is generated in addition to the recorded lateral component and these two time-history excitations are used as input motions applied at the base of the torsionally reactor building. The floor response spectra are determined and analysed with the objective of evaluating the influence of the estimated torsional ground motion on the response parameters.</p>