DYNAMIC SOIL-STRUCTURE INTERACTION INCLUDING SLIP AND SEPARATION

Abstract

In Nuclear Power Plants, the equipments located on various floors of Reactor Building are designed for floor motions. Inclusion of soil-structure interaction modifies the response of the structure as well as the response of various floors. The Nuclear Reactor Buildings tend to be very massive and stiff, and frequently they have to be located on relatively softer soils. The slip and separation at the base and the embedded sides may significantly influence the earthquake response of the building. The study of dynamic soil-structure interaction behavior allowing slip and separation at the soil-foundation interface under severe earthquake motion has been carried out in the present work. The incorporation of interface between the reactor and the surrounding soil plays an important role to account for slip and separation phenomena. The tensile forces are not transmitted between the planes representing structure and the soil. The dynamic properties governing the sliding have been determined by the Mohr-Coulomb failure law from the adhesion and the friction angle between the soil and the structure. The modelling of dynamic soil-structure interaction (DSSI) is carried out by using finite element approach. The structure and soil are modelled by 4-noded finite elements and the interface by a linear interfacial finite element. A stiffness degradation model has been used which includes the adhesion at the interface. The shear and normal stiffness parameters of the model are estimated logically. The adopted model and the procedure for estimation of joint stiffness have been validated by numerical simulation of experimental results. The procedure of estimation ofjoint stiffness is general and can be employed for any soil-structure interaction problem. Formulation of dynamic problem by finite element method results into aset ofdifferential equations. The temporal discretization reduces the differential equations into a set of simultaneous equations. Adirect time integration scheme has been used to solve the simultaneous equations.

Two reactor buildings are considered for the slip and separation studies under lateral loads. The first building is assumed to be resting on ground surface whereas second building is analyzed with about Va embedment and as located on ground surface. It is observed that the maximum vertical stresses are reduced at heel and increased towards toe in reactor as well as soil. In case of embedded model the separation and loss of contact at the reactor base reduce to 12.2% and 27.1% respectively in comparison with Reactor Building located on ground surface. The seismic response studies of the first building are carried out under North-South and vertical components of El-Centro earthquake motion simultaneously. The response of second building is carried out under the transverse and vertical components of Uttar Kashi (India) earthquake motion. Rayleigh damping parameters are computed for 10 %equivalent damping in the considered first and second modes. In order to compare the responses under severe earthquake motion the buildings are subjected to 1.5 times the respective earthquake motions. The implicit time integration scheme has been used with a time step of 0.00025s. The slip and separation increase with the increase in the earthquake intensity and significantly reduce with the embedment. The stress responses are considerably affected with the inclusion of interface, severity of earthquake motion and embedment.