Translational and rotational excitation for the seismic analysis of base-isolated structures

The development of a complete ???6-dimensional??? seismic input model is addressed for the analysis of large structures resting on rigid mat foundations. The topic is first illustrated within the framework of the linearized theory of dynamic soil-structure interaction and the nature of the kinematic transformation between the free-field seismic motion and the motion of the rigid mat is cleared and discussed. On this basis, the complete spectral description (direct and cross power density functions) of the 6D motion of the rigid mat is derived from the space-time stochastic model of the free field displacement. To this aim different forms of the coherency function relating the horizontal and vertical motion at a point are proposed and tested. The rigid-mat excitation model is then applied to the dynamic analysis of the reactor building designed within the IRIS international project, this being a classical example of rigid system, i.e. of a structural system which, once base-isolated, tends to behave as a rigid body. The effect of the rotational input components is studied, by a classical random vibration approach, in view of characterizing their effect on the isolators, especially in term of imposed relative displacement and of applied axial forces. The result show how the torsional rotation input has a negligible effect on the horizontal relative displacement of the isolators, while rocking inputs are responsible for a more significant increase in the axial load variation. The effect of the model assumed for the correlation between the horizontal and vertical free-field motion at a point is also enlightened.