Abstract: Dynamic response of a light equipment item attached to a non‐linear and torsionally coupled main system is evaluated under bi‐directional earthquake excitation. To account for the effect of translations and torsion, each story of the building is modelled as three degrees‐of‐freedom (DOFs), with two DOFs for translation in two orthogonal directions and third DOF for torsion. The responses (relative displacement between the equipment system and the floor of the primary system on which the equipment system is mounted and absolute acceleration of the equipment system itself), are determined under random ground motion in two orthogonal directions, which is idealized as a stationary random process represented by a white noise excitation. The responses are obtained by time domain simulation procedure. The response behavior of the light equipment is examined under a set of parametric variations. These parameters include the uncoupled lateral frequency of the primary and the equipment systems, the ratio of uncoupled lateral to rotational frequencies of the primary system, eccentricity ratios of the primary and the equipment systems in X and Y directions, damping ratio of the primary and the equipment systems and the mass ratio of the two systems. Results of the study indicate that under some parametric conditions the responses of the equipment system are significantly affected by torsional coupling and non‐linearity of the primary system. It is also observed that the responses of the equipment system can be alleviated by increasing the damping ratio of the equipment system.
