Decoupling of Secondary Systems for Seismic Analysis

Interest in the decoupling of such secondary systems as floor-mounted equipment centers on two decoupled models, the “cascading” model and the “lumped” model. A set of decoupling criteria is derived which addresses both models. The solution is based on the two-degree-of-freedom system. Error in predicted frequency given by each decoupled model is presented. Similar results for response error are derived based on broad-band input to the supporting system. The broad-band input would be applicable to subsystems supported by primary structures. Recommendations are made for extension of the problem to arbitrary multiple-degree-of-freedom systems. The findings of this study will provide the analyst with the information necessary to choose the best decoupled model and will indicate whether decoupling should be attempted.