SEISMIC DESIGN CONSIDERATIONS FOR SECONDARY STRUCTURAL SYSTEMS

In practice, light secondary systems attached to the massive primary system are treated independently in the seismic design. Evaluation of the secondary systems and their design forces are performed as equivalent static lateral forces applied to the approximate centers of gravity of the systems being analyzed. These design forces, since they are statics based, do not lend themselves to providing realistic design guidelines of secondary systems. In this paper, possible dynamic design considerations for primary-secondary (P-S) systems are studied. A simple yet efficient design procedure is developed. It is shown that, while an increase in damping ratio of the secondary system can always decrease the relative displacement between the primary system and the secondary system, there is an optimum damping ratio for minimizing the maximum acceleration of the secondary system. Compromises thus need to be made when there is a conflict in achieving the best global secondary-system performance. For illustration purposes, a series of design surfaces for different system configurations under the case of white-noise input are presented for obtaining the lowest maximum response of secondary systems.