Robert McNamara
RJ McNamara LLC, Philadelphia

A system composed of an arrangement of energy dissipating devices (shock absorbers) that reduces the dynamic wind or seismic response of a structural system is becoming a very popular and economical method to improve dynamic performance of modern buildings. The system creates a set of dynamic “resistive forces” through the generation of velocity dependent forces. Much of the literature relevant to the design of these systems has been aimed at describing their effect on the building behavior as “equivalent damping”. This presentation addresses the actual action of the shock absorber system on the structure thru a force-related interpretation, providing the designer a more familiar aspect of the action. By observing the force nature of these systems, one can get a better understanding of how the response reduction is achieved and an insight to the conceptual design of the resistant system.The importance of the local load paths and the time effect of the shock absorbers is also explained.

A detailed look at the force behavior of the combined structural lateral force-resisting system and the dynamic force resistive system (derived from the shock absorber supplement) with a simple model demonstrates the important aspect of the viewing of the force nature of the response reduction developed. Some interesting aspects of the force approach in the layout of the resisting system enable new layouts to be achieved.