Design of Mass Isolated Structures with Consideration of Stability Constraints

Vertical mass isolation is one of the new techniques in the seismic design of structuresthat consists oftwo stiff and softsubstructures connected by viscous dampers. Adding to the flexibility and energy dissipation potential of the system is the main feature ofsome newapproaches in the seismic design ofstructures. Extra flexibility helps to reduce earthquake-induced forces and accelerations in the building and provides higher energy dissipation potential for the system (by creating large relative deformations in the structure). Mass subsystem possesses low lateral stiffness but carries the major part of the mass system. Stiffnesssubsystem, however, controls the deformation of the mass subsystem and attributes with much higher stiffness. In this paper, the aim is to find the limitation of the stability of a soft structure and to obtain the maximum period available for a soft structure. According to the studies, the most important obstruction in increasing the period of the soft structure, assuming control of its deformation by connecting to the stiff substructure, is to maintain the stability of the structure. In this paper, first, a relationship has been presented to calculate the period of the structure in terms of the stability factor that estimates the period of structure with good agreement by analytical results. This paper deals with presenting a procedure for designing the Mass Isolation System(MIS) with consideration ofstability constraints. To this end, the paper presents mathematical solutions to calculate the period of the structure followed by proposing a design procedure of the soft substructure.