DETERMINATION OF STABILITY MARGIN FOR FLEXIBLE SUBSTRUCTURES WITH LIMITED LATERAL DEFORMATION

A variety of new approaches in seismic design of structures are based on adding to the flexibility and energy dissipation potentials of a structural system instead of relying on its ductility capacities. In this case higher flexibility reduces the level of lateral force on the structural system and energy dissipation controls its lateral deformation duringearthquake excitation (Ziyaeifar, 2002). On the other hand, expanding the flexibility of a structural system is limited to its stability margin in sustaining vertical loads. In this work it is attempted to find the required level of stability margin for a flexible structure that its lateral deformation subjected to earthquake actions is restricted by damping forces. Such flexible structural systems (mass subsystems) are connected to a much stiffer structure (stiffness subsystems) by the means of large energy dissipaters for reducing their lateral deformation (Ziyaeifar et al., 2012). Similar arrangement of soft and stiff structural systems also exist in case of connecting two adjacent buildings with viscous dampers (Christenson et al., 2003; Yuji et al., 2004). Increasing the flexibility of structural system and its natural period reduces the stability margin of the system (asdescribed by Bernal, 1998). This can be understood from the inverse relationship between natural period and stability index of typical structural systems ( where T is natural period of the system and l is its stability index). Considering the role of lateral deformation of structural systems on their stability margin (due to P-D effect), the highestflexibility of mass subsystems has to be determined based on stability margin for the system at its expected lateral deformation (that is restricted by high damping forces).