Damping for the Nonlinear Static Procedure
Abstract
The Nonlinear Static Procedure in ATC 40 is based on the capacity spectrum method. With this approach the deflection of a yielding structure resulting from seismic excitation is assumed to be equal to that of a viscously damped linear model with the secant stiffness of the yielding structure. The required level of damping is based on the "equivalent" viscous damping theory of "Jacobsen", in which the damping level is determined by equating the energy dissipated viscously in the linear model in a cycle to that dissipated by the yielding structure when subjected to the same maximum displacement. As it was found that equivalent viscous damping is not totally appropriate for all cases, the ATC 40 procedure multiplies the equivalent viscous damping value by an empirical factor, ?, which varies with ductility level and hysteretic form.