where � = c/2mw is the fraction of critical damping coefficient; it is a dimensionless measure of the damping coefficient c for the system. For brevity, we will refer to � as the damping ratio. Damping has the effect of decreasing the natural circular fre• quency of vibration and increasing the natural period of vibra-
tion. Fig. 10 shows that for damping ratios less than 0.2, a range which includes most structures, these effects are negligible, i.e., w0 is approximately equal to w and hence T0 is approximately equal to T.
The displacement amplitude decreases progressively because of damping. As shown in Fig. 9, this decay in amplitude is ex-