Abstract | The seismic behavior of numerous single-degree-of-freedom (SDOF) systems is investigated through Incremental Dynamic Analysis (IDA), a computer-intensive procedure that offers thorough (demand and capacity) prediction capability by using a series of nonlinear dynamic analyses under a suitably scaled suite of ground motion records. The oscillators are of moderate period with pinching hysteresis and feature backbones ranging from simple bilinear to complex quadrilinear with an elastic, a hardening and a negative-stiffness segment plus a final residual plateau that terminates with a drop to zero strength. The results of the analysis are summarized into their 16%, 50% and 84% fractile IDA curves which are in turn fitted by flexible parametric equations. The final product is SPO2IDA, an accurate, spreadsheetlevel tool for Performance-Based Earthquake Engineering that is available on the web. It offers effectively instantaneous estimation of demands and global-instability collapse capacities, in addition to conventional strength reduction R-factors and inelastic displacement ratios, for any SDOF system whose Static Pushover curve can be approximated by such a quadrilinear backbone.