Summary | The influence of the elastic spectral shape on the limit-state capacities of a 9-story steel moment-resisting frame is investigated through the use of Incremental Dynamic Analysis (IDA). IDA is a parametric analysis method that has recently emerged to estimate more thoroughly structural performance under seismic loads. It involves subjecting a structural model to several ground motion records, each scaled to multiple levels of intensity (measured by the Intensity Measure, IM), thus producing curves of response parameterized versus intensity level, on top of which limit-states can be defined and corresponding capacities can be calculated. When traditional IMs, such as the peak ground acceleration or the 5%-damped first-mode spectral acceleration, are used, the IM-values of limit-state capacity can display large record-to-record variability. Thus, a large number of ground motion records has to be used to achieve a given level of confidence in the results. By testing a multitude of single spectral values as well as scalar combinations of spectral ordinates on the 9-story frame, several candidate IMs are found that significantly reduce such dispersion and consequently the needed number of records. Furthermore, such results are used to determine the most influential regions (or periods) of the elastic spectrum for each limit-state of the building. Thus, we are able to observe the evolution of such influential periods as the seismic intensity and the response of the building increase, from first yield all the way to global collapse. In conclusion, the ordinates of the elastic spectrum and the spectral shape of each individual record are found to significantly influence the seismic performance of the building and they are shown to provide promising candidates for new, more efficient IMs.