Structure-specific, multi-modal and multi-level scalar intensity measures for steel plane frames

Abstract

Two new structure-specific scalar intensity measures for plane steel frames under far-field earthquakes are proposed. These intensity measures of the spectral acceleration and spectral displacement type are multi-modal as they take into account the effect of the first four natural periods and multi-level as they are defined for four performance levels and consider inelasticity and period elongation up to the collapse prevention level. This is accomplished with the aid of the equivalent modal damping ratios of a structure previously developed by the authors for performance-based seismic design purposes. These modal damping ratios are period, soil type and deformation dependent and associate the equivalent linear structure to the original nonlinear one. The proposed intensity measures are conceptually simple, elegant and include all the aforementioned features in a rational way without artificially combining terms, defining period ranges and adding coefficients to be determined by optimization procedures as it is the case for all the existing measures, which try to take into account more than one mode and inelasticity. Comparison of the proposed intensity measures against some of the most popular ones existing in the literature, with respect to efficiency (β), practicality (b), proficiency (ζ), sufficiency in terms of seismic magnitude (M) and source-to-site distance (R), scaling robustness and the range of their values at any damage or performance level demonstrates their very good performance as indicators of the destructive power of an earthquake.