Seismic Demand in Columns of Steel Frames

Principal Investigator: Michel Bruneau

PhD Students: Lisa Shrestha

Abstract: Axial force demand for column design of a seismic load resisting systems is obtained by considering load transferred from all the energy dissipating elements assuming that they have simultaneously reached their ultimate capacity. Time history analyses from the past studies have, however, indicated that considering simultaneous yielding of all the stories is possibly overly conservative in tall structures and may result in large uneconomical sections. Yet, no studies have proposed a method to quantify the number of simultaneously yielding stories in buildings, other than by empirical observation of results from multiple non-linear inelastic time history analyses. This research is conducted in an attempt to provide one such quantification method.  Starting from the idea that an earthquake ground excitation can be represented by a pulse or series of pulses, concept of wave propagation is applied to investigate the response of a generic tall shear-type building to pulse base excitation. Relationship between the value of the velocity wave propagating along the building and the inter-story drift of the building is obtained, and mathematical expressions are derived to predict the beginning and end of the story yielding due to the incident wave in a shear-type building subjected to full-sine velocity pulse base excitation. Using these expressions, number of simultaneously yielding stories NSYS at any time as the incident wave propagates up the building can be obtained.