A COMPARATIVE STUDY ON THE SEISMIC VULNERABILITY OF 1970S VS MODERN TALL STEEL MOMENT-RESISTING FRAME BUILDINGS

This paper outlines the methodology followed in the risk-based assessment of two archetype tall buildings in downtown San Francisco: a 50 story steel moment resisting frame (MRF) office building designed following the requirements of the Uniform Building Code of 1973 and a 50 story steel MRF office building designed following modern code requirements (International Building Code 2012). The methodology enables the development of the vulnerability function for the archetype buildings under consideration, highlighting loss contribution from (1) collapse, (2) irreparable damage from excessive residual deformations and (3) reparable damage. The goal of this study is to benchmark the performance of older existing steel MRF buildings against modern designs, providing an overall comparison of their seismic vulnerabilities. The results illustrate that existing tall steel MRF buildings from the 1970s are drastically more vulnerable to earthquakes than tall steel MRF buildings designed to modern standards. The vulnerability function for the 1970s archetype building highlights that collapse potential is the highest contributor to the losses, with a collapse fragility characterized by a relatively low median spectral acceleration. The resulting vulnerability function of the modern archetype building indicates that: i) at low ground motion intensities of shaking, losses are influenced by repairable damage; ii) at medium intensities of shaking losses are equally dominated by repairable damage and residual drift rendering the building irreparable; iii) collapse only starts contributing to the loss at large spectral amplitudes, but even then losses are largely dominated by residual drifts. The collapse fragility of the modern archetype building is in agreement with the design objective of modern building codes, which is to produce designs with low probability of collapse under a Maximum Considered Earthquake.