Comparison of Seismic Performance & Recovery Metrics for a 1970s vs Modern Tall Steel Moment Frame Building

This study benchmarks the performance of older existing tall steel moment resisting frame (MRF) buildings designed following historic code-prescriptive requirements (1973 Uniform Building Code) against modern design standards (2012 International Building Code). The comparison is based on risk-based assessments of alternative designs of a 50-story archetype office building, located at a site in San Francisco, CA. The following metrics are compared: (i) mean annual rate of collapse, λc (ii) average annual loss (AAL), and (iii) average annual downtime (AAD). The mean annual frequency of collapse of the the 1973 archetype building is 28 times greater than the equivalent 2012 building (28·10−⁴ versus 1·10−⁴), or approximately 13% versus 0.5% probability of collapse in 50 years. The expected AAL is 65% higher for the 1973 than the 2012 building (0.66% versus 0.40% of building replacement cost); and the AAD to re-occupancy is 72% greater for the 1973 than the 2012 building (8.1 vs 4.7 days). The AAD to functional recovery for the 1973 building is twice that of the 2012 building (10.4 vs 5.0 days). An evaluation of the results at various earthquake ground motion shaking intensities suggests that existing 1970s tall steel moment frames are far from complying with modern design requirements in terms of both collapse safety under extreme ground motions and damage control in small to moderate magnitude earthquakes. Furthermore, while modern building code requirements provide acceptable seismic collapse safety, they do not ensure a level of damage control to assure a swift recovery after a damaging earthquake.Applied Science, Faculty ofNon UBCCivil Engineering, Department ofReviewedFacult