Comparative risk-based seismic assessment of 1970s vs modern tall steel moment frames

This study benchmarks the performance of older existing tall steel moment resisting frame buildings designed following historic code-prescriptive requirements (1973 Uniform Building Code) against modern design standards (2015 International Building Code). The comparison is based on seismic risk assessments of alternative designs of a 50-story archetype office building, located at a site in San Francisco, CA. The mean annual frequency collapse risk of the 1973 building is 28 times greater than the equivalent 2015 building (28 × 10^{-4} versus 1 × 10^{-4}), or approximately 13% versus 0.5% probability of collapse in 50 years. The average annual economic loss (based on cost of repair) is 65% higher for the 1973 as compared to the 2015 building (0.66% versus 0.40% of building replacement cost). The average annual downtime to re-occupancy for the 1973 building is 72% longer (8.1 vs 4.7 days) and to functional recovery is about 100% longer (10.4 vs 5.0 days). Building performance evaluations at the design basis earthquake (DBE) and the maximum considered earthquake (MCE) shaking intensities further suggest that 1970s tall steel moment frames have much higher risks of collapse under extreme ground motions and risks of damage and building closure in moderate earthquakes. Furthermore, while modern building code requirements provide acceptable seismic collapse safety, they do not necessarily ensure a level of damage control to assure a swift recovery after a damaging earthquake due to extensive downtime. A set of vulnerability functions are proposed for both archetype buildings considered in the assessment