Performance Quantification of Tall Steel Braced Frame Buildings Using Rupture-To-Rafters Simulations

What is the probability of collapse of tall steel braced frame buildings under San Andreas earthquakes in the next 30 years? Using more than 38000 ground motion histories from simulations of 60 scenario earthquakes on the San Andreas fault with magnitudes in the range of 6-8, 3-D nonlinear analysis of several variants of an 18-story steel braced frame building is conducted to address this question. Results are combined with the 30-year probabilities of the scenario earthquakes using the PEER performance based earthquake engineering framework to determine the exceedance probabilities of five performance limit states over the next 30 years.

3-component ground motions from the 60 scenario earthquakes, rupturing at five distinct fault locations, with two directions of propagation are generated. 30-year time-independent probabilities of occurrence for the scenario earthquakes are estimated using UCERF. For each scenario earthquake, 3-component ground motion time histories at 636 target sites on a 3.5km grid in southern California are computed using earth models. These ground motions are used to perform 3-D nonlinear response analyses of several site-specific variants of an 18-story steel braced frame (BF) building, a redesigned version of an existing 18-story steel moment frame (MF) building whose connections fractured in the 1994 Northridge earthquake. Model performance is classified into Immediate Occupancy IO, Life Safety LS, Collapse Prevention CP, Red-Tagged RT, and Model Collapse CO. The results are combined with the 30-year probability of occurrence of the scenario earthquakes using the PEER performance based earthquake engineering framework to determine the probability of exceedance of these limit states over the next 30 years.

It is found that model collapse probabilities in the next 30 years range from 1.71% (1997 UBC design) to 3.34% (1994 UBC design), both significantly higher than code's intent of limiting collapse probability to 2% in 50 years. Clearly, the 1997 UBC has been quite effective in lowering the collapse risk to the braced frame buildings. Collapse probabilities in Downtown LA are 3.5% and 2.3% for the 1994 and 1997 BF designs, and 5.3% for a 1997 MF design. These would likely be higher if all regional faults (not just the San Andreas) or time-dependent earthquake probabilities are considered.