Comparing Seismic Retrofit Design via RSA and NLTHA for a Multi-Story Non-Ductile Reinforced Concrete Building in Downtown Los Angeles

摘要

A comparison of the computed structural response, of a retrofitted 15-story non-ductile reinforced concrete building in downtown Los Angeles (LA), by means of a Response Spectrum Analysis (RSA) and a Non-Linear Time-History Analysis (NLTHA), is presented. The RSA approach is compliant with the LA Building Code (LABC) seismic provisions and its mandatory ordinance for earthquake hazard reduction of existing non-ductile concrete buildings. The NLTHA performance-based approach is compliant with ASCE 41-13 requirements. In the RSA approach, the strength of the lateral-force resisting system was designed to 75% of the base shear specified in the current LABC. For the NLTHA performance-based approach, the performance objective was the BPOE, i.e. Life Safety and Collapse Prevention under the Basic Safety Earthquakes (BSE-IE & 2E respectively). Thirteen (13) earthquake records, suitable for sites in downtown LA, were selected and scaled to the BSE-1E & 2E response spectra using amplitude matching per ASCE7. According to the results, the calculated base reactions at the foundation, and story drifts along the height of the building, were generally lower for the NLTHA approach. The results infer lesser demands can be achieved by means of a NLTHA. A further comparison of the results is discussed within the scope of a probable maximum loss (PML) analysis. The loss estimate results were compared between using structural responses from the NLTHA and approximated responses based on FEMA P-58 prediction methods. The estimated mean loss or Scenario Expected Loss (SEL) for a 10% in 50-year earthquake was approximately 26% (for the existing 1927 building) and 8% (assuming ASCE7-10) for the prediction method and 14% for NLTHA. The results show a clear difference in the building response, when comparing inter-story peak drift demands, which has a significant impact on loss estimates. The retrofit designed per the RSA approach meets the BPOE performance objective through the NLTHA. Most new strengthening elements are well below the acceptance criteria which suggests that a more cost-effective strengthening solution may be found, however this needs to be verified with a NLTHA-only approach.