Assessment of ASCE 7-16 Seismic Isolation Bearing Torsional Displacement

摘要

Base isolation provisions in ASCE 7 Standard have been historically shown to be conservative in estimating seismic demands. New torsional bearing base isolation displacement expressions have been proposed recently by the latest ASCE 7-16 Standard. This study compares the accuracy of the new ASCE 7-16 static-based isolation expressions for additional bearing displacement due to plane torsion to the response obtained using simplified structural dynamics' plane torsion theory expressions. In addition, it conducts a parametric study to assess the effect of accidental eccentricity, damping ratio, and plan aspect ratio on the accuracy of ASCE 7-16 bearing displacement expression. The results showed that the ASCE 7-16 undamped displacement estimations improved compared to the significant conservatism of ASCE 7-10 by 7-33% depending on the eccentricity condition and ratio, which may further promote the use of base isolation in the US as a seismic hazard mitigation solution. However, the study also revealed that a considerable conservatism of the new base isolation bearing displacement provisions of ASCE 7-16 still exists in some cases, which ranged from 52 to 105% in the case of three equal fundamental frequencies and 5-20% in the case of equal lateral frequency and distinct torsional frequency. The study also showed that the ASCE 7-16 conservatism is proportional to the eccentricity and is more pronounced with biaxial eccentricity compared to single eccentricity. Furthermore, the results show that ASCE 7-16 expression accuracy significantly declines with damped systems with a possibility of un-conservative estimation of bearing displacements that can reach 25-40% with larger eccentricities. In addition, the ASCE 7-16 torsional displacement was shown to be inversely proportional to the plan aspect ratio with a discrepancy that can reach +/- 20%. The study also exhibited that the ASCE 7-16 torsional displacement conservatism is slightly affected by increasing damping ratio above 4%.