Quantication of Seismic Performance Factors for Concrete Filled Steel Tube Diagrid Structural System

Abstract

The structural system designed using linear methods of analysis responds in inelastic range under the effect of actual seismic event. For estimation of strength and deformation demands of structural systems, Seismic Performance Factors (SPF) are used in current building codes and standards. Many new structural systems have never undergone any substantial earthquake shaking. As a result, their seismic design performance objectives and seismic response characteristics, both are untested and unknown. As a result, it is necessary to determine the seismic response factors of newly emerging seismic force resisting systems proposed for inclusion in building codes, which will lead to equivalent safety against collapse during earthquake when incorporated in the seismic design process.

The objective of present study is to understand the process of determining seismic response factors for structural systems. For the quantification of the seismic performance factors for structural systems, FEMA P695 guidelines are used. For modeling and numerical analysis, Open System for Earthquake Engineering Simulation (OpenSees), an open source software is used. Step-by-step procedure for modeling frame structure using OpenSees is presented in this report. Nonlinear static analysis is carried out to obtain the overstrength factor and period based ductility. Incremental dynamic analysis and fragility analysis is carried out to evaluate the response reduction factor. For the verification of process, an illustrative example of 4-storey steel moment frame available in FEMA P695 is solved and the analysis results obtained from the present study using OpenSees software are compared with the same. Seismic performance factors such as overstrength factor, period based ductility and response reduction factor are evaluated for concrete filled steel tube (CFST) diagrid structural system based on FEMA P695 guidelines. For evaluation of seismic response factors, performance group consisting of 4, 8, 16 and 24 storey CFST diagrid building with the plan dimensions 18 m × 18 m is considered. The inclination of diagonal columns on periphery of building is considered as 50. For modeling and numerical analysis, OpenSees software is used. Nonlinear static analyses are performed to obtain overstrength factor and period based ductility. Nonlinear static analysis results indicate that as the height of building increases overstrength factor decreases and period based ductility increases. Incremental dynamic analyses are performed to obtain collapse margin ratio (CMR) for each building. Adjusted collapse margin ratio (ACMR) is calculated as the product of collapse margin ratio and spectral shape factor which takes into account the effects of spectral shape of different ground motions. The values of calculated ACMRs are compared with the acceptable values given in FEMA P695 to assess reliability of presumed seismic performance factors of diagrid system. Incremental dynamic analysis results indicate that the value of response reduction factor (R) equal to 5 used for designing the CFST diagrid structural system satisfies the acceptance criteria of FEMA P695. The outcome of present study will be useful to professional structural engineers, while seismic design of structural system, not covered in present code of practice.