The Benefits of Linear Time-History Analysis in Consulting Practice and the Precision of the Code Calculated Period

Abstract eng:
Seismic design of structural systems has long been recognized as the component of the design posing the highest levels of risks, challenges and uncertainties to the design. A common code-based seismic design approach for determining the seismic loads for regular structures is the equivalent static force procedure (ESFP). Like all simplified methods, the ESFP has limitations. Some of the limitations of the ESFP include the estimation of the fundamental period of the structure, excluding the contribution of higher modes, deficiency in providing means for calculating response quantities other than force and displacements, and failure to account for the multi-directionality nature of seismic events. Linear dynamic methods of analysis such as linear modal response spectrum analysis (MRSA) and linear time history analysis (LTHA) have been used to reduce the uncertainties and reduce the limitations associated with the ESFP. Although practicing time history analysis has been commonly incorporated in research, it is not fully recognized as an effective analysis method by structural engineers. The increased processing power of computing hardware and the availability of progressive computer aided design tools has diminished the computational effort involved in performing time history analysis for the design and retrofit of structures. Therefore, time history analysis can now be incorporated in the industry as one of the primary methods of analysis, leading to more economical and efficient designs. This paper outlines the benefits of incorporating LTHA, as the primary method of analysis, in seismic design. The emphasis is on regular structures, for which the codes of practice do not require a dynamic analysis for. First a large building database has been studied. The empirical periods, as suggested by the codes of practice, and the actual analytical periods are compared and evaluated. Next, available guidelines for adopting a linear time history analysis as the method of analysis and the selection and scaling of ground motions, provided by the codes of practice, are reviewed. The results of 4 case study buildings analyzed and retrofitted using LTHA is presented. The buildings vary in sizes and lateral load resisting systems (LLRS) and were designed according to the provisions of the 2010 National Building Code of Canada (NBCC 2010) [1]. The seismic base shears obtained by doing a LTHA are compared to the corresponding values determined from code-based ESFP and MRSA. Finally, the advantages and limitations of doing a LTHA are evaluated.

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