Reappraisal of code provisions for structures subjected to seismic loading

Abstract

Seismic code provides criteria for the design and construction of new buildings subjected to earthquakes in order to minimise the hazard to life for all buildings, to increase the expected performance of higher occupancy structures as compared to ordinary structures, and to improve the capability of essential facilities to function after an earthquake. In the present study, efforts have been made to critically evaluate seismic code provisions and identifY areas in which investigations could be carried out to update the code provisions. Efforts have been made to formulate response spectra, which may be used in dynamic analysis. A suitable numerical coefficient have been derived from these response spectra for using in equivalent static analysis. Response spectra have been fonnulated using four soil types from rock to soft. For the development of the response spectra, several recent free field earthquake records have been used. In absence of free field earthquake records, earthquakes are simulated taking the appropriate properties of soils into consideration; To incorporate the effect of soil and the effect of attenuation in simulated earthquakes Kanai-Tajimi power spectra and Shinozuka- Sato (1967) envelope have been incorporated in the available subroutines. Perfonnance of the developed spectra have been tested against Uniform Building Code (1994) spectra by analysing various moderately high moment resisting framed structures. Additionally, a limited study has been conducted to calculate the predominant time period for Dhaka City soil to validate the applicability of the proposed simulated response spectra for Dhaka City soils. It has been found that for calculating the base shear in the equivalent static method, almost all codes of practice adopt similar defmitions for the various numerical coefficient of the base shear formula. It appears that further improvement in the equations pertaining to the calculation of time period of the buildings may not be rewarding. It has been found that the maximum amplitude of the acceleration spectra decreases as the soil changes from rock to soft type. It has been observed that largest amplification occurs at the vicinity of natural time period of soil. It has also been noticed that the rate of attenuation of rock spectra was faster than stiff soil spectra. Rate of attenuation of the spectra developed on the basis of recent free field earthquake records has been found to be faster than the spectra embodied in the present day codes. It is understood that efforts should be made to increase the value of structural damping so that amplification of seismic forces is less. It is imperative to install suitable number of seismic stations in Bangladesh so that local spectra based on site specific real earthquake records can be developed.