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MODE ACCELERATION APPROACH FOR GENERATION OF FLOOR SPECTRA INCLUDING SOILSTRUCTURE INTERACTION
"... A mode acceleration formulation is presented for the transfer function of absolute acceleration response of a singledegreeoffreedom oscillator which is supported on a baseexcited, classically damped and flexiblebase primary system. The primary system response has been described in terms of fixe ..."
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A mode acceleration formulation is presented for the transfer function of absolute acceleration response of a singledegreeoffreedom oscillator which is supported on a baseexcited, classically damped and flexiblebase primary system. The primary system response has been described in terms of fixedbase primary mode shapes, with the response in last few modes assumed to be pseudostatic. Base flexibility has been assumed to be described by complexvalued impedance functions, and the effects of kinematic interaction have been assumed to be negligible. The proposed formulation can be used within the framework of any power spectral density functionbased random vibration approach to generate floor response spectra of desired level of confidence. A numerical study is carried out with the help of an example primary system and bandlimited white noise to illustrate the proposed formulation. It has been shown that the proposed formulation gives very accurate estimates of floor response spectra if pseudostatic response is assumed to be in those primary modes only which are stiff enough to the base excitation. It has also been shown that neglecting soilstructure interaction may give too overconservative or unconservative estimates, depending on the damping ratio, natural period, and location of the oscillator, energy distribution in the excitation process, and shear wave velocity of soil.
A MODAL COMBINATION RULE FOR PEAK FLOOR ACCELERATIONS IN MULTISTORIED BUILDINGS
"... It is useful to estimate peak floor accelerations consistent with the specified seismic hazard for ensuring the safety of rigid nonstructural components in structural systems. A modal combination rule is formulated here to estimate peak floor accelerations in a multistoried building directly in term ..."
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It is useful to estimate peak floor accelerations consistent with the specified seismic hazard for ensuring the safety of rigid nonstructural components in structural systems. A modal combination rule is formulated here to estimate peak floor accelerations in a multistoried building directly in terms of the dynamic properties of the building and pseudo spectral acceleration ordinates of the base excitation. The formulation is developed under the framework of stationary random vibration theory for a linear, lumpedmass, classically damped, multidegreeoffreedom system with the help of some approximations. A numerical study shows that the proposed rule performs well with the maximum average absolute error in any combination of building and excitation being less than 20 % in case of 5 % damping. Two simpler SRSStype variants of the proposed rule, one considering modal crosscorrelation and another ignoring this, are also shown to perform reasonably well, particularly when the building is not flexible to the ground motion.
DESIGN FORCE RATIO SPECTRUM FOR PERFORMANCEBASED DESIGN IN CASE OF MULTIPLE EVENTS
"... The earthquakeresistant design methodology in most existing codes of practice is based on ensuring “no collapse ” during the most severe event expected at the given site while most of the input energy is dissipated through inelastic deformations. Evolution of the performancebased design over the l ..."
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The earthquakeresistant design methodology in most existing codes of practice is based on ensuring “no collapse ” during the most severe event expected at the given site while most of the input energy is dissipated through inelastic deformations. Evolution of the performancebased design over the last decade has seen a few performance levels added up to this so that the structure remains functional even after a moderately strong event. This methodology however overlooks the possibility that in case of multiple earthquake events expected during the design life of the structure, the structure may get gradually damaged and that it may not be feasible to carry out repairs in the structure after every damaging event. As a result, the structure may collapse earlier than expected and perhaps during an event of moderate intensity. To address such a concern, a new spectrum, called as design force ratio (DFR) spectrum, is proposed in this paper. DFR spectrum gives the ratio by which the design yield force level of a conventionallydesigned singledegreeoffreedom structure should be raised such that the damage caused by all earthquake events expected to occur during its lifetime is limited to a specified level. A numerical study is carried out for a hypothetical seismic region by following a simple procedure based on several assumptions, and DFR spectra are obtained for elasticperfectly plastic oscillators when the return periods of earthquakes follow exponential distribution over the entire range of magnitudes.