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no.10 Design Spectrum-Based Scaling Of Strength Reduction Factors
[Debasis Karmakar, July 2003, Supervised by V. K. Gupta]
Strength reduction factors (SRFs) continue to play a key role in obtaining
inelastic spectra from elastic design spectra (through response reductions factors)
for ductility-based earthquake-resistant design. Despite several years of
sustained research efforts, it remains interesting to explore how best to estimate
these factors for a given design situation. This study proposes a new model to
estimate the strength reduction factor (SRF) spectrum, in terms of a pseudo
spectral acceleration (PSA) spectrum (normalized with respect to peak ground
acceleration, PGA) and ductility demand ratio, with the help of two coefficients.
The proposed model is illustrated for an elasto-plastic oscillator, in case of ten
recorded accelerograms and three ductility ratios. The proposed model is more
convenient to use in real-life applications and is able to predict SRF spectra
reasonably well, particularly at periods up to 1.0 sec. Its parameters may be
determined either by matching with the SRF spectrum for a single accelerogram
or from the SRF spectra for a suite of accelerograms that are compatible with the
same response spectrum. Those parameters are estimated in this study with the
help of 1274 accelerograms recorded in western U.S.A., in case of design spectra
for different combinations of earthquake magnitudes, site conditions and
epicentral distances. A parametric study is also carried out for the explicit
dependence of SRF spectra on strong motion duration, earthquake magnitude,
site conditions, and epicentral distance. The Trifunac-Brady definition of strong
motion duration is generalized for this purpose, and it is found that there is no
clear and significant dependence of SRF spectra on strong motion duration. The
parametric dependence on earthquake magnitude, site conditions, and epicentral
distance conforms to the trends reported by earlier investigations. In particular,
this study confirms that the dependence of SRF spectra on earthquake magnitude
should not be ignored. |
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