en Research Paper Research Paper <i><font size="1" face="TimesNewRomanPS-ItalicMT"><font size="1" face="TimesNewRomanPS-ItalicMT"><p align="left">Future design procedures for civil structures, especially those to be protected from extreme and blast related</p><p align="left">loads, will need to account for temporal evolution of their frequency content. There are, however, several instances</p><p align="left">where acceleration time histories are required as seismic input. For example, to determine the ultimate resistance and</p><p align="left">to identify modes of structures’ failure, a nonlinear time history analysis is needed. In other cases, acceleration time</p><p align="left">histories are required for linear analyses. Many seismic codes require this type of analysis for buildings which have</p><p align="left">irregularities. The process of time-frequency analysis made possible by the wavelet transform provides insight into the</p><p align="left">character of transient signals through time-frequency maps of the time variant spectral decomposition that traditional</p><p align="left">approaches miss. In this paper an approach is examined and a new method for processing the ground motion which is</p><p align="left">modeled as a non-stationary process (both in amplitude and frequency), is proposed. This method uses the best basis</p><p align="left">search algorithm with wavelet packets. In this approach, the signal is expressed as a linear combination of timefrequency</p><p align="left">atoms which are obtained by dilations of the analyzing functions, and are organized into dictionaries as</p><p align="left">wavelet packets. Several numerical examples are given to verify the developed models.</p></font></font></i> wavelet packet transform; artificial accelerogram; best-basis algorithm frequency content. 248 257 http://ijce.iust.ac.ir/browse.php?a_code=A-10-156-7&slc_lang=en&sid=1 G. Ghodrati Amiri 180031947532846006101 180031947532846006101 Yes A. Asadi 180031947532846006102 180031947532846006102 No