Simplification of Earthquake Accelerograms for quick time history Analyses by Using Their Modified Fourier Inverse Transforms

There are several cases in seismic design of structures in which the simplified seismic analysis code is not applicable. In such cases most codes recommend time history analysis, which is very time consuming, mainly since the time step size of the digitized accelerograms is usually very small. Therefore, if the time history analysis can be performed by large time steps without losing much precision, it would be very helpful. In the present study, a method is introduced for simplification of accelerograms based on the modification of their fourier analyses. For this purpose, at first the fourier spectrum of the accelerograms is calculated. Then, by using a computer program, created by the authors, a modified version of fourier spectrum is produced and then the corresponding inverse fourier transform is calculated using a relatively large time step, which is computed based on the all major earthquake frequencies (which is usually 5 to 10 times larger than the original accelerograms time step) in order to create the simplified accelerograms.  It is obvious that time history analysis by using this simplified accelerograms will be much less time-consuming. Numerical results indicate that the maximum response values obtained by using the simplified accelerograms have approximately 5 to 10 percent error depending on the type of the structure and earthquake characteristics.