3. Matching Design Spectra at Minimum

In this method, a target time is specified in such a way that the ETEFs average spectra does not fall bellow the desired design spectra in the vicinity of fundamental mode of vibration. Usually this vicinity is considered from 0.2T0 to 1.5 T0, where T0 is the fundamental period of vibration of the structure. For this purpose, response spectra of ETEFs at different times should be used. The time where the average response spectrum of ETEFs is just above the design spectra in the desired range of T should be specified. This can be done with some simple programming. Alternatively, an Excel worksheet can be used for manual try and error searching for target times.

Figure 2. shows an example matching target time at t=9.7 s.

Evidently, the applied intensity is somewhat higher than method 2 and therefore some overestimation in the response can be expected. However, this method is more consistent with code recommendations for practical applications.