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If a set of ground motions are available as the design basis, setting target times to directly match the representative ground motions is probably the most appropriate approach. This procedure starts with scaling the ground motions into proper intensity level based on code requirements. For example, Figure 1. shows FEMA P695 far field record set strong components scaled to PGA=1.
Figure 1. FEMA P695 far field spectra
The ground motions are usually scaled in such a way that their average response does not fall bellow the desired design spectra in 0.2T0 to 1.5T0. An example scaling for this purpose is shown in Figure 2.
Figure 2. Scaled average response spectra
In this matching method, target times are set to match with the average response spectra in the same interval of 0.2T0 to 1.5T0. This is shown in Figure 3.
Figure 3. Matching the target time
As can be seen in this figure, the average response spectra of ETA20inx series matches the ground motions at t=8.28 s.
This can be repeated at other intensities of interest as well. For example, for MCE level, the scaling is shown in Figure 4.
Figure 4. MCE level design spectra
The target time matching is shown in Figure 5.
Figure 5. Matching Target time
The procedure can be repeated for any number of intensity levels of interest. Note that only a single intensifying ET analysis is needed to cover all intensities.
The process for choosing the target time can be further simplified if A215 data are available for the particular ETEF set.
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