3. Use A215 data
Data files with _A215 suffix provide the mean of Sa values from 0.2T to 1.5T, at a period T. This is the periodic range that is considered to cover most significant periods for a structure with fundamental period of vibration T by various seismic design codes. The data provided in A215 matrices can simplify the process of target time selection as explained below. Sample data from ETA20inx_Sa_Avr_A215 file is shown in Figure 1.
Figure 1. Sample _A215 data
In order to demonstrate the application of these data, consider that we are interested in choosing a target time that matches a particular design spectra or spectra of a set of scaled ground motions. The first step is to calculate the average spectral intensity of the target spectra. For example, Figure 2 shows the mean response spectra of 44 FEMA p695 far field ground motions scaled to 0.5 g. The average from 0.2T to 1.5T curce for this spectra is also shown in this figure.
Figure 2. FEMA p695 GMs scaled to 0.5g and their 0.2-1.5T average
Now consider that we want to select a target time using ETA20inx ET function for a structure with fundamental period of vibration of T=1.0 s. As can be seen in Figure 2., the average Sa from T=0.2 s to T=1.5 s should be 0.7529g. Now, using ETA20inx_Sa_Avr_A215 data, we plot average Sa at T=1.0 s (row 101 in ETA20inx_Sa_Avr_A215).
Figure 3. ETA20inx average 0.2-1.5T (A215) at T=1.0
As can be seen in this figure, average Sa equals 0.7529 at t=8.37. This is considered the right target time based on Sa intensity. The mean Sa of ETA20inx at t=8.37 is plotted along with scaled GM spectra in Figure 4.
Figure 4. ETA20inx average Sa at t=8.37 s and GM spectra