1. Template Spectra Based

Specifying the target time can be based on the template spectra that is originally used in order to produce the ETEF. This is the simplest and most practical procedure. However, ETEFs with a template spectra that matches the shape of the desired design spectra may not be available. In practice, the shapes of design spectra are not very different from each other for a particular soil type. On the other hand, theoretically, there can be infinite number of different design spectra. Anyway, in case that ETEFs matching a particular design spectra are already available, or can be produced, current procedure can be the most straightforward and convenient. If such ETEFs are not available, it may steel be possible to use a particular ETEF by considering spectral intensity and shape in the vicinity of the structure's significant periods of vibration.

In this approach, target times are specified directly according to the scale factor that has been used in producing the ETEFs. For examples ETEFs series ETA40g have been produced based on a particular ASCE design spectra with Ss=1.5, S1=0.6, Fa=1.0, Fv=1.3 and TL=8 s. The scale is linear with a value of 1.0 at t=10 s. Therefore, a target time of t=10 s is considered to represent DBE intensity. Similarly, t=15 sec is considered equivalent to MCE level earthquake. Target spectra and ETA40g spectra at t=10 s are shown in Figure 1.

Note that no factor of safety regarding the slight variations of ETEF spectra around template spectra has been considered in this procedure. See following reference for application:

Estekanchi, H.E., Valamanesh, V. and Vafai, A. (2007), ''Application of Endurance Time Method in Linear Seismic Analysis'', Engineering Structures, v29, n10, p2551-2562, doi:10.1016/j.engstruct.2007.01.009