Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 484

- For structures with T/T p < 1.0 the standard design methodology is not a good
substitute for the dynamic time-history analysis, because the spectral analysis
may not capture all important response characteristics of pulse-type ground
motions, once the higher-mode effects become important.
As an example for checking the possibility to use artificial ground motions for
the evaluation of the structure behavior during a pulse earthquake, a one span-six
levels frame (Fig 10.7) has been designed by considering the first mode in
accordance with the Romanian seismic code, neglecting the effects of the superior
modes. The DRAIN 2D time-history program has been used for the elasto-plastic
analysis of the frame, by using a peak acceleration A g = 0.35g. The study
parameter was the pulse period T p , selected to cover the first three vibration modes
of the frame: T 1 = 1.72 sec, T 2 = 0.61sec, T 3 = 0.5 sec (Gioncu and Mateescu,
1999, Gioncu et al, 2000). The results are presented in Figure 10.8. One can see
that the structure behaves elastically for pulse periods T p smaller than 0.35 sec.
The first plastic hinge occurs at the top of the frame for 0.4 sec, near to the third
vibration mode. The increasing of the pulse period moves down the formation of
the plastic hinges. For T p > 0.7 sec, corresponding to the second vibration mode, a
plastic mechanism occurs at the middle of the frame. For T p = 1.8 sec,
corresponding to first vibration mode, a global mechanism is formed, but
associated with some story mechanism at the middle part of the frame.
Figure 10.7 The analyzed frame (Gioncu et al, 2000)