Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 512

10.6.5 Problems of Structural Response Control for Low-to-Moderate
Earthquakes
There are many important problems in the structural response control for low-to-
moderate earthquakes, which are different from the characteristics of the other
earthquake types. First, being produced by surface sources, all the manifestation of
near-source effects are present, but with some particularities: compared to the case
of interplate earthquakes, all the characteristics are reduced, like magnitude, pulse
periods, duration, etc. The numerical analyses presented in the previous section
have clearly shown that the main characteristic for the structural response control is
the pulse period, which is shorter than the period of the first vibration mode of the
structure. This implies the influence of the higher vibration modes on the structure
response, generally reducing the seismic demands, but moving them in different
zones of the structure (i.e. at the building top). Since the demands are not as severe
as in high seismicity regions, large inelastic deformations are not required even
under the maximum credible earthquakes. The seismic design based on limited
ductility design seems appropriate in moderate regions (Chang et al, 2001).
Therefore, the use of the ductility demands, determined for the Californian
earthquakes, also in case of moderate earthquakes is a mistake. Due to the short
duration of moderate earthquakes, the structure has not time enough to dissipate a
sufficient quantity of induced seismic energy. Therefore, the behavior factor, to be
used in case of moderate earthquakes for considering this reduced capability of
energy dissipation, has to be different from the one used in case of the other
earthquakes. So, a reduced behaviorfactor is used in the design of structures under
moderate earthquakes.
From the main results of the recent research works, the specialists are fully
aware that the design philosophy to be used in regions of low-to-moderate
earthquake must be different from the philosophy in regions of high seismicity.
But, unfortunately, this philosophy is not developed as well. There are many
countries belonging to low-to-moderate seismicity regions. Even in a country with
strong earthquakes, there can be some zones of moderate seismic hazard level.
Therefore, results and experiences on the new seismic design for low-to-moderate
seismicity zones will be beneficial for many countries. In the last period, the
increasing interest in this problem is documented by papers, reports, organized
symposia and sessions in world conferences devoted to this subject.
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10.7 STRUCTURE RESPONSE TO INTRASLAB DEEP EARTHQUAKES:
LONG DURATION GROUND MOTIONS
10.7.1 Elastic Spectra and Duration
The intraslab earthquakes are capable of generating large magnitude between 7.5 to
9.5 and long duration earthquakes until 240 seconds. For these earthquakes, the
number of the important cycles is very large. In addition, the influence of site soil
conditions produced very important amplifications in peak accelerations. The