Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 522

The first requirement for the next generation of codes is to complete the map of
the seismic zonification in function of ground motion accelerations, by indicating
on the map the earthquakesourcetype :
- Interplate crustal source:
subduction (thrust)
strike-slip;
- Intraplate crustal source:
convergence
crust fracture;
- Intraslab sub-crustal source.
The second step for the next generation of codes is to consider in seismic design
the main particularities of each earthquake type.
The main problem is to select a set of spectra corresponding to earthquake type,
normalized to the acceleration (Fig. 10.41) (Gioncu and Mazzolani, 2002) for the
three earthquake types and for the three limit states. The values in this figure,
concerning amplifications and corner periods, are given just as an example, the
exact values being established for each case. One can see that the spectra for the
limit states are different, contrary to the pattern of the present code spectra. For
serviceability limit states, the spectra are characterized by high amplification for
short periods, with a steep reduction in amplification for vibration periods
exceeding the corner period. For damageability limit states, the amplification is not
so high and the reduction more gentle. For ultimate limit states, no amplification
occurs, because the structure is transformed into a plastic mechanism, which works
as the ground motions impose, without any interaction due to lack of any vibration
mode.
The differences depending on earthquake types are very large. The interplate
and intraslab earthquakes are in the first category (Fig. 10.41a), being characterized
by longer corner periods (especially for strike-slip and intraslab earthquakes). The
amplification is higher for strike-slip and especially for intraslab earthquakes in
comparison to thrust earthquakes, due to presence of a higher number of cycles. In
case of intraplate, especially for crust fractures, the patterns of spectra are very
different (Fig. 10.41b).
A very important step in improving the next code generation is the topic dealing
with different design philosophy in function of earthquake type. The seismic-
resistant structures are usually designed relying on their ability to sustain high
plastic deformations. The earthquake input energy is dissipated through the
hysteretic behavior of plastic hinges during a high number of cycles of seismic
loading. According to this design philosophy, the structure may be designed for
lower forces than those it has actually to resist. This seismic design philosophy has
proved to be valuable in many great earthquakes and it corresponds to the
methodology commonly included in the modern codes. But the recent events
occurred in near-source areas produced a widespread and unexpected brittle
fracture of joints with little or no evidence of plasticization of members, as it is
assumed in code provisions. As a consequence, the amount of seismic input