Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 470

- In case of regions with low to moderate levels of seismicity, due to the lack of
earthquake records, the required data are very scarce. It is a real mistake to use
the data from other seismic zones, characterized by another source type,
without checking if they are available for the zones under consideration.
The improvement of these aspects gives rise to new approaches in seismic
design of structures, for which a deep knowledge of each earthquake type
characteristics is required. In the last period, a very intensive activity in analytical
and numerical modeling of the earthquake generation has been developed, giving a
great contribution in the process of understanding the intricate phenomena
produced during an earthquake and allowing the different earthquake types to be
identified. The main task of specialists is to transfer this new knowledge to the
structural practice.
10.3.4 Needs and Challenges for the Next Design Practice
After these critical points of view concerning the current design methodologies, it
may be concluded that the process to setup a reliable design philosophy is very
complex, due to the reason that so many factors, very difficult to be accurately
accounted, are involved. Considering the conservative attitude of the design
community, which does not like to accept some radical changes in design practice,
a justified question arises: What must be the solution of this matter of fact,
considering that the use of a methodology based on design spectra must remain
available for current design in to the future? The answer is not simple and
satisfactory, due to the complexity of the structural response during an earthquake.
Considering current and future development, the proposed solution is based on
three points (Gioncu and Mazzolani, 2002, 2006):
- Verification at three levels of performances: serviceability, damageability and
ultimate limit states.
- Diversification of design spectra and behavior factor in function of the
earthquake types for serviceability and damageability limit states.
- Introduction of explicit ductility and fracture demands for ultimate limit state
in function of the earthquake type, duration and number of large yield cycles,
ground motion velocities, effects of strain-rate, etc.
After a long period when only the ultimate limit state is considered in the design
practice, now all codes consider two levels, the serviceability and ultimate limit
states. It is a reasonable approach, which has been accepted by the design
practitioners. However, for the next code generation, it is an imperative issue to
pass to a further step, considering three performance levels: (i) the serviceability
for low and frequent earthquakes to protect against damage both structural and
non-structural elements; (ii) the damageability for moderate and rare earthquakes,
in order to limit the damage at a repairable level; (iii) the ultimate limit state for
large and very rare earthquakes to protect the structure against collapse.
The second approach to have different spectra in function of the earthquake
type is already given in some codes in different forms, but the application rules are
not always clear. The above-mentioned UBC 97 code introduced, for the first time,
the possibility to consider the effects of near-source ground motions for crustal