Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 453

was an evolution from an arbitrary set of forces based on the earthquake damage to
a set of forces applied as static loads. It would approximately reproduce the peak
dynamic response of the structure to the design earthquake. Linear elastic response
spectra provided to estimate the level of forces and deformations present in
structures. The following step in the code development was the dissipation of
seismic energy through plastic deformations and the use of the inelastic spectra.
The control factor of seismic energy dissipation was the ductility, defined as the
ability of a structure to undergo deformations after its initial yield, without any
significant reduction of the ultimate strength. These two items, i.e. the calculation
of lateral design forces and the means of providing sufficient ductility, constituted
the two most effective provisions in seismic design. After the Northridge and Kobe
earthquakes , new design aspects were recognized for structures situated in near-
source areas: the importance of velocity and displacement spectra, instead of
acceleration spectra, as well as the importance of superior modes of vibrations.
Period of personal computers. Since the early 1970s, a drastic change in
seismic codes has taken place, thanks to the availability of personal computers and
the implementation of a great number of programs for structural engineering,
which very easily perform static and dynamic analyses in elastic and elasto-plastic
ranges. These technological advances allow obtaining more refined results, giving
to the researchers the perspective to improve the design methodology. The code
recommended structural analysis, instead of providing practical procedures, turns
to the examination of phenomenological aspects. As a consequence, the modern
codes have to contain recommendations for the proper use of different
methodologies of analysis, in function of structure and earthquake types. In this
situation, the principal task of the seismic codes is to provide the seismic loads and
general rules for designing an optimal structural configuration.
International codes. The new initiative to create an international system of
codes started in the last period. This international code system includes the
principles for the determination of seismic actions, the seismic design methodology
and the main factors to be considered. But the seismic codes today, in comparison
with the codes for other structural actions, have some very important peculiarities.
Different from other codes, the development of seismic provisions is strongly
based on the observations of some damaging earthquakes. Consequently, the
current set of design factors found in national standards are based on a measured
combination of historical seismic events, state-of-the-art research works and
engineering judgments, very different in each country, being a function of its local
constructional experience in seismic areas, coming from the nature of ground
motions, as well as from traditions and jurisdictions. Recognizing the specific
aspects of seismic design, connected to the regional particularities, all documents
elaborated in the frame of an international action could serve as a guideline for all
issues belonging to regional or national standards.
10.1.2 The Long Way from Theory to Practice
Today, thanks to the development of computers, the actual performance of a
structure during a strong ground motion can be satisfactorily evaluated by