Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 386

Chapter9
AdvancesinSeismicDesign
Methodologies
9.1CHALLENGESIN SEISMICDESIGN
9.1.1DesignConceptsbeforeNorthridgeandKobe
The beginning of modern seismic design may be fixed during the 1950s, when the
dissipation of seismic energy through plastic deformations was considered by Housner.
His method develops a limit design type analysis for ensuring a sufficient energy-
absorbing capacity to guarantee an adequate safety factor against collapse in case of
extremely strong ground motions. The first study on the inelastic response spectrum
was made by Velestos and Newmark in 1960. They obtained the maximum response
deformation for elastic-perfectly plastic structures. Since this first application in
seismic design, the response spectrum has become a standard measure of the demand
of ground motion. The utility of the response spectrum lies in the fact that it gives a
simple and direct indication of the overall displacement and acceleration demands of
the earthquake ground motion, for structures having different period and damping
characteristics, without needing to perform detailed numerical analysis. A new concept
was proposed in 1969 by Newmark and Hall, by constructing spectra based on
accelerations, velocities, and displacements, in the short, medium and long period
ranges, respectively (Gioncu and Mazzolani, 2002).
Based on these developments, the first practical seismic design philosophy stated
the requirements for a minimum level of safety of building. Specifically, the designed
structures are expected:
- To resist minor level of earthquake without significant damage.
- To resist moderate level with some non-structural damage and repairable
structural damage.
- To resist major level of earthquake without collapse.
However, the majority of the seismic design methodologies today explicitly
consider only one performance objective, in case of rare major earthquakes, defined as
protection of occupants against injury or loss of life. Criteria for checking buildings
against minor or moderate earthquakes, which may occur relatively frequently in the
life of the building, are not explicitly specified.