Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 389

Advances in Seismic Design Methodologies 389

- Elasto-plastic analysis for rare moderate earthquakes. The non-structural elements
are partially damaged, so the analysis must consider only the structure behavior,
without any interaction with the non-structural elements.
- Kinematic analysis for the very rare earthquakes, considering the behavior due to
the possible formation of plastic mechanisms, must be performed.

9.1.3RequiredPerformancesandSeismicHazards

A key feature of seismic design is the presentation of the earthquake actions to be
considered in design. The required structural performance can be established in
function to the given earthquake actions. Generally, it is accepted the following
framing for relating seismic hazard to magnitude M:
- Low seismic hazard, M < 4.5.
- Moderate seismic hazard, M = 4.5 - 6.5.
- High seismic hazard, M > 6.5.
Looking to the world maps with the seismic areas (see Section 6.5), one can see that
the high seismic hazard areas are concentrated mainly near to known faults, especially
around the tectonics plate boundaries, where interplate earthquakes occur. The
remainding areas, diffuse seismic zones, characterized by intraplate earthquakes, can
by framed into low to moderate seismic hazard areas. The required structural
performances must consider these differences in seismic hazards. Therefore, the
seismic design must be divided in to two different approaches:
- Seismic design for high seismic risk areas , where the required performance must
be very high. Especially after the seismic decade 1989-1999, a very important
stock of valuable research works has been developed, giving rise to consistent
improvements in seismic design methodologies for these areas. High performance
levels are required for structures situated in these areas.
- Seismic design for low to moderate seismic risk areas , for which the research
results are not at the same level as for the high seismic risk areas. A wrong
tendency to use the results obtained for the high risk areas was noticed. But in the
last decade it was remarked that the design concept in low and moderate seismic
areas cannot be the same as in the high seismicity areas and, as a consequence, a
very important change has been observed in this aspect. Limited performance
levels are required for the structures situated in these areas.
This division in seismic design approaches is crucial from the economical point of
view, allowing the designer to optimize the costs of seismic protection measures.
Design considerations for the seismic resistance of buildings in areas of low and
moderate seismic risk are in many ways different than the ones in areas of high seismic
risk. As the earthquake design procedures are generally based on considerations
coming from areas of high seismicity, the current seismic design rules are not readily
adaptable to areas of low to moderate seismic risk.