Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 418

418 Earthquake Engineering for Structural Design

9.4.6 Criticism of Using Response Spectra

Due to its simplicity, the response spectra representation provides a very reliable tool
to estimate the ground motion effects on the ordinary structures. The criticism of the
use of this representation is based on the following points (Gioncu and Mazzolani,
2006):
-The assumption that the use of only one or a reduced number of ground motions is
sufficient to describe the earthquake effects is wrong, because other records can
provide a response spectra shape, which is considerably different from the one of the
considered records. This is, especially, the case of intraplate earthquakes, for which the
number of records on the same location is very small.
- The shapes of spectra for distinct ground motion levels are very different on the
same site. Therefore, the use of the same spectra for structural checking at different
levels is an unsuitable procedure.
- The records near to source are very different from the ones recorded far from the
epicenter, both for interplate and intraplate earthquakes. Unfortunately, there is a
reduced number of near source records.
-The damping effects are very different in function of the level of the considered
earthquake. For reduced level, the effects are also much reduced, so use of the
standardized 5% critical damping ratio is an inaccurate procedure.
-One of the most important factors in determining the structural response, the
ground motion duration, is completely ignored in the set-up of response spectra.
Generally, the duration is very different for interplate, intraplate or intraslab
earthquakes, so the number of cycles able to dissipate seismic energy is also very
different and the reducing factor q must consider this fact.
-The proper consideration of superior vibration modes is very difficult, because the
response spectra are constructed on the base of the first vibration mode. Some
particular response spectra must be considered, especially in case of intraplate
earthquakes, where the superior vibration modes are dominant.
Despite these criticisms, the use of response spectra remains the most practical
method. Therefore, the improving of this methodology is a task for research (see
Chapter 10).

9.5SEISMIC ANALYSIS PROCEDURES

9.5.1MainCharacteristics

Seismic analysis is a subset of structural analysis, whose scope is the evaluation of the
response of buildings under earthquakes. It is part of the process of structural design in
regions where earthquakes are prevalent (Wikipedia, nd). The primary objective of
seismic analysis is to determine the dynamic response of structures under seismic
actions.