Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 409

Advances in Seismic Design Methodologies 409

9.4.2MainCharacteristicsofAccelerationSpectra

The form of the elastic response spectrum, S e (T), which is introduced in the building
codes, is shown in Figure 9.14. It presents the amplification of ground accelerations in
function of T(s), which is the vibration period of a linear single-degree-of-freedom
system. The main parameters of this spectrum are:
- g, ground acceleration ;
- max, amplification of ground acceleration;
- b, corner period at the beginning of the plateau (generally T b = 0.2 T c). For
periods shorter than about 0.03 sec, the response spectrum follows the constant
PGA line;
- T c, corner period at the end of the plateau, equivalent to the predominant period of
ground motion (ranging approximately from 0.3 to 0.7 sec);
- T d , value defining the beginning of constant displacement response range of the
spectrum (ranging approximately from 3 to 4 sec).
All the values of these parameters depend on the ground motion values. The main
factors influencing the shape of the response spectra will be presented in the
followings sections.

9.4.3 Influence of DifferentFactors on Spectra

There are many factors which influence the form of the response spectral acceleration.
They are:
Structural damping. The reduction of elastic response due to damping has been
studied since long ago. The important fact worth pointing out here is that the effect of
damping depends on the vibration period of the system, being more pronounced for
short periods (0.2 to 0.7 sec) (Fig. 9.15). Most of building codes have design spectra
implicitly based on 5% of the critical damping.
Earthquake types. It is very important to notice the great difference between
intraplate and interplate earthquakes and the importance of the distance from source
(especially for intraplate earthquakes) (Fig. 8.16), which must be reflected into the
elastic spectra (Lam et al, 1996). The differences are given by the natural period of
ground motions. For intraplate earthquakes, characterized by low to moderate
earthquakes and short ground periods, an important increasing of amplification is
noticed for short structure periods. At the same time, the corner periods are much
reduced for intraplate earthquakes (0.3 to 0.5 sec) in comparison with the interplate
earthquakes, for which the natural periods are longer (0.5 to 1.5 sec) (especially in
function of site soil conditions).