The excitement corresponding an earthquake is result of the overlapping of a great quantity of waves of different frequencies and of different amplitude that constitute the accelerogram [Sarria & Bernal, 19??].
In a diagram, named response spectrum, for different frequencies and damping, the maximum values of response developed during the action of certain earthquake are plotted. It represents in a single graph the combined influences of terrain acceleration amplitudes, frequency components of the movement and, in certain measure, the duration of the earthquake [Seed, 1974]. Then, a description of the most important characteristics in the response is achieved [Barbat & Canet, 1994].
At the present time the spectral accelerations are the parameters of the seismic action considered to be better related with the damages and the possible structural failure [Ordaz et. al., 1989]. The response spectrum is a convenient medium to evaluate the maximum lateral force developed in structures subject to a given movement in the base [Seed, 1974]. This constitutes, perhaps, the most important application of the response spectra, to such a degree that is the base of analysis procedures of very widespread employment [Prince et. al., 1983]. It is an element indispensable today that engineers should introduce in their calculations [UN, 1978].
On the other hand, since the time history of the seismic excitement in a certain site (seismogram) is characterized by the corresponding response spectrum, the differences among the time histories of the movements at different places can be analysed by the comparison of their response spectra [Seed, 1974].
Other advantages are product of the fact that the response spectra can be averaged or even modified to include the ground conditions, even though when the details of the excitement process are not known [Barbat & Canet, 1994].
From the analysis of several response spectra, the essential characteristics of the response in a whole seismic region can be estimated, and sometimes even for a particular type of earthquake. The result obtained is a smoothened spectrum, normalized to a certain level considered appropriated for design purposes.
Defining the design spectrum requires to adjust the smoothened spectrum to the different seismic zones within the region for which it is valid, and then to adjust it to the structure that is designed. Such adjustments are made multiplying the spectrum for different coefficients [Barbat & Canet, 1994].
To adjust the spectrum to a sub-zone, coefficients that characterize that sub-zone are used, which are usually empirical, obtained from approaches such as seismicity, seismic hazard, socio-economic importance of buildings within that zone, geology, among others.
The coefficients to adjust the spectra to the structures have as defining criteria the structure type, their rigidity and ductility, the characteristics of materials used, the foundation type, the characteristics of damping, the importance of the structure immediately after the earthquake (for example, hospitals), etc.
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Last updated: Sat, 31 Jan 2004