Reliability of Response Spectrum Curves at Low Frequency Region

The seismic design of low frequency systems differs considerably from the normal practical situation, in the sense that is based on spectral velocities and displacements rather than on spectral accelerations. This fact imposes some limitations on the standard spectral analysis that must be considered. First, the amount of processing spurious noise present in the strong motion records must be known and kept bounded at all times. The above is particularly true for analog accelerograms, for which the instrument characteristics and the procedures used to retrieve the original data affect significantly the reliability of the long period range of response spectra. Other sources of enor, however, are not man-induced and therefore escape human control. This is the case of uncertainties that emanate from geologic or geophysical phenomena, like anomalous attenuation laws (e.g., Vrancea earthquakes in Romania), fault directivity, site effects, and so on. Actually, all this factors decrease the reliability of the spectral ordinates at low frequencies and need to be carefully evaluated. Besides they are important in assessing the seismic performance of base-isolated structures at high risk conditions -such as epicentral areas, soft soil sites, etc.— for which special caution and restrictive design measures are specified in the corresponding building codes