On the reliability of long-period response spectral ordinates from digital accelerograms

Using records from co-located broad-band and digital strong motion instruments, it is first shown that the displacement waveforms obtained by double integration of the accelerogram need not be free of unrealistic baseline drift to yield reliable spectral ordinates up to at least 10 s. Secondly, to provide objective criteria for selecting reliable digital strong motion records for ground motion predictions at long periods, a set of synthetic accelerograms contaminated by random long period noise has been used and the difference between the original accelerograms and the spurious ones in terms of response spectra has been quantified, by introducing a noise index that can be easily calculated based on the velocity waveform of the record. The results of this study suggest that high-pass filtering the digital acceleration record from a cut-off period selected to suppress baseline drifts on the displacement waveform appears to be in most cases too conservative and unduly depletes reliable information on long period spectral ordinates.