Response of a historical short-span railway bridge to passing trains: 3-D deflections and dominant frequencies derived from Robotic Total Station (RTS) measurements

Based on measurements with a Robotic Total Station (RTS), we computed the displacement characteristics (pattern, amplitude, frequency) of the Gorgopotamos Bridge, a short-span, historical bridge in central Greece, in response to passing trains. Vertical deflections of selected points of the midspan were found to range up to 6 mm for heavy cargo trains, exceeding the noise level of 1–2 mm, and were analyzed in nearly semi-static and dynamic components. Dominant frequencies of the bridge span, in the range 3.18–3.63 Hz, were also computed. Evidence of possible lateral and longitudinal deflections was also found. Measurement noise and quality of results were controlled on the basis of the analysis of apparent displacements of reference points during excitation and of non-excitation intervals, as well as on the basis of structural constraints. The amplitude of measured deflections was found to correlate with the load, and a somewhat distinct oscillation waveform was observed in the cases of large oscillations, despite clipping affecting highly dynamic RTS recordings. Observed deflections are large for modern standards, but reasonable for a >100 years old bridge partly destroyed and rebuilt twice during World War II.