Rapid Decay of a Timber Footbridge and Changes in Its Modal Frequencies Derived from Multiannual Lateral Deflection Measurements

Abstract

Lateral deflections of a timber pedestrian bridge in Patras, Greece, excited by pedestrians were measured using a robotic total station (RTS) in six annual surveys. Analysis of the data indicated a drop of 1.6 Hz in the natural frequencies of the bridge between 2007 and 2008 and then a gradual drop of approximately 8% between 2008 and 2012. This frequency drop was consistent with signs of decay, accelerometer data, and a drop in the natural frequencies along the vertical axis, and was most probably prompted by a strong earthquake and especially by an extraordinary (for the area) icing event. The overall changes in the response of the bridge to dynamic loads were consistent with the feelings of discomfort for pedestrians crossing this bridge. This is probably the first case of repeated measurements of deflections documenting deterioration of the structural health of a bridge at this intensity and scale of time.