Use of mode shape ratios for pier scour monitoring in two-span integral bridges under changing environmental conditions

In this paper, a novel pier scour indicator is introduced, which uses the ratio between mode shape amplitudes identified at two points on an integral bridge structure to monitor the progression of scour erosion. The mode shape ratio (MSR) is investigated as an additional parameter to complement the use of changes in natural frequency as a scour indicator. The approach is demonstrated using numerical modelling and the MSR is extracted from acceleration signals arising in the structure due to modelled ambient and vehicle-induced vibrations. The MSR shows higher sensitivity to scour erosion than the more commonly researched natural frequency. Furthermore, the variation in MSR under temperature fluctuations is inversely related to that of frequency, in that it increases with increasing temperature whereas frequency decreases with increasing temperature. This inverse relationship potentially enables the separation of the scour effect from the temperature influence on the dynamics of the system.