Modal systems identification of an eleven-span concrete motorway off-ramp bridge using various excitations

This paper addresses and evaluates the influence of different types of dynamic excitation sources on identifiability and reliability of modal parameter identification of an eleven-span, post-tensioned, curved concrete motorway off-ramp bridge. The excitation sources included ground vibration waves generated by traffic on nearby motorways, people jumping on the bridge deck, broad-band linear chirp excitation induced by two light electro-dynamic shakers, and stepped sinusoidal sweeping forcing induced by a rotating eccentric mass shaker. Experimental modal analysis, operational modal analysis (OMA) and OMA with exogenous input system identification by a subspace state-space identification algorithm were carried out to extract the modal characteristics of the bridge. Numerical frequencies and mode shapes were also calculated from a detailed shell-element bridge model with the purpose of serving for the corroboration of the experimental modal properties. Through comprehensive cross-comparisons, it was revealed that the natural frequency discrepancies across different excitation methods were small (within ±1.5% relative difference), whist the consistency of the estimated damping ratios was poorer (up to 1.4% absolute difference). Eccentric mass shaker testing enabled identification of all modes predicted by the numerical model but used very heavy equipment. On the other hand, in ambient testing about one-third of the modes were missed, including the fundamental lateral mode. Electrodynamic shaker testing proved attractive as it only missed one mode and achieved its good performance with shakers that could be moved on site with relative ease.