Comparison of techniques for modal analysis of concrete structures

This paper describes different experimental techniques for obtaining modal parameters of structures. Attention is focused on those techniques that may be applicable to in situ concrete structures (e.g. bridges). In a first stage, experiments are made on reinforced concrete beams of 6 meters length. The beams are excited using three types of excitation methods: impact hammer excitation and two different electromagnetic shaker signals: pseudo-random and swept-sine signals. The modal parameters are determined either by performing curve-fitting procedures on series of measured frequency response functions or by applying the stochastic subspace identification technique to the time response signals of the structure. The influence of the non-linear behaviour of the concrete beams is investigated by performing measurements at different excitation amplitudes. It appears that modal parameter estimates are affected by excitation techniques, data acquisition parameters and processing methods. The main cause of this is the non-linear behaviour which is observed even at very low vibration amplitudes. However, the influence on resonant frequencies and mode shapes is negligible. This is not the case for the modal damping ratios, so that the estimation of these parameters is unreliable.