A detailed database, containing geometric, mechanical, and vibrational characteristics of the most common footbridges in Portugal, was developed, with a total of 79 footbridges, classified into 5 typologies. Thirtyone are reinforced concrete (RC) structures. The main vibrational properties, such as frequency and damping of first modal shapes, and amplitude of vibration for certain loading patterns, were obtained through expedite in situ measurements using single 3-component accelerometric stations. Ten base-isolated, single-span, RC footbridges (25-46-m long), with a channel or U cross-section, were selected from the database to be the object of a more detailed study. From those ten footbridges, two were selected for additional in situ testing using multiple 3-component accelerometric stations, which led essentially to the same results as those obtained with a single 3-component station. By taking into consideration the accumulated (prior) knowledge embodied in the database, we conclude that frequencies and damping of the main modes, which usually lie in the range of influence of pedestrian loading (1.5-3.5 Hz), are easily obtained from expedite measurements with a single 3-component station. Frequencies obtained with finite element models were also compared with in situ measurements. In the vertical and longitudinal directions, both analytical models and in situ measurements give similar results, whereas in the transversal direction, the finite element models do not reproduce well the in situ results. In the vertical direction, the base-isolated devices cause a slight decrease in the frequency, increasing the potential for amplification of response in loading conditions close to resonance. The obtained results indicate that vertical amplifications may be on the limit of acceptance. For the range of pedestrian loading patterns, the transversal and longitudinal frequencies do not cause amplification for these structures. Formulae to estimate the main vertical and transverse frequencies were obtained from the in situ testing. In the vertical direction, a power function shows very good correlation with the results for the ten footbridges and for the other 21 RC footbridges present in the database. For the transverse direction, there was good correlation only for the in situ results of the ten footbridges.
