Using prior knowledge on the dynamic performance of footbridges in Portugal to estimate the main characteristics of based-isolated single-span RC structures from in-situ measurements

A data-base with the geometric and mechanical characteristics of most common footbridges in Portugal, typified in 5 main categories, was used in estimating vibration properties of based-isolated single-span RC footbridges (25-46 m long) subjected to in-situ measurements of the response for simple dynamic loadings. Frequencies and damping of those modes with frequencies in the range of influence of pedestrian loading (1.5-3.5 Hz), as well as the amplification factors, are easily obtained from expedite measurements of responses if we take into consideration the accumulated (prior) knowledge from working with the data-base information. In the vertical direction these devices cause slightly decrease in the frequency, increasing the potential for amplification of response in loading conditions close to resonance. For horizontal directions, the base isolator device introduces a decoupled very low frequency around 0.04 Hz (T=25 sec) due to its low shear rigidity. For the range of pedestrian loadings, the transversal and longitudinal frequencies do not pose a problem of amplification.