Frequencies and damping ratios of bridges through Operational Modal Analysis using smartphones

Seismic Force Assessment in structural design of bridges requires a good estimation of the fundamental period. Estimated periods, using current methodologies in seismic provisions, are approximations that may significantly differ from the real periods. These differences are related to assumptions and simplifications made in the model development. Through Operational Modal Analysis (OMA) it is possible to estimate operational frequencies of a bridge and provide evidence about its current dynamic behavior. In this paper, vertical operational frequencies of 451 structures, 285 pedestrian bridges and 166 vehicular bridges, and horizontal operational frequencies of 111 structures, 101 pedestrian bridges, 9 vehicular bridges and 1 railway bridge of the transportation network of Santiago de Cali, Colombia, were estimated using ambient vibration tests through OMA. Data were obtained using smartphones and processed using NExT-ERA and SSI techniques. For a sample of thirteen bridges, the results were compared with OMA results using seismic sensors. The results show that the smartphones used are appropriate as instrumentation systems for modal identification of flexible structures. The experimental frequencies identified using both time-domain techniques are similar among them and to equations proposed in technical literature. The damping ratios associated to the identified operational frequencies do not show relation with the geometric characteristics of the bridges or a clear difference between materials. Equations relating fundamental frequencies with structural characteristics of the bridges are presented.