On The Time Variability Of Eigenfrequencies And Viscous Equivalent Damping In Historical Masonry Structures: A Case Study

The main Vara Bridge is a multispan masonry arch bridge built at the end of XIX century. In the last years its dynamical behaviour has been extensively studied. During the tests the bridge was impulsively excited by a weighted lorry that jumped on a step placed across the deck. The free oscillation were recorded through accelerometers and seismometers. For the identification of the main structural dynamic properties a time-scale technique based on wavelet func-tions was experienced instead of the classical time domain or frequency domain techniques. The use of continuous wavelet transform highlighted an unexpected variation of natural frequencies and viscous equivalent damping coefficients with time. As the oscillations recorded decayed with time it means that frequencies and damping varies with the amplitude of the oscillations. In this paper this variation is studied in detail. The possibility of describing this phenomenon through a single analytical expression is also explored.