Capturing the long-term dynamic properties of concrete cable-stayed bridges

A cable-stayed bridge can be analysed by finite element method with the bridge deck and pylons modelled as beam elements taking into account geometric nonlinearities, and the stay cables as truss elements with equivalent Ernst moduli to account for cable sag. The long-term dynamic properties of concrete cable-stayed bridges are governed by the time-dependent material behaviour, including ageing, creep and shrinkage of concrete, and steel relaxation. A design code can be regarded as a summary of experience and local material properties of a certain country or geographical location. Results of analyses using five design codes show that, although different codes give different long-term material properties, the trend of long-term dynamic characteristics is largely independent of the code adopted. An empirical formula with undetermined coefficients is hence proposed to model the long-term development of structural frequencies, which will be useful to engineers conducting structural health monitoring of concrete cable-stayed bridges irrespective of the geographical locations. Moreover, an experiment has been conducted to monitor the long-term development of dynamic properties of post-tensioned concrete beams for verification. The combined effects of time-dependent behaviour and structural damage on the structural frequencies are also investigated.