Eigenfrequencies of continuous prestressed concrete bridges subjected to prestress losses

The paper presents a numerical procedure to evaluate the consequences of prestress loss on the frequencies of vibration of continuous prestressed concrete (PSC) bridges. The importance of being able to identify an excessive loss of prestressing, caused for instance by the breaking of some strands, is evident considering that currently the majority of collapses occurred in prestressed bridges is due to the stress-corrosion cracking of the prestressing steel.The method allows to solve the structural problem taking into account the following mutually interconnected features: non-linearity of the materials mechanical properties, redistribution of stresses based on the actual flexural stiffness of the beam segments in statically indeterminate structures and loss of prestressing. The application of the proposed procedure to two case studies found in the literature indicates that the variation of the first three eigenfrequencies for a prestress loss of 30% turned out to be in the range 1.5%–2.5%. The procedure could, thus, be included in continuous monitoring systems of PSC bridges that already take into account the frequency variations produced by other causes (temperature, humidity, …) for a timely detection of relevant prestress losses.