Operation modal analysis on prestressed concrete beams with shear or flexural cracks

This study aimed to apply Operation Modal Analysis (OMA) techniques on prestressed concrete beams (PCBs) in order to identify early warning signs of structural damages, such as changes in natural frequencies and/or relevant mode shapes. The main novelty, compared to past similar investigations, is the employment of OMA to these particular damage-scenarios: (1) a PCB equipped with bonded strands affected by “almost pure” shear-cracks; (2) a PCB equipped with unbonded strands affected by small and reversible flexural-cracks. It was found that, especially in the second case, variations of the natural frequencies may not be an effective index of damage. On the contrary, a qualitative comparison between undamaged and damaged identified mode shapes can be useful for the spatial location of the cracks. Although not among primary objectives, it was also ascertained that OMA techniques are not suitable to assess the actual residual strands’ tensile load within typical variation ranges during bridges’ service life.