Estimation of stiffness changes of post-tensioned concrete beams subjected to quasi-static loading through modal testing and finite element model updating

Post-tensioned concrete structures are frequently used in the construction industry because of their improved load-deformation behaviour, higher strength, and durability. However, they are susceptible to many forms of degradation during their service life that compromise the structural integrity and require periodic monitoring to avoid catastrophic failure. One popular monitoring technique is offline modal testing, which periodically measures the dynamic response of post-tensioned structures to detect potential damage by estimating stiffness loss. Experimental modal testing was conducted in the laboratory on three simply supported post-tensioned concrete beams, each 3.3 metres long, to investigate changes in dynamic behaviour resulting from damage induced by quasi-static loading in a four-point loading configuration. An inverse eigen-sensitivity approach was implemented to minimise the weighted difference between the experimentally measured frequencies and mode shapes and their numerically correlated counterparts in a least-squares sense, estimating localised stiffness loss due to progressive damage. The methodology can reproduce the observed global dynamic responses at each damage state by effectively estimating locally reduced stiffness parameters, which, in turn, match well with experimentally observed flexural crack patterns.