Non-linear vibrations of cracked reinforced concrete beams

As part of a larger project considering the use of dynamic testing in the structural health monitoring of bridges, a series of laboratory experiments was performed to investigate the non-linear behaviour of reinforced concrete beams. Eight beams were incrementally loaded to failure - four in flexure and four in shear. At each load increment the behaviour of the beam was investigated by exciting the beam harmonically at different amplitudes. The results confirmed previous studies, which indicated a significant non-linearity in the dynamic response of cracked reinforced concrete beams that negated the use of absolute changes in natural frequency alone as a damage indicator. A simple single degree of freedom model was used to simulate the response to harmonic excitation at a given natural frequency. Here, the non-linearity was introduced both as a bilinear "open crack" model and using a hyperbolic tangent function to describe the transition between the crack open and the crack closed states. The results of the model supported the findings of the experimental work.