Test study on modal deflection and modal stiffness of prestressed concrete simply supported beams under damage state

Here, to quickly detect and evaluate load-bearing capacity of existing concrete bridges in different damage states, a 16 m-span prestressed concrete single piece simply supported hollow plate-girder dismantled in a certain actual highway renovation and expansion project was taken as the study object, modal testing technique was combined with static loading and unloading tests to explore the applicability and correctness of measured modal deflection and modal stiffness in evaluating load-bearing capacity of existing bridges in practical engineering. Simply supported test beams' static load test loading and unloading working conditions and environmental excitation modal testing conditions with 14 different damage degrees were designed, their static load deflections and test modal parameters of beam bodies with 14 different damage degrees were measured, modal deflections of simply supported beams under static loads at different levels were predicted, and tangent stiffnesses of load-displacement curves and modal stiffnesses of test beams with different damage degrees were analyzed contrastively. The results showed that before cracking, test beams' measured loading and unloading static deflections are roughly linearly related to loads; after cracking, with increase in damage degree, linear elastic stage of load-displacement curves gradually shrinks, while nonlinear stage gradually increases to reveal an obvious trend of stiffness degradation; when test beams are in a lighter damage state before cracking and a larger damage state in initial loading stage after cracking, relative errors between predicted modal deflections of the first two measured vibration modes and static load deflections are basically not more than 10%; relative errors between modal stiffnesses and initial stiffnesses of static load-displacement curves are basically within 10%; modal stiffness of test beam with a damage degree lower than larger damage is approximately equal to initial tangent stiffness of beam body; relative error between modal stiffness of a lightly damaged simply supported beam before cracking and tangential stiffness of the designed bending moment is within 10% to reveal the measured modal stiffness of structure can more accurately evaluate initial stiffness state of a prestressed concrete simply supported beam subjected to 4 times the design load with larger damage, and also accurately evaluate the design structural stiffness of a lightly damaged uncracked simply supported beam bridge subjected to 2 times the design load; the study results have important reference and guidance significance for practical application of measured modal deflection and modal stiffness in engineering, and expand application range of modal testing in technical evaluation of bridge health conditions. © 2023 Chinese Vibration Engineering Society. All rights reserved.