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The safety and sustainability of prestressed concrete bridges can be improved with accurate prestress loss prediction. Considerable loss of the prestress force may imply damages hidden in the bridge. In this study, a prestress force identification method was implemented for concrete beams. Based on the Euler–Bernoulli beam theory, the procedure estimates the prestress force by using one or a set of static displacements measured along the member axis. The implementation of this procedure requires information regarding the flexural rigidity of the beam. The deflected shape of a post-tensioned concrete beam, subjected to an additional vertical load, was measured in a short term in several laboratory experiments. The accuracy of the deflection measurements provided favorable prestress force estimates. In particular, the “compression-softening” theory was validated for uncracked post-tensioned concrete beams.

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Vertical displacements are one of the crucial parameters defining, for example, the loadcarrying capacity of a bridge deck in short and long terms monitoring. Bridge managers are always looking for an easy way to measure vertical displacements of bridges. However, such measurements are difficult to perform. With the advancement of fiber-optic technologies, Fiber Bragg Grating (FBG) sensors are more commonly used in structural health monitoring due to their outstanding advantages including multiplexing capability as well as high resolution and accuracy. In this study, FBG-Differential Settlement Measurement (DSM) sensors, connected by hydrostatic leveling system of communicating vessels, were used for the displacement measurements along a largescale Prestressed Concrete I (PCI) beam. Specifically, the member was subjected to a set of threepoint bending tests in the laboratory. The measured displacements matched well with the corresponding experimental values using Linear Variable Differential Transformers (LVDT). In addition, in situ experiments on Bridge No. 24 of Highway No. 86 in Taiwan indicated that FBG-DSM system can be effectively employed to measure vertical displacements along span bridges. In conclusion, the proposed FBG-DSM system can be applied referring to an absolute reference and 2 without any external physical reference.