A long-gauge fiber Bragg grating (FBG) strain sensor with enhanced strain sensitivity is proposed, which is encapsulated with two T-shaped metal blocks. Its fabrication method is described briefly, and the strain sensitivity can be flexibly adjusted through changing its packaging method. A series of experiments are carried out to study the packaging and its sensing properties. The experimental results show that the strain and temperature sensitivity coefficient of the sensor are three times larger than the common FBG sensors. The linearity coefficients of the FBG sensor are larger than 0.999, and the relative error of the repeatability of all sensor samples is less than 1%. Through the stability test on the actual bridge, it is revealed that the long-term stability of the sensor is excellent, and the maximum error is less than 1.5%. In addition, the proposed FBG strain sensors are used to conduct a shear strengthening experiment on a reinforced concrete (RC) beam to verify its working performance. The experimental results show that the strain change and crack propagation of the RC beam are well monitored by the sensors during the loading process.

In order to improve the accuracy of load identification and study the influence of transverse distribution, a novel method was proposed for the moving load identification based on strain influence line and the load transverse distribution under consideration. The load identification theory based on strain influence line was derived, and the strain integral coefficient was proposed for the identification. A series of numerical simulations and experiments were carried out to verify the method. The numerical results showed that the method without considering the load transverse distribution was not suitable for solving the space problem, and the method with the load transverse distribution under consideration has a high identification accuracy and excellent anti-noise performance. The experimental results showed that the speed identification error was smaller than ±5%, and the vehicle speed had no obvious influence on the identification results of the vehicle weight. Moreover, the average identification error of the vehicle weight was smaller than ±10%, and the error of more than 90% of samples was smaller than ±5%.

A new method was proposed for the damage identification of box girder bridges under moving load, wherein the difference of strain influence line (DSIL) was taken as an index to represent the long-gauge strain difference before and after damage. The damage identification theory based on long-gauge strain influence lines was derived for box girder bridges with shear lag effect under consideration, and a regularized index DSIL was proposed for the quantitative identifications of damage location and extent. A series of experiments were carried out to study the influences of speed, vehicle type, and vehicle weight on the damage identification, and the experimental data were obtained by long-gauge fiber Bragg grating strain sensors. Moreover, numerical simulations were performed to confirm the method. The experimental and numerical results show that the method can locate the damage accurately, and quantitatively identify the damage extent under different working conditions. The experimental damage extent is generally slightly higher than the theoretical, with an average identification error smaller than 5%. Additionally, the relative error of damage extent is smaller than 3% under different working conditions. Thus, the effectiveness of this method was verified.

In this study, a finite element (FE) model of the Xiuzhen River bridge was developed, and the validity of the model was verified using monitoring results, load tests and theoretical calculations. Subsequently, a series of FE simulations were conducted on a model of the Xiuzhen River Bridge strengthened using K-brace composite trusses (K-BCTs). Static, eigenvalue and response spectrum analyses were carried out for 16 prestressed concrete box-girder (PCB) bridge strengthening models with different K-BCT stiffness and spacing. The results revealed that K-BCTs could reduce the girder deflection, improve the load distribution, and reduce the distortion stress. A slightly better-strengthening effect in the K-BCTs with higher stiffness and smaller spacing. The dynamic analysis revealed that the K-BCT stiffness and spacing influenced the higher vibration modes but had little effect on the lower modes. Furthermore, the K-BCT had no significant effect on the maximum internal force of the model under different seismic waves, and the maximum deviation was within 5%. Finally, two K-BCTs made of concrete-filled steel tubes, with a spacing of 3 m and an elastic modulus of 50 GPa, were used to strengthen the existing bridge. The experiment results indicated that the strain and deflection in the Xiuzhen River Bridge were reduced by 53.6% and 20.6%, respectively.