Safety Monitoring and Effect Evaluation of Variable Cross-Section Continuous Box-Girder Bridges Strengthened with External Prestressing

External prestressing technology has been widely applied to mitigate the cracking and deflection of variable cross-section continuous box-girder bridges. However, it remains a challenging task to establish a reliable safety monitoring and evaluation method in engineering applications. This paper takes a practical project as the research background and installs a sensing monitoring system on a bridge before strengthening. The evolutions of the strain at the characteristic sections, crack width, and deflection of bridges before and after strengthening were compared and analyzed. The construction process’s safety and strengthening measures’ effectiveness were monitored and evaluated. According to the construction process monitoring, the prestress tensioning process did not bring torsion to the bridge, and the strain at each monitoring point was within safe boundaries. After strengthening, the bridge appeared to camber with a value of 3–6 mm. The typical cracks were mitigated to different extents, and the closure degree of the crack width was (2.5–4)×10−2 mm. The strain distribution of the bridge was more stable after strengthening, and the frequency of large strain was significantly reduced, which reflected the improvement of section stiffness. The study can provide meaningful references for the optimization design and strengthening strategies of similar projects.