Investigation of the Causes of Web Cracking and Strengthening Methods for Continuous Box-Girder Bridges with Large Cantilevers and Thin Web

To address structural deficiencies—including web cracking, transverse cracking at the girder bottom, and inadequate load-bearing capacity—in existing prestressed concrete continuous box-girder bridges with large cantilevers and thin webs, this study employed field inspections, static and dynamic load tests, and numerical simulations. Longitudinal frame models, transverse solid models, and refined interface models were developed to systematically investigate the effects of prestress loss, vehicle live loads, and temperature on crack formation. Furthermore, the effectiveness of a combined strengthening strategy involving web cross-section enlargement and internal prestressing was evaluated, with particular emphasis on the transfer efficiency of the added prestress across the interface. The results indicate that long-term prestress loss reduces the compressive stress reserve in the box-girder section and is a primary contributor to transverse cracking at the girder bottom, whereas inclined web cracking is primarily caused by the combined effects of shear stress and vertical tensile stress. Eccentric vehicle loading has a pronounced effect on the vertical tensile stress in the web; at the mid-span section, the maximum vertical tensile stress on the inner web face reached 4.24 MPa, of which 2.23 MPa was attributable to eccentric loading. After implementation of the combined strengthening strategy, the bottom-edge stress at the mid-span section decreased from 4.40 MPa to 1.74 MPa, representing a 60.45% reduction, while the mid-span deflection decreased from 30.6 mm to 20.13 mm, corresponding to a 24.54% reduction. In addition, the shear safety factor at the support increased from 0.78 to 1.10, demonstrating substantial improvements in crack resistance and overall structural stiffness. Finally, the interface stress-transfer efficiency reduction in the added internal prestress was 12.87%, indicating that the effect of interface transfer efficiency on the actual strengthening performance should be fully considered when applying the combined cross-section enlargement and internal prestressing method.