Restrained Torsional Response of Composite Box Girder Bridge with Corrugated Steel Webs During Balanced Cantilever Construction

During the balanced cantilever construction of long-span composite box-girder bridges with corrugated steel webs (CSWs), eccentric construction loads induce restrained torsion in addition to bending and shear. If this torsional contribution is not adequately accounted for, the resulting warping normal stresses in the concrete slabs and shear stresses at the CSW–slab interfaces may be significantly underestimated—particularly during stages when the cantilever section remains partially open and torsional stiffness is reduced. Although restrained torsion has been extensively studied for prismatic or fully closed box sections, the response of variable-depth composite girders with CSWs under asynchronous pouring construction (APC)—where the cantilever tip may be temporarily unclosed—has not been systematically clarified. To address this gap, this study proposes an equivalent modeling approach that incorporates the orthotropic characteristics of CSWs. Based on the restrained-warping theories of Umanskii and Vlasov, the governing differential equations for an eccentrically loaded cantilever are derived and solved via a finite-difference scheme with appropriate end-boundary conditions, yielding closed-form expressions for warping normal and shear stresses. The proposed analytical method is validated against three-dimensional finite-element (FE) simulations of an actual bridge. The validated FE model is then used to simulate the cantilever erection process, systematically evaluating the effects of construction scheme, diaphragm casting sequence, and critical APC stages on the torsional response. Results indicate that the equivalent CSW model accurately captures the torsional behavior of variable-depth composite girders. Restrained torsion from eccentric loading produces substantial secondary stresses: warping normal stresses in the bottom and top slabs reach up to 26% and 18% of their bending counterparts, respectively, whereas warping shear stresses in the CSWs account for approximately 22% of the shear stress. Torsional resistance is highly sensitive to the construction method; APC sequences, in particular, induce highly variable warping stresses near the cantilever tip, where eccentric loading should be avoided. Critical-stage analysis further reveals that the long-cantilever stage with an unclosed section represents the most vulnerable condition, owing to abrupt changes in torsional stiffness and local stress concentrations at the end-section transition. Internal diaphragms promote a more uniform distribution of warping deformation from the fixed support to the cantilever end; accordingly, it is recommended that each diaphragm be cast promptly upon completion of its corresponding segment. Overall, the proposed analytical–numerical framework offers a practical tool for rapid restrained-torsion assessment, identification of critical construction stages and regions, control of eccentric construction loads, and rational selection of diaphragm casting sequences—thereby supporting construction-stage risk mitigation for long-span bridges with CSWs.