Non-uniform torsion on a curved girder in the girder system analysis is determined by dividing the primary bending moment by the radius of curvature. The lateral distributed load on the flanges then is obtained by converting the torsion to a force couple acting on the flanges. If the torsion is acting on a composite section, the upper force in the couple is assumed to act at mid-depth of the slab.
Lateral bending moment in the flange is determined as if the bracing locations act as fixed ends to the beam segments between bracing. This practice is according to guidance provided by the USS HIGHWAY STRUCTURES DESIGN HANDBOOK, VOL II, section II/6. At points between the bracing the varying lateral bending moment then is determined by statics using lateral moment in the flange at each of the straddling braces, and the trapezoidal distribution of the lateral force q on the side of the flange. Lateral bending moment in top flanges of box girders also includes that due to the inclined webs.
Finally, lateral bending stress is determined by dividing the lateral bending moment in the flange by the section modulus of the flange taken about an axis parallel to the web.