Abstract: A method for calculating a bending moment resistance of an internal unbonded post-tensioned composite beam with corrugated steel webs (CSWs) and a double-concrete-filled steel tube (CFST)lower flange includes: determining a degradation law of sectional flexural rigidity of the internal unbonded post-tensioned composite beam with CSWs and a double-CFST lower flange based on numerical analysis, and establishing a sectional flexural rigidity degradation model of the composite beam. The method can include segmenting a bending moment diagram of the composite beam based on the sectional flexural rigidity degradation model, and establishing a segmented integral equation of IUPS strain increment. The method can include establishing an equilibrium equation of force and a bending moment by considering contributions of concrete, the steel tubes, the upper steel flange, the IUPSs, and reinforcement in the composite beam.