AbstractIn this study, the flexural behavior of noncompact and slender concrete-filled U-section (CFU) beams was investigated focusing on local buckling of U-section walls and shear transfer across the beam-to-slab interface. Seven large-scale CFU beam specimens were tested under negative bending. The width-to-thickness ratios of U-section walls (i.e., flanges and webs) and stiffening details for preventing local buckling were considered as the test variables. The tests showed that, due to excessive slab reinforcement, all specimens were failed by shear failure at the composite interface of beam and slab after flexural yielding. The ultimate strength and deformation capacity of CFU beams were degraded by flange and web local buckling. Buckling modes of U-section walls were affected by the stiffening details. The ultimate strengths of noncompact and slender CFU beams were compared with the nominal strengths calculated in accordance current standards. In addition, the flexural behavior of CFU beams affected by the local buckling and partial debonding of U-section walls and slip behavior at the composite interface was further investigated through fiber-based section analyses based on the effective stress–strain models of materials.