Experimental study on the mechanical behaviour of castellated composite beams under a negative bending moment

In this study, mechanical tests were performed on eight castellated composite beams with hexagonal openings under a negative bending moment. The effects of concrete slab, opening rate, and transverse stiffener on the failure mode, bearing capacity, and stress distribution around the openings of the castellated composite beams were examined. The test results revealed that when the negative moment acts alone, the castellated composite beams fail owing to buckling of the compression flange. When the negative moment and shear force act together, the castellated composite beams fail because of buckling of the web post. The development curve of the stress around the opening can be used as a reference for determining the yield load of the specimen. The transverse stiffener can effectively limit the buckling of the web post of the composite beam. As the opening rate increases from 0.47 to 0.6, the bearing capacities of the castellated composite beams under the two working conditions decrease by 8.7% and 10.5%, respectively. Following the addition of the concrete slab, the bearing capacities increase by 16.8% and 7.2%, respectively. Accordingly, a calculation method for the bearing capacity of castellated composite beams considering coupling between the bending moment and shear force under a negative bending moment was proposed. The calculation results were found to be consistent with the experimental results.