Composite effects on rotational constraints of a double-beam system reinforced with beam-end concrete

Composite beam systems have been widely used for building structures since the combination of two materials can provide the effective bending capacity. However, it is still challenging to estimate the constraint effects of concrete material in beam-column joints. In this study, the flexural behavior and composite constraint effects of a novel steel double-beam system with composite joints are investigated for the use of high vertical loads. First, the flexural performance of four 1/3-scale test specimens was experimentally examined according to the presence of beam-end concrete reinforcement in terms of the initial stiffness, ultimate strength, and failure modes. Second, the composite joints were idealized as beam-end rotational springs and the degree of rotational constraints were analytically estimated to quantify the negative moment demand at beam-end and positive moment demand at mid-span. The results showed that the composite joint of the double-beam system made an effective and efficient way to increase the bending capacity and to decrease the moment demands by providing significant rotational constraint effects. It was also found that the test specimen exhibits the constraint effect similar to the rigid connection defined in the ANSI/AISC 360 criteria, thereby reducing the moment demand by 89.3%.