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Partially encased composite beams (PECBs) have advantages over conventional steel–concrete composite beams in load-carrying capacity, flexural stiffness and fire resistance. In order to determine whether the shearing force is sufficient to ensure the yield of the tensile reinforcement in the case of a large tensile reinforcement ratio, as well as the influence of encasing concrete strength and the addition of studs on the steel web, three PECB specimens were tested under bending. The results show that, in the case of a 5% tensile reinforcement ratio, natural bonding and friction forces ensure the yield of tensile reinforcement whether studs are added on the steel web or not. The encasing concrete strength and the addition of studs on the steel web have no obvious effect on both the elastic and plastic bending resistance of PECBs. The addition of studs on the steel web significantly slows down the stiffness deterioration of PECBs within the elastoplastic stage, while the flexural stiffness is not obviously affected by the strength of encasing concrete. The simplified plastic theory is proved to be applicable to predict the flexural capacity of PECBs with a large tensile reinforcement ratio. It is also indicated by calculation that, by increasing the tensile reinforcement ratio from 2% to 5%, the flexural capacity of PECBs has a significant increase, by about 32%.