In the ancient concrete building, concrete beams with plain steel reinforcement under the load will evolve from flexural failure to shear failure with the increase of service time. The existing bearing capacity analysis model of the concrete beam with plain steel reinforcement couldn’t distinguish these two failure modes. By selecting concrete beam reinforced with plain bars as research object, the calculated formula of bonding strength between plain steel bar and concrete is proposed and the effect of relatively small bond strength on the load bearing capacity of concrete beams was analyzed in detail. Based on the strut-tie model and the standard truss model, the bearing capacity of the concrete beams was calculated. The results show that the average ratio of the test values to the analytical values of the strut-tie model and the standard truss model was 1.460 and 1.309, respectively, and the variance was 0.858 and 0.781, respectively. the variable angle truss calculation model was established which could distinguish the failure mode of the concrete beams with plain steel reinforcement. The suggested model was validated by the experimental data of 32 concrete beams. The average ratio of the test results of the bearing capacity of the concrete beams to the calculated values of the variable angle truss model was 1.034 and the variance was 0.100, which indicated that test results and prediction results were in a good agreement. The modified variable angle truss model could precisely predict the failure mode of the reinforced concrete beam with plain steel bar. Thus, the suggested model could be used to calculate the load bearing capacity and analyze the failure mode of concrete beams reinforced with plain steel bars.
