<p>This study aims to understand the shear resistance mechanism of reinforced concrete (RC) beams as a combination of arch action and beam action, and to propose and validate a new truss model that numerically simulates this mechanism. By introducing the arch coefficient (<i>α</i>), a simplified and effective method for describing the internal force flow of RC beams was developed. Analysis of experimental results demonstrated that the new truss model incorporating the arch coefficient predicts shear strength more accurately than existing design codes (ACI 318 Code, Eurocode 2). The shear strength prediction formula considering arch action exhibited particularly high accuracy for beams with a shear span-to-depth ratio (<i>a/d</i>) greater than 2.5, addressing the conservatism or overestimation issues of current codes. Furthermore, the proposed model's applicability and versatility were validated for beams without shear reinforcement and T-shaped beams with shear reinforcement. These findings are expected to contribute to the improvement of future design codes and the development of high-performance RC structures.</p>