A modified single degree of freedom method for the analysis of local-global coupling response of reinforced concrete beams under close-in explosions

Existing single-degree-of-freedom (SDOF) methods inadequately capture the influence of localized damage on the global response of reinforced concrete (RC) structures subjected to close-in blast loading. This study aims to addresses this limitation. Close-in explosion experiments were conducted on RC beams at five scaled height of burst, demonstrating that localized effects are non-negligible in the structural response. A validated numerical model was subsequently developed, and regression analysis of the simulation data was used to establish a predictive model for local damage characteristic dimensions. Furthermore, a modified SDOF framework was formulated to capture the coupled local–global response of RC beams under close-in explosions. A local response model for RC beams under close-in explosions was first formulated based on the traveling hinge concept. A structural resistance-deflection relationship was then developed to account for cross-sectional loss induced by localized damage. The influence of local damage evolution on the mass transformation coefficient and stiffness transformation coefficient was subsequently analyzed, and an equivalent SDOF model was established to characterize the global structural response of RC beams under varying degrees of cross-sectional damage. By integrating the local response model with the equivalent SDOF approach, a global response model incorporating localized damage effects was established and validated against experimental data. The model improves the prediction accuracy, especially for scaled heights of burst below 0.45 m/kg^1/3. For maximum displacement prediction, the traditional SDOF model yields a mean absolute percentage error of 23.28%, while the improved model reduces this error to 6.49%.