Numerical Investigation of Failure Modes of Reinforced Concrete Beams Under Eccentric Near-Field Air Blast Loading with Experimental Validation

As primary structural components, the damage characteristics and failure modes of reinforced concrete (RC) beams under near-field blast loads are essential for blast-resistant design and vulnerability analysis. To address the research gap regarding the failure modes and blast performance of RC beams under eccentric explosions, this study systematically investigates the effects of charge mass and eccentric distance on structural damage. This was achieved through three near-field air blast tests with varying charge masses and explosion locations, supplemented by LS-DYNA numerical simulations. The experiments utilized 1/2-scale RC beam specimens, and the numerical simulations were conducted using the ALE fluid–structure interaction (FSI) algorithm. A classification criterion for beam failure modes was established using a deformation decoupling method, based on the shear deformation ratio (δ). Results indicate that under eccentric explosions that do not trigger significant local damage, the beams primarily exhibit global deformation. Under a charge mass of 2 kg TNT, as the eccentric distance (e) increases from 0 (mid-span) to 0.90 m, the maximum vertical displacement of the RC beam decreases from 3.50 cm to 1.37 cm (a reduction of approximately 60%). The shear deformation ratio δ at the point of maximum displacement first decreases from 0.3117 at mid-span to a minimum of 0.0670 at e = 0.90 m, then rises to 0.2635 at e = 1.05 m, exhibiting a clear “V-shaped” trend. Increasing the charge mass from 2 kg to 2.5 kg for mid-span explosions raises the maximum displacement from 3.50 cm to 8.22 cm (an increase of 135%) and causes δ to increase from 0.3117 (flexural-shear failure) to 0.4428 (shear-like failure). The inflection point of the “V-shaped” δ curve shifts inward from e = 0.90 m (2 kg) to approximately e = 0.45 m (2.5 kg), indicating a transition toward shear-dominated failure modes with increasing charge mass. As the equivalent increases, the failure mode gradually shifts toward a shear-dominated mode, and the inflection point of the deformation ratio shifts toward the mid-span. These findings provide a theoretical foundation and technical support for the damage assessment and blast-resistant design of RC structures.