Influence of Standoff Distance on the Response of RC Columns Subjected to Close-In Explosions

Threats involving terrorist attacks using large amounts of explosives are less likely due to the complexity of acquiring them without alerting security agencies. A plausible threat tactic is to detonate small quantities of concealed explosives in contact with critical load-bearing structural members to compromise the structure’s stability, leading to its partial or complete collapse. This paper investigates the response of reinforced concrete (RC) columns subjected to blast loads from a small standoff. Columns in buildings are commonly cladded for aesthetics or concealment and can thus be used to create a small standoff distance between the explosive and the column. In this study, three full-scale columns were subjected to blast loads from 115-g and 500-trinitrotoluene (TNT) charge masses placed at 50- and 100-mm standoff distances. The postblast residual axial load capacities of the damaged columns were obtained experimentally. The results were compared to benchmark results from RC columns subjected to the same explosive mass detonated in contact with the columns. The results of the experimental studies indicate that the small standoff distance significantly reduced the column damage. Furthermore, a validated LS-DYNA numerical model has been developed and used in parametric studies to evaluate the effects of various design parameters on the behavior of columns under blast loading. The predominant failure mode of columns subjected to blast loading at scaled distances below ${0.1\mathrm{m}/\mathrm{kg}}^{1/3}$ was a local material failure in the core and shear/flexural failure when the scaled distance was above ${0.1\mathrm{m}/\mathrm{kg}}^{1/3}$. The numerical results indicate that the Hopkinson–Cranz scaling law is not applicable at small-scaled distances.