Modeling the effects of detonations of high explosives to inform blast-resistant design
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Blast-resistant design has traditionally been performed using simplified methods. The most widely used method models structural members such as columns and beams as single-degree-of-freedom (SDOF) systems and characterizes the blast load using a peak reflected overpressure and a reflected impulse, where both are calculated using empirical design charts such as those provided in UFC-3-340. A uniform loading over the height (length) of the component is assumed. Moreover, the empirical design charts assume either a spherical (free-air) burst of a hemispherical (surface) burst and do not consider variations in charge shape, charge orientation and point of detonation within the charge. A comprehensive survey of different strategies to model detonations in commercially available finite element (FE) codes LS-DYNA and AUTODYN and the computational fluid dynamics (CFD) code Air3D was conducted. A robust modeling strategy capable of modeling different charge shapes and fluid-structure interaction was identified in LS-DYNA and the use of remapping capabilities of AUTODYN and Air3D was demonstrated. A near-field detonation of a spherical charge of TNT and the subsequent fluid-structure interaction with a rigid column was modeled using the three codes. The resulting peak reflected overpressure and impulse monitored on the face of the column were compared with those computed using the empirical design charts in the UFC. There were significant differences between the results of the FE and CFD analysis, and between these results and the predictions of UFC-3-340. The reasons for the differences include the change in the value of the constant specific heat ratio, γ, of air at high temperatures, the effect of afterburning on the reflected impulse, and the effect of blast-wave clearing on both the reflected peak overpressure and impulse. A numerical study was performed using AUTODYN to study the influence of charge shape, charge orientation and point of detonation within the charge on the overpressure distributions and the response of an A992 Grade 50 W14×257 column. A set of analyses was performed with cylindrical charges with different aspect ratios. Results were compared with those involving a baseline analysis of a spherical charge. The resulting peak incident overpressure and impulses, and the pressure contours were compared in the near-, mid- and far-fields. In the near-field, Z 0.33 </super>, the overpressure distributions are influenced significantly by charge shape and the point of detonation in the charge. The influence of these variables diminishes with distance. The loading and subsequent response of the W14×257 column to a detonation of 1000 kg of TNT at a standoff distance of 3 m showed significant dependence on the charge shape and charge orientation and clearly demonstrated that SDOF assumptions are inappropriate for blast-resistant design against detonations of improvised explosive devices at small standoff distances.