Blast Load Prediction for Deflagration of Low Explosives in Confined Structures

Presented By: Ming Liu, NAVFAC - EXWC

Description: According to Section 5.4.3 of ACI 370R-14, when design of concrete structures involves containment of internal explosion effects, both shock waves and gas pressures should be considered. For high explosives (HE) detonations, empirical relationships for predicting gas loads are provided in UFC 3-340-02. Logically, the TNT equivalencies for low explosives (LE) such as propellants and pyrotechnics are used in some cases to predict the internal gas pressure-time histories, as mentioned in Section 5.2.2 of ACI 370R-14. Section 5.2.2 also points out that “the designer should insist on an adequate determination of TNT equivalency, including energy, pressure, and impulse equivalents.” However, the confined burns of LE without venting generates deflagration so that the gas pressures can last tens of minutes or even hours as long as the internal temperature decays are slow enough. Thus, any dynamic analysis such as single degree-of-freedom (SDOF) is not applicable herein. The confined burns of LE with venting involve complex convective combustion processes where chemical/combustion and aerodynamic experts should play an important role in predicting gas loads. As a result, this paper provides the evidence on why the TNT equivalency should not be used in blast design for containment of confined burns of LE. This paper also provides the simplified procedures in estimating the quasi-static gas loads for the confined burns of LE without and with venting, after briefly comparing intrinsic characteristics in HE detonations with those in LE burns.