# SimEx: A Tool for the Rapid Evaluation of the Effects of Explosions
## 1 Introduction  
Unlike the slow energy release exhibited by deflagrations, the instantaneous energy deposition associated with the detonation of a high explosive produces an extremely rapid increase in temperature and pressure due to the sudden release of heat, light, and gases [1]. The gases produced by the explosion, initially at extremely high temperatures and pressures, expand abruptly against the surrounding atmosphere, vigorously pushing away any other object that may be found in their path. This gives rise to the two most notable effects of explosions: the aerial, or blast, wave [2], and the projection of shell fragments or other items (i.e., secondary fragments) located in the surroundings of the charge [3]. If the explosive device is located at a ground level, a fraction of its energy is effectively coupled to the ground, generating seismic waves and a well distinguished surface crater that results from the ejection of the shattered ground materials in direct contact with the charge [4]. Quantifying these phenomena and assessing their effect on the environment, including structural elements, vehicles, objects, or people located around the blast site, is a highly complex task that requires a thorough knowledge of the physical-chemistry of explosions [1; 5; 6; 7] and their dynamic interactions with nearby structures [8] or the human body [9]. As a result of the growing terrorist threat experienced in the last few decades [10], estimating the effects of explosions has become a critical issue in the design, protection, and restoration of buildings and infrastructures, both civil and military [11]. However, this task is far from trivial, in that it involves transient compressible flows, nonlinear structural response, and highly dynamic fluid-structure interactions. These phenomena can be described with some accuracy using multiphysics computational tools, also known as hydrocodes [12], such as Ansys Autodyn, LS-Dyna, or Abaqus, based on the explicit finite element method [13]. In the simulations, all the critical components are modeled, including the detonation of the explosive charge, the resulting blast wave, the induced dynamic loads, and the nonlinear structural response. However, the enormous computational effort required to complete detailed computational analyses, which includes not only the calculation time itself, but also complex pre- and post-processing stages, remains a critical issue. For instance, simulating the effect of an explosive charge on a full-scale bridge may require more than 10 million finite elements [14]. For this reason, most engineering analyses still make use of simplified models for determining the explosive loads and estimating the resulting dynamic structural response in a timely manner. This enables the fast computation of damage diagrams in the Charge Weight-Standoff distance (CW-S) space, of utility to determine the level of protection provided by an input structural component loaded by blast from an input equivalent TNT charge weight and standoff [15]. In this regard, the American Unified Facilities Criteria UFC 3-340-02 [16], which supersedes the former ARMY TM 5-1300, establishes the requirements imposed by the US Department of Defense in the tasks of planning, design, construction, maintenance, restoration, and modernization of those facilities that must be protected against explosive threats. In the absence of similar regulations in other countries, UFC 3-340-02 [16] is widely used by engineers and contractors outside the US, as it provides a valuable guide for calculating the effects of blast-induced dynamic loads, including step-by-step procedures for the analysis and design of buildings to resist the effects of explosions. To facilitate the application of the procedures set forth in the UFC 3-340-02 [16], as well as other analyses established in classic references of explosives engineering [3; 5; 6; 9; 17; 18; 19], fast evaluation software tools have been developed that incorporate the vast amount of data available as tables or graphs in the literature [7].