Discrete Element Modeling and Analysis of Structural Collapse/Survivability of a Building Subjected to Improvised Explosive Device (IED) Attack

Abstract

Within the present work, the problem of structural integrity (including potential collapse/survivability) of a model building when subjected to a blast attack by a close-proximity vehicle-borne improvised explosive device (VBIED) is investigated using advanced transient, non-linear dynamics, discrete-element modeling (DEM) and simulation computational methods and tools. Since the DEM approach is highly sensitive to the details of the constitutive-material (and contact-interaction) models, a significant portion of the work is devoted to explaining the formulation and the physical basis for the material models used. In particular, since concrete is the key material used in the construction of a building, a critical assessment is provided of the DEM concrete material model employed. To quantify blast-survivability of the building and, in particular, its key structural components, the concept of the so- called design basis threat, DBT (as quantified by the TNT-equivalent charge mass and the associated VBIED standoff distance) is utilized. To help quantify DBT, a parametric study is carried out involving the following design parameters: (a) TNT-equivalent charge mass; (b) VBIED standoff distance; and (c) the degree of concrete reinforcement with steel. The results obtained in the present work also revealed (in a qualitative fashion) the role that the phenomena such as the interaction of the VBIED-induced soil- borne shock waves with the building underground support structure as well as the interaction of the vehicle fragments and the detonation products with the structural elements of the building play in causing damage (and potential collapse) of the building. Within the present work, advanced transient, non-linear dynamics, discrete-element modeling (DEM) and simulation computational methods and tools are used to investigate potential collapse/survivability of a model building when subjected to a blast attack by a close-proximity improvised explosive device (IED). Thus, the main aspects of the present work include: (a) discrete element modeling and simulation methods and tools; (b) impulse loading resulting from the interaction of shrapnel produced during detonation of IEDs, detonation products, explosive casing fragments and soil/pavement ejecta with stationary target structures such as buildings; and (c) constitutive models for the materials (such as concrete) used in the construction of the key structural elements of the targeted buildings. In the remainder of this section, a brief overview of the first two aspects of the current problem will be presented. Then, in Section II, a more detailed account is given of the third aspect of the present problem. A. The Basics of the Discrete Element Method (DEM) The Discrete Element Method (DEM) is a computational modeling and simulation technique within which a material is treated as an assembly/collection of mobile and interacting discrete elements, and the behavior of the material at the macro- length-scale is deduced from the statistical analysis of the (normal, tangential, rolling and twisting) contact-interactions and motions of these elements (e.g. 1) . The state (i.e. position, velocity, acceleration, force and torque) of each discrete particle in the system and its temporal evolution is governed by the basic physical laws, e.g. Newtons Second Law, Hookes Law, and Coulomb friction law. The DEM offers a new way of inferring the basic macroscopic behavior of a material and the formulation of the appropriate material constitutive model. That is, the behavior of the material is prescribed locally through different laws governing the discrete-element interactions while the macroscopic behavior of the material is simply the outcome of the discrete-element interactions. This approach is sharp contrast to the conventional finite element modeling and simulation approach within which macroscopic behavior/properties of the participating materials are prescribed at the outset and the details related to the local kinematic and deformation response are revealed during computational analysis which involves solving the governing mass, momentum and energy conservation equations. Since detailed accounts of the DEM can be found in many sources (e.g. 2) , only a brief overview of this method will be presented in this section. For improved clarity, the DEM is overviewed below by describing separately each of its key components/constituents.