Simulating Blast Effects In Steel Skeletal Structures

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This research embodies a synergistic application of sophisticated commercial finite element codes and internally developed software tools. The contribution to the state of the art is the development of a new approach to constitutive modeling, within a computationally expedient framework, which enables the consideration of blast effects in steel skeletal structures composed of wide-flange members. This research is split into three main parts: (1) development of an air blast load generation code, derived from openly available sources, to calculate spatially and temporally varying blast loads on a structure for a given explosive scenario, (2) formulation and implementation of a blast beam-column element (macro-element) that encapsulates the effects of an explosion on the capacity of a single steel wide-flange member, and (3) testing of the blast beam-column element approach, as implemented in a nonlinear explicit dynamic finite element code, through various validation and application examples. Regarding the first part of the research, blast loading from explosive air burst is modeled using a series of empirical parameters and equations derived from open literature sources, where loads are represented by overpressure time histories that are applied in a spatially varying manner over a finite element domain. Sensitivity studies carried out for dynamic behavior of a plate and girder subjected to blast loading with variations in load parameters show that structural response is sensitive to small changes in load definition. Regarding the second part of the research, a new method for modeling the reserve capacity of a blast-damaged structural component is derived that uses bounding surface plasticity models, defined in stress-resultant space, to represent the effects of the blast by way of a plastic reduction matrix. Each bounding surface is created by generating numerical failure data for a given component and explosive scenario through a series of virtual load tests (using a high-resolution model of a wide-flange member in LS-DYNA), and then fitting the data to a continuous function. For each test, a member that has already been damaged by blast loading (within a dynamic nonlinear finite element analysis), is statically loaded to failure in separate collapse analyses, each with a unique combination of proportional moment-thrust loading. Given a specific blast location and explosive yield, the locus of all force points (moment and axial loads) at failure defines the bounding surface for this member, for a particular blast, where this bounding surface is approximated by a continuous function that is a linear combination of real-valued spherical harmonic functions. Regarding the third part of the research, an automated process is implemented to create a library of bounding surface plasticity models representing the reserve capacity for various wide-flange members under different explosive scenarios. The library of bounding surfaces is coupled to a developed nonlinear explicit dynamic finite element code, written in C, that includes the blast beam-column element. The code does not explicitly model the explosion itself but rather calculates a series of parameters that define the explosion location relative to a given member, and then accesses the library of bounding surfaces to find the bounding surface with parameters that most reasonably approximate the actual physical explosive scenario, and member geometry. Finally, the macro-element-based code is used to predict the post-blast collapse load of a structural system in a few representative test problems, in order to assess accuracy of results and computational savings.

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