Browsing by Author "Jeffers, Ann E."

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    A Fiber-Based Approach for Modeling Beam-Columns under Fire Loading
    Jeffers, Ann E. (Virginia Tech, 2009-06-19)
    The work described herein emphasizes a new fiber-based approach to modeling the response of structural frames subjected to realistic fire conditions. The proposed approach involves the development and validation of two finite elements that can be used collectively to simulate the thermal and mechanical response of structural frames at elevated temperatures. To model the thermal response, a special-purpose fiber heat transfer element is introduced. The first of its kind, the fiber heat transfer element uses a combination of finite element and finite difference methods to provide an accurate and highly efficient solution to the three-dimensional thermal problem. To simulate the mechanical response, a flexibility-based fiber beam-column element is used. The element presented here extends the formulation of Taucer et al. (1991) to include thermal effects, geometric nonlinearities, and residual stresses. Both fiber elements are implemented in ABAQUS (2007) using the user-defined element (UEL) subroutine. The element formulations are verified by analyses of benchmark experimental tests and comparisons with traditional finite elements. Results indicate that both elements offer superior accuracy and computational efficiency when compared to traditional methods of analysis. Analyses of structures subjected to non-uniform heating emphasize the advantages of the fiber-based approach. To demonstrate a realistic application of the proposed approach, the work concludes with an investigation of the response of unprotected steel beams subjected to localized fires. Because realistic fires are considered, the treatment of strain reversal upon cooling is also addressed. The analyses are used to demonstrate that the standard fire test is generally unconservative at predicting the time at failure of a structure subjected to realistic fire conditions, since failure depends more on the evolution of temperatures within the steel beams than the duration of fire exposure. The analyses also show that critical temperatures from the standard fire test are conservative and thus offer a better means for predicting failure in steel structures within the scope of the standard fire test.
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    Vibration Isolation of a Horizontal Rigid Plate Supported by Pre-bent Struts
    Jeffers, Ann E. (Virginia Tech, 2005-12-08)
    The purpose of this research is to analyze a new type of vibration isolator consisting of two pre-bent struts which are clamped at both ends and intermediately bonded with a viscoelastic filler. The proposed isolation device has the ability to support a relatively large static load with little deflection and offers a low axial resistance under dynamic excitation, making it ideal for isolating vertical vibrations. In this research, four of these vibration isolators are used to support a rigid, square plate. The symmetric case is analyzed first. Then the plate has a center of mass which is located at some distance from the geometric center of the plate. When the system is subjected to vertical harmonic base excitations, this eccentric weight introduces rotational as well as vertical motions of the plate. This research will investigate the effects of various eccentricities on the efficiency of the vibration isolators in the configuration described. The displacement transmissibility will be the measure of the isolators' effectiveness at mitigating vibrations transmitted from the base to the rigid plate. For each case, the nonlinear equilibrium equations and the governing equations of motion for small vibrations about equilibrium are numerically solved, and the transmissibility is calculated and plotted over a wide range of frequencies. These plots are used to recognize ranges of frequencies for which isolation is achieved and frequencies at which resonance occurs in the system. At the resonant frequencies, the physical behavior of the system is analyzed to determine the types of vibration modes which occur in the system. A free vibration analysis is also performed to obtain a better understanding of resonances in the system.