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Browsing by Author "Wei, Gengrui"

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    Computational and Experimental Study on the Behavior of Diaphragms in Steel Buildings
    Wei, Gengrui (Virginia Tech, 2022-02-03)
    The lateral force resisting system (LFRS) of a steel building consist of two parts, i.e., a vertical LFRS such as braced frames or shear walls, and a horizontal LFRS with diaphragms playing a crucial role. There are various types of floor and roof diaphragms in steel buildings, such as concrete-filled steel deck diaphragms for the floor system and bare steel deck diaphragms for the roof system of a typical steel braced frame building, and standing seam roof diaphragms for a typical metal building. Compared to vertical elements of a building's LFRS, our understanding of the horizontal elements, i.e., the diaphragms, is grossly lacking. The motivation for this work comes from the gaps identified in the research, including the lack of generally adopted acceptance criteria and modeling protocols for seismic performance-based design of bare steel deck and concrete-filled steel deck diaphragms through linear and nonlinear analysis, the need to better understand the complex behavior of concrete-filled steel deck diaphragms with irregular configurations such as reentrant corners and openings under lateral loading, the absence of appropriate Rs values for the alternative diaphragm seismic design approach in the current building code that considers diaphragm inelasticity, and the demand for understanding the in-plane behavior of a standing seam roof system and its use in lateral bracing of rafters in metal buildings. A series of computational and experimental studies were conducted to investigate the behavior of diaphragms in buildings systems, including: 1) development of acceptance criteria and modeling protocol for performance-based seismic design of bare and concrete-filled steel deck diaphragms using a database of existing cantilever diaphragm tests; 2) a computational study on the nonlinear behavior of diaphragms with irregular configurations under lateral loading using high-fidelity finite element models validated against experiment test results; 3) investigation of the seismic behavior and performance of steel buildings with buckling restrained braced frames that considers different diaphragm design approaches and diaphragm inelasticity using nonlinear three-dimensional (3D) computational models; and 4) an experimental study that investigated the in-plane behavior of full-scale standing seam roof assemblies and their use in lateral bracing of rafters in metal building systems. The results of these studies contribute to a better understanding of the behavior of diaphragms in steel buildings and lead to several recommendations for diaphragm design. Firstly, a series of m-factors (ductility measures) and nonlinear modeling parameters (multi-linear cyclic backbone curves) were determined for bare steel deck diaphragms and concrete-filled steel deck diaphragms. These new provisions are recommended for adoption in ASCE 41 / AISC 342, which allows the use of ductility in steel deck diaphragms for their design and retrofits. Secondly, results of the finite element analysis on concrete-filled steel deck diaphragms revealed a concentrated distribution of shear transfer through the shear connections on the collectors of the diaphragm near braced frames and a stress concentration in the composite slab near reentrant corners and openings. Thirdly, results of eigenvalue analyses with nonlinear 3D building models showed that the consideration of diaphragm flexibility led to an increase in first mode period between 13% and 48%. A comparison of results from pushover analyses and response history analyses indicated that even though the pushover analyses (based on a first mode load pattern) identified the BRBF as being weaker than the diaphragms and therefore dominating the inelastic pushover behavior, response history analyses demonstrated that the diaphragms can experience substantial inelasticity during a dynamic response. The response history results also suggest that there would be a significant difference in seismic behavior of buildings modeled as two-dimensional (2D) planar frames as compared to the 3D structures modeled herein. Furthermore, the observed final collapse mode involves an interaction between large BRBF story drifts combined with diaphragm deformations that are additive and exacerbate second order effects leading to collapse. The computed adjusted collapse margin ratios for all buildings satisfied the FEMA P695 criteria for acceptance. Therefore, it is concluded that the alternative diaphragm design procedure with the proposed Rs values (Rs = 2 for concrete-filled steel deck diaphragm and Rs = 2.5 for bare steel deck diaphragm) are reasonable for use in design of these types of structures. Lastly, the effects of different standing seam roof configurations (panel type, clip type, thermal insulation, and purlin spacing) on the in-plane stiffness and strength of the standing seam roof system were investigated through an experimental testing program, and a method was described to use these experimental results in the calculations of required bracing for metal building rafters.
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    Computational Study of Tension Field Action in Gable Frame Panel Zones
    Wei, Gengrui; Koutromanos, Ioannis; Murray, Thomas M.; Eatherton, Matthew R. (2018-04-01)
    A computational study was conducted to evaluate the extent to which the column panel zone in a gable frame knee joint can develop tension field action when subjected to positive bending (top flange of the rafter is in tension). Past testing was reviewed and used to validate finite element models. The validated finite element modeling approach that utilized shell elements, was then used in a parametric study to evaluate the post-buckling strength of the panel zone beyond the shear buckling strength. In addition, a plastic mechanism model was used to derive an equation for the post-buckling shear strength of the panel zone and the derived equation was validated against the results of the parametric study. The result was a proposed equation for tension field action strength of the panel zone when subjected to positive bending that had an average error of 1% compared to the model results.