Modified Elastofiber Element for Steel Slender Column and Brace Modeling
Publication: Journal of Structural Engineering
Volume 136, Issue 11
Abstract
An efficient beam element, the modified elastofiber (MEF) element, has been developed to capture the overall features of the elastic and inelastic responses of slender columns and braces under axial cyclic loading without unduly heavy discretization. It consists of three fiber segments, two at the member ends and one at midspan, with two elastic segments sandwiched in between. The segments are demarcated by two exterior nodes and four interior nodes. The fiber segments are divided into 20 fibers in the cross section that run the length of the segment. The fibers exhibit nonlinear axial stress-strain behavior akin to that observed in a standard tension test of a rod in the laboratory, with a linear elastic portion, a yield plateau, and a strain-hardening portion consisting of a segment of an ellipse. All the control points on the stress-strain law are user defined. The elastic buckling of a member is tracked by updating both exterior and interior nodal coordinates at each iteration of a time step and checking force equilibrium in the updated configuration. Inelastic postbuckling response is captured by fiber yielding, fracturing, and/or rupturing in the nonlinear segments. The key features of the element include the ability to model each member using a single element, easy incorporation of geometric imperfection, partial fixity support conditions, member susceptibility to fracture defined in a probabilistic manner, and fiber rupture leading to complete severing of the member. The element is calibrated to accurately predict the Euler critical buckling load of box and I sections with a wide range of slenderness ratios ( , 80, 120, 160, and 200) and support conditions (pinned-pinned, pinned-fixed, and fixed-fixed). Elastic postbuckling of the Koiter-Roorda L frame (tubes and I sections) with various member slenderness ratios ( , 80, 120, 160, and 200) is simulated and shown to compare well against second-order analytical approximations to the solution even when using a single-MEF element to model each leg of the frame. The inelastic behavior of struts under cyclic loading observed in the experiments of Black et al., Fell et al., and Tremblay et al. is accurately captured by single-MEF-element models. A FRAME3D model (using MEF elements for braces) of a full-scale six-story braced frame structure that was pseudodynamically tested at the Building Research Institute of Japan subjected to the 1978 Miyagi-Ken-Oki earthquake record is analyzed and shown to closely mimic the experimentally observed behavior.
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Acknowledgments
The writer is grateful to Professor Chia-Ming Uang (University of California, San Diego) for providing the data from the pseudodynamic test of a full-scale six-story structure, conducted in the years 1982–1984, to Professor Robert Tremblay (Ecole Polytechnique, Montreal, Canada) for providing data from his tests on concentrically braced steel frames with tubular braces, and to Professor Amit Kanvinde (University of California, Davis) and Professor Benjamin Fell (California State University, Sacramento) for extensive discussions on brace behavior and for providing the data from their NEESR brace-testing project. The writer appreciates the continued support by the National Science Foundation (NSF), the Southern California Earthquake Center (SCEC), and the United States Geological Survey (USGS) to his research program. Finally, the writer is thankful for critical comments from the reviewers and the journal editors who have helped enhance this paper significantly.UNSPECIFIED
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Published In
Journal of Structural Engineering
Volume 136 • Issue 11 • November 2010
Pages: 1350 - 1366
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© 2010 ASCE.
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Received: Jun 22, 2009
Accepted: Apr 23, 2010
Published online: Oct 15, 2010
Published in print: Nov 2010
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