Framework for Simplified Live Load Distribution-Factor Computations
Publication: Journal of Bridge Engineering
Volume 16, Issue 6
Abstract
The live load distribution-factor (LLDF) equations in the AASHTO-LRFD specifications were developed under National Cooperative Highway Research Program (NCHRP) Project 12-26. These equations include limited ranges of applicability, and when these ranges are exceeded, a refined analysis must be used. Additionally, the multiple-presence factors, bias, and variability with respect to the rigorous estimates are obscurely embedded. Herein, a simplified LLDF framework is provided. The effects of analysis uncertainty, variability, and multiple presence are separated and distinctly defined. This separation provides specification writers with the opportunity to use different multiple-presence and variability models. LLDFs were calculated using several simplified methods and grillage analyses for over 1,500 bridges. Based on the comparison, two simplified methods were further studied: adjusted uniform distribution and adjusted lever rule methods. Calibration factors were used to their improve accuracy. Seventy-four “test-the-limits” bridges (11 steel) were designed to test wide ranges of girder spacing, relative transverse to longitudinal stiffness, and bridge flexural-to-torsional stiffness. The focus of this paper is on the framework. The procedure and results are similar for other bridge types.
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Acknowledgments
The research reported herein was performed under NCHRPNCHRP Project 592 by BridgeTech, Inc., Department of Civil Engineering, Tennessee Technological University, Department of Civil Engineering, University of Delaware, and HDR, Inc. The support of NCHRP is gratefully acknowledged. The Wyoming Department of Transportation provided BRASS-GIRDER (LRFD) and permitted programming of various simplified methods. The content and ideas presented herein are those of the writers and not those of NCHRP. The NCHRP 592 panel is especially acknowledged for the helpful suggestions and thoughtful guidance during the project.
References
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Information & Authors
Information
Published In
Journal of Bridge Engineering
Volume 16 • Issue 6 • November 2011
Pages: 777 - 791
Copyright
© 2011 American Society of Civil Engineers.
History
Received: Jul 20, 2010
Accepted: Jul 6, 2011
Published online: Jul 8, 2011
Published in print: Nov 1, 2011
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