Statistical Significance of Less Common Load Combinations
Publication: Journal of Bridge Engineering
Volume 12, Issue 3
Abstract
The purpose of this work is to provide a simple, rational basis for combining bridge loads in the extreme event limit state. Classic methods are used to evaluate the probabilities of traffic, seismic, and storm-related bridge loads occurring simultaneously and in various intensities. The loads are modeled using a Poisson distribution, which circumvents problems encountered when using normal- or log-normal distributions. The hazard level is evaluated directly using a negative exponential function in the time domain. An acceptable hazard level for combined events is subjectively based on that deemed acceptable for the strength limit state and extreme events occurring individually, as well as fiscal prudence. It is shown that (1) application of seismic loads to a structure already subjected to the combined effects of degradation, local pier scour, and contraction of the waterway is not justifiable; (2) live loads reduced from the anticipated 75-year maximum to a 2-week maximum are appropriate when designing a bridge in its 100-year “scoured-out” storm configuration; and (3) vehicular live loads are likely to be on a bridge during a seismic event, but other issues need to be considered.
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Acknowledgments
The writer thanks the reviewers of this paper for their valuable suggestions, and Vinacs Vinayagamoorthy and Anthony Gugino of Caltrans Structures Maintenance and Investigation for their expertise in heavy vehicles. Prof. Frank Samaniego, University of California–Davis; Mr. Paul Benson, Caltrans Material Evaluation and Testing (since retired); Professor James T. P. Yao, Texas A&M University (since retired); Ms. Cathy Avila, Caltrans Hydraulics and Hydrology (since separated from Caltrans) are also acknowledged for helpful guidance on earlier versions of this paper. The views in this paper do not necessarily reflect those of the California Department of Transportation.
References
AASHTO. (2004). AASHTO LRFD bridge design specifications, 3rd Ed., Washington, D.C.
AASHTO. (2005). AASHTO LRFD bridge design specifications, 2006 Interims, Washington, D.C.
Ang, H-S., and Tang, W. H. (1975). Probability concepts in engineering planning and design. Vol. 1: Basic principles, Wiley, New York.
Ghosn, M., Moses, F., and Wang, J. (2003). “Design of highway bridges for extreme events.” NCHRP Rep. 489, Transportation Research Board of the National Academies, Washington, D.C.
Moses, F. (2001). “Calibration of load factors for LRFD bridge evaluations.” NCHRP Rep. 454, Transportation Research Board of the National Academies, Washington, D.C.
Novak, A. S. (1999). “Calibration of LRFD bridge design code.” NCHRP Rep. 368, Transportation Research Board of the National Academies, Washington, D.C.
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© 2007 ASCE.
History
Received: Aug 30, 2005
Accepted: Apr 10, 2006
Published online: May 1, 2007
Published in print: May 2007
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