United States Department of Transportation
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Determination of barge impact probabilities for bridge design.
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2016-04-01
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Abstract:Waterway bridges in the United States are designed to resist vessel collision loads according to design provisions released by the American Association of State
Highway and Transportation Officials (AASHTO). These provisions provide detailed procedures for calculating design vessel impact loads within the context of a
comprehensive risk assessment. One of the primary subcomponents of this process is the calculation of probabilities that estimate the likelihood that a barge-to-bridge impact
event will occur. However, the expressions used to predict the frequency of barge-to-bridge collisions were developed from a limited number of data sets. Furthermore, the
technology employed by the maritime industry at the time the original AASHTO provisions were developed—in the early 1990s—has been significantly improved in
subsequent decades. As a consequence of these factors, current estimates of barge-to-bridge collision probabilities may differ from presently-employed AASHTO estimates.
The focus of the research described in this report was the development of a revised barge impact probability expression particularly applicable for the design of bridge
structures located on Florida waterways. Specifically, the existing AASHTO expression for the base aberrancy rate (BR)—used to estimate the likelihood that a barge flotilla
will stray from the intended transit path—was recalibrated and updated. Barge flotilla traffic data and barge-to-bridge collision (casualty) data for Florida bridge locations
were collected and used to compute historical barge-to-bridge collision probabilities. These probabilities were then utilized in conjunction with additional supplementary
parameters specified in AASHTO—quantified using bridge site-specific information—to back-calculate BR values for each bridge location. A subset of BR estimates from
several bridge sites were then utilized to produce a single design value of BR that may be used in risk assessments for new and existing bridge structures. Based on results
from the recalibration process, the updated BR estimate was 55% smaller than the current value prescribed by AASHTO. To demonstrate the effect of the recalibrated BR
parameter, annual frequency (AF) of collapse values from risk assessments of two previously-investigated bridge structures—the Bryant Grady Patton Bridge (Apalachicola
Bay, FL) and the LA-1 Bridge (Leeville, LA)—were recomputed using the updated BR expression. Despite the reduction in BR, values of AF estimated using UF/FDOT
methods and the updated BR expression remained high relative to AF estimates produced by existing AASHTO methods. It was noted in this study that bridge locations with
low volumes of barge traffic corresponded to high estimates of BR. This finding was a consequence of utilizing less data in the statistical calibration process, which reduced
the accuracy of the resulting predictions. Consequently, only Florida bridge locations with significant levels of barge flotilla traffic were utilized to produce the recommended
design value of BR. However, additional out-of-state locations exist with more highly trafficked bridge locations, as well as a more comprehensive source of barge traffic
data. Inclusion of such locations in a similar recalibration effort could result in a lower design value of BR.
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