United States Department of Transportation
i
Assessment and Evaluations of I-80 Truck Loads and Their Load Effects: Final Report
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2016-12-01
[PDF-1.07 MB]
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Edition:Final report
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Abstract:The research objective is to examine the safety of Wyoming bridges on the I-80 corridor considering the actual truck traffic on the
interstate based upon weigh in motion (WIM) data. This was accomplished by performing statistical analyses to determine reliability indices for a
set of archetype bridges. This set of bridges include simple-span bridges with lengths between 30 ft and 200 ft (positive moments) and two-span
bridges with equal spans lengths of 30 ft to 200 ft (negative moments). Adequate safety, as defined by AASHTO Bridge Specifications, is a
reliability index of at least 3.50. Wyoming DOT has several years of truck characteristics that were used to develop a live load model in a manner
similar to the NCHRP used to calibrate the LRFD Specifications. The results are the live load bias values and coefficients of variations for the
different bridge archetypes that are used to determine the 75-year maximum load statistical properties for the reliability analyses.
Three optimally designed steel bridges from the NCHRP 20-7/186 report with varying ratios of dead, wearing surface, and live loads are used to
perform the reliability analyses and assess safety. Truck traffic along I-80 creates more demand than that assumed in the AASHTO LRFD bridge
design procedures. The greater demand results in reliability indices that do not meet target safety levels and have reliability indices significantly
less than 3.5. Two issues should be addressed: (1) the unacceptably low reliability indices for short multi-span bridges (2) the overall low reliability
indices for all span lengths.
The “optional” (low-boy) dual tandem load where there is a tandem in adjacent spans in the AASHTO LRFD commentary significantly increases
the negative design live load moments. Using the dual tandem, the reliability indices for the shorter two-span bridges increase to 3.00 and above,
placing this bridge type into the range of the reliability indices for the other bridge span lengths. However, indices are below the target. Raising
the design live load factor, yL, directly and fairly uniformly increases reliability indices. An increase in yL to 2.00 (from 1.75) increases almost all of
the reliability indices above 3.50.
In summary, the “optional” low boy load should be used for design and will control shorter multiple-span bridges in the negative moment region.
Alternatives to address this situation are outlined.
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