United States Department of Transportation
i
Load distribution and fatigue cost estimates of heavy truck loads on Louisiana state bridges.
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2013-11-01
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By Saber, Aziz
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Abstract:The bridge in this study was evaluated and a monitoring system was installed to investigate the effects of heavy loads and
the cost of fatigue for bridges on state highways in Louisiana. Also, this study is used to respond to Louisiana Senate
Concurrent Resolution 35 (SCR-35). The superstructure of the bridge in this study was evaluated for safety and reliability
under four different kinds of truck configuration and loads hauling sugarcane. The bridge model was verified by performing
live load tests using 3S3 trucks with a gross vehicular weight (GVW) of 100,000 lb. on the structure. The bridge finite
element model was analyzed under the different kinds of loading and the effects were listed and compared. The results of
the analyses show that the pattern of response of the bridge under the different cases follows the same trend. Among the
four different cases of loading configurations, case 4, which was GVW =148,000 lb. and a vehicle length of 92 ft., produced
the largest tensile and compressive stresses in the members. The results from the bridge deck analyses confirm that the
bridge deck is under a stable stress state, whether the stresses are in the tension zone or the compression zone. The heavy
load as indicated in SCR-35 will cause damage to bridges.
The data from the monitoring system indicates that the average number of heavy loads during October, November, and
December is 3.5 times higher than the rest of the year. The bridges are exposed to high cycles of repetition of heavy loads
that will reduce the life span of the bridges by about 50%. The bridges that are built to last 75 years will be replaced after
about 40 years in service. This seasonal impact is due to the sugarcane harvest and confirms the cost of fatigue, $0.9 per
truck per trip per bridge, as determined in the previous study.
Based on the results of the studies presented in this report, increasing the gross vehicle weight of sugarcane trucks is not
recommended. The heavy loads indicated in SCR-35 will cause premature fatigue damage to the main structural members
and could cause their eventual structural failure. In addition, the majority of the Louisiana bridges currently in service were
designed to accommodate lower loads than the bridge tested on this project. Therefore, based on the test results, one should
expect that the proposed trucks to significantly shorten the remaining life span of Louisiana bridges. All these bridges
should be rehabilitated prior to implementing SCR 35. The data from the monitoring system will provide a good source of
information to review the current serviceability criteria used by the Louisiana Department of Transportation and
Development (LADOTD) for the design of prestressed concrete bridge girders.
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