Calibration of the live load factor in LRFD design guidelines : [revised].
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Calibration of the live load factor in LRFD design guidelines : [revised].

Filetype[PDF-2.04 MB]

  • English

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      Final report.
    • Abstract:
      The Load and Resistant Factor Design (LRFD) approach is based on the concept of structural reliability. The approach is

      more rational than the former design approaches such as Load Factor Design or Allowable Stress Design. The LRFD

      Specification for Bridge Design has been developed through 1990s and 2000s. In the development process, many factors were

      carefully calibrated such that a structure designed with LRFD can achieve a reliability index of 3.5 for a single bridge girder

      (probability of failure of about 2 in 10,000). As the initial development of the factors in the LRFD Specification was intended to

      be applied to the entire nation, state-specific traffic conditions or bridge configuration were not considered in the development

      process. In addition, due to lack of reliable truck weigh data in the early 1990s in the U.S., the truck weights from Ontario,

      Canada measured in the 1970s were used for the calibration. Hence, the reliability of bridges designed with the current LRFD

      specification needs to be evaluated based on the Missouri-specific data and the load factor needs to be re-calibrated for optimal

      design of bridges.

      The objective of the study presented in this report is to calibrate the live load factor in the Strength I Limit State in the

      AASHTO LRFD Bridge Design Specification. The calibration is based on the Missouri-specific data such as typical bridge

      configurations, traffic volume, and truck weights. The typical bridge configurations and the average daily truck traffic of the

      bridges in Missouri are identified from statistical analyses of 2007 National Bridge Inventory. The Weigh-In-Motion (WIM) data

      from 24 WIM stations in Missouri are used to simulate realistic truck loads. Updated material and geometric parameters are also

      used to update the resistance distributions.

      From this study, it was found that most representative bridges in Missouri have reliability indices slightly lower than 3.5

      mainly due to the adopted projection method to predict 75 year load. For many bridges in rural areas with Average Daily Truck

      Traffic (ADTT) of 1,000 or less, the average reliability indices are higher than the average reliability index of bridges with ADTT

      of 5,000. This study proposes a table of calibration factors which can be applied to the current live load factor of 1.75. The

      calibration factor is developed as a function of ADTT such that bridge design practitioners can select a calibration factor

      considering the expected ADTTs of a bridge throughout its life span. Impact of the calibration factor on the up-front bridge

      construction cost is also presented.

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