Calibration of the live load factor in LRFD design guidelines.

682572706Final report.PDFTech ReportOR11-003Load factorDesign loadCalibrationLive loadsGuidelinesBridgesMissouriMissouri. Dept. of TransportationKwon, Oh-SungOrton, SarahSalim, HaniKim, EungsooHazlett, TimMissouri. Dept. of Transportation. Organizational ResultsNTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and StructuresNTL-HIGHWAY/ROAD TRANSPORTATION-DesignUS Transportation CollectionThe Load and Resistant Factor Design (LRFD) approach is based on the concept of structural reliability. The approach ismore rational than the former design approaches such as Load Factor Design or Allowable Stress Design. The LRFDSpecification for Bridge Design has been developed through the 1990s and 2000s. In the development process, many factors werecarefully 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 tobe applied to the entire nation, state-specific traffic conditions or bridge configuration were not considered in the developmentprocess. 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 LRFDspecification needs to be evaluated based on the Missouri-specific data and the load factor needs to be re-calibrated for optimaldesign 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 theAASHTO LRFD Bridge Design Specification. The calibration is based on the Missouri-specific data such as typical bridgeconfigurations, traffic volume, and truck weights. The typical bridge configurations and the average daily truck traffic of thebridges in Missouri are identified from statistical analyses of 2007 National Bridge Inventory. The Weigh-In-Motion (WIM) datafrom 24 WIM stations in Missouri are used to simulate realistic truck loads. Updated material and geometric parameters are alsoused to update the resistance distributions.From this study, it was found that most representative bridges in Missouri have reliability indices higher than 3.5. Formany bridges in rural areas with Average Daily Truck Traffic (ADTT) of 1,000 or less, the average reliability indices are higherthan 5.0. This study proposes a table of calibration factors which can be applied to the current live load factor of 1.75. Thecalibration factor is developed as a function of ADTT such that bridge design practitioners can select a calibration factorconsidering the expected ADTTs of a bridge throughout its life span. Impact of the calibration factor on the up-front bridgeconstruction cost is also presented