Calibration of Load and Resistance Factors in LRFD Foundation Design Specifications

7044043982011FinalPDFTech ReportNUTC R23700022979DTRT06-G-0014Tables (Data)Bridge superstructuresLoad factorResistance (Mechanics)Bridge foundationsBridge designSpecificationsReliability indexRandom variableProbabilistic distributionTolerable settlementGirder bridgePrestressed concrete bridgeSolidslab bridgeConcrete diaphragmMissouriMissouri University of Science and Technology. Center for Transportation Infrastructure and SafetyWang, ZuocaiChen, GendaKwon, Oh-SungOrton, SarahUniversity Transportation Centers Program (U.S.)NTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and StructuresNTL-HIGHWAY/ROAD TRANSPORTATION-HIGHWAY/ROAD TRANSPORTATIONNTL-HIGHWAY/ROAD TRANSPORTATION-DesignUS Transportation CollectionThis report summarizes the findings and recommendations on the impact of foundation settlements on the reliability of bridge superstructures. As a collaborative effort of an overall initiative for the development of LRFD foundation design specifications, this study is focused on the investigation of pros and cons for including foundation settlements in bridge designs under gravity loads. Settlement was modeled both probabilistically and deterministically. In the case of a random settlement variable, a lognormal distribution was used in reliability analysis with a fixed coefficient of variation of 0.25. Dead and live loads were modeled as random variables with normal and Gumbel Type I distributions, respectively. Considering the regional traffic condition on Missouri roadways, the effect of a live load reduction factor on bridge reliability was also investigated. Therefore, a total of eight cases were discussed with a complete combination of settlement modeling (mean and extreme values), design consideration (settlements included and excluded), and live load reduction (unreduced and reduced live loads). Based on extensive simulations on multi-span bridges, bridges designed without due consideration on settlements can tolerate an extreme settlement of L/3500 - L/450 under unreduced live loads and up to L/3500 under reduced live loads without resulting in a reliability index below 3.5 (L=span length). Depending upon span lengths settlements and has potential to reduce overall costs in bridge design. The second method may result in oversized foundations.93