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More than 15 years have passed since the U.S. transportation industry started its transition from allowable stress design (ASD) to load and resistance factor design (LRFD). For geotechnical design, the AASHTO LRFD Bridge Design Specifications includes provisions that allow state departments of transportation (DOTs) to develop their own design methods and resistance factors. The TRB National Cooperative Highway Research Program's NCHRP Synthesis 601: Practices for Local Calibration of LRFD Geotechnical Resistance Factors documents the extent to which state DOTs have developed agency-specific geotechnical design methods and resistance factors and also details the challenges of the development and benefits resulting from implementation of the methods.
"Over the past two decades the load resistance factor design (LRFD) has been accepted by the American Association for Transportation and Highway Officials (AASHTO) for the design of bridges. This approach is now gaining widespread popularity in the United States for substructure bridge design, including the design of driven pile foundations, as the states calibrate the geotechnical resistance factors for local geological conditions and practices. This study presents the geotechnical resistance factors calibrated for axially driven pile using the first-order reliability method (FORM) for the target reliability index of 2.33 and 3.0 based on 64 end-of-drives (EOD) and 22 beginning-of restrike (BOR) pile driving analyzer (PDA) test data from nine bridge sites in Missouri. Three static pile capacity prediction methods were used, i.e. the Nordlund, Meyerhof and Beta methods. The work investigated the efficiency of each method based on the bias factor, the ratio of the measured to the predicted capacity of the pile, ([lambda] = R[subscript m]/R[subscript p]), the coefficient of variation (COV), and efficiency factor or the ratio of the resistance factor to the bias factor, ([phi]/[lambda]). It verified that the Beta and Nordlund methods provide better predictions than the Meyerhof method. In addition, a comparison of the resistance factor in current AASHTO LRFD with the calibrated resistance factor shows that validating the resistance factors in the AASHTO may result in less reliable design. Finally, the recommended resistance factors for LRFD design are provided for use in Missouri. In addition, further refinement of the developed resistance factors is recommended to improve the resistance factors using large quantity and high quality of data that cover wide areas the glaciated plain and southeast lowland geological regions"--Abstract, leaf iii.
The ground is one of the most highly variable of engineering materials. It is therefore not surprising that geotechnical designs depend on local site conditions and local engineering experience. Engineering practices, relating to investigation and design methods site understanding and to safety levels acceptable to society, will therefore vary between different regions.The challenge in geotechnical engineering is to make use of worldwide geotechnical experience, established over many years, to aid in the development and harmonization of geotechnical design codes. Given the significant uncertainties involved, empiricism and engineering
The Federal Highway Administration (FHWA) mandated utilizing the Load and Resistance Factor Design (LRFD) approach for all new bridges initiated in the United States after October 1, 2007. As a result, there has been a progressive move among state Departments of Transportation (DOTs) toward an increased use of the LRFD in geotechnical design practices. For the above reasons, the Iowa Highway Research Board (IHRB) sponsored three research projects: TR-573, TR-583 and TR-584. The research information is summarized in the project web site (http://srg.cce.iastate.edu/lrfd/). Two reports of total four volumes have been published. Report volume I by Roling et al. (2010) described the development of a user-friendly and electronic database (PILOT). Report volume II by Ng et al. (2011) summarized the 10 full-scale field tests conducted throughout Iowa and data analyses. This report presents the development of regionally calibrated LRFD resistance factors for bridge pile foundations in Iowa based on reliability theory, focusing on the strength limit states and incorporating the construction control aspects and soil setup into the design process. The calibration framework was selected to follow the guidelines provided by the American Association of State Highway and Transportation Officials (AASHTO), taking into consideration the current local practices. The resistance factors were developed for general and in-house static analysis methods used for the design of pile foundations as well as for dynamic analysis methods and dynamic formulas used for construction control. The following notable benefits to the bridge foundation design were attained in this project: 1) comprehensive design tables and charts were developed to facilitate the implementation of the LRFD approach, ensuring uniform reliability and consistency in the design and construction processes of bridge pile foundations; 2) the results showed a substantial gain in the factored capacity compared to the 2008 AASHTO-LRFD recommendations; and 3) contribution to the existing knowledge, thereby advancing the foundation design and construction practices in Iowa and the nation.
Communication of risks within a transparent and accountable framework is essential in view of increasing mobility and the complexity of the modern society and the field of geotechnical engineering does not form an exception. As a result, modern risk assessment and management are required in all aspects of geotechnical issues, such as planning, desi