Load and resistance factor design (LRFD) for deep foundations.

Author(s)
Paikowsky, S.G. Birgisson, B. McVay, M. Nguyen, T. Kuo, C. Baecher, G. Ayyub, B. Stenersen, K. O’Malley, K. Chernauskas, L. & O’Neill, M.
Year
Abstract

NCHRP Project 24-17 was aimed at rewriting AASHTO’s Deep Foundation Specifications. The AASHTO specifications are traditionally observed on all federally aided projects and generally viewed as a national code of U.S. highway practice; hence they influence the construction of all the deep foundations of highway bridges throughout the United States. This report presents the results of the studies and analyses conducted for that project. The development of load and resistance factors for deep foundations design is presented. The existing AASHTO specifications, similar to others worldwide, are based on Load and Resistance Factor Design (LRFD) principles. The presented research is the first, however, to use reliability-based calibration-utilizing databases. Large databases containing case histories of piles tested to failure were compiled and analyzed. The state of the art was examined via a literature review of design methodologies, LRFD principles, and deep foundation codes. The state of the practice was established via a questionnaire, distributed to and gathered from state and federal transportation officials. Large databases were gathered and provided. Analyses of the data, guided by the state of practice led to findings detailing the performance of various static and dynamic analyses methods when compared to recorded pile performance. Static capacity evaluation methods used in common design practices were found overall to over-predict the observed pile capacities. Common dynamic capacity evaluation methods used for quality control were found overall to under-predict the observed pile capacities. Both findings demonstrate the shortcoming of safety parameter evaluation based on absolute values (i.e., resistance factors or factors of safety) and the need for an efficiency parameter to allow for an objective measure to assess the performance of methods of analysis. The parameters that control the accuracy of the predictions were researched and analyzed for the dynamic methods. A set of controlling parameters was established to allow calibration of the prediction methods. Target reliability magnitudes were researched and values were recommended considering the action of piles in a redundant or non-redundant form. Statistical analyses compatible with common practice in the structural area were utilized for the development of LRFD resistance factors. Parameters that control the size of a testing sample and site variability were researched and incorporated. Recommended design parameters offering a consistent reliability in design were then presented and discussed. The need for the modification of LRFD for use in geotechnical applications through knowledge-based parameters accounting for subsurface variability, quality of soil parameters estimation, and previous experience as well as amount and type of testing during construction is presented. (Author/publisher) This report may be accessed by Internet users at http://gulliver.trb.org/publications/nchrp/nchrp_rpt_507.pdf

Publication

Library number
20041974 ST S [electronic version only]
Source

Washington, D.C., National Research Council NRC, Transportation Research Board TRB / National Academy Press, 2004, 76 p., 144 ref. + CD-ROM; National Cooperative Highway Research Program NCHRP Report ; 507 - NCHRP Project 24-17 FY’99 - ISSN 0077-5614 / ISBN 0-309-08796-1

Publication (link)

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