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Development of preliminary load and resistance factor design of drilled shafts in Iowa.
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  • Publication/ Report Number:
    InTrans Project 11-410
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  • Abstract:
    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. To achieve part of this goal, a database for Drilled SHAft Foundation

    Testing (DSHAFT) was developed and reported on by Garder, Ng, Sritharan, and Roling in 2012. DSHAFT is aimed at assimilating

    high-quality drilled shaft test data from Iowa and the surrounding regions. DSHAFT is currently housed on a project website

    (http://srg.cce.iastate.edu/dshaft) and contains data for 41 drilled shaft tests.

    The objective of this research was to utilize the DSHAFT database and develop a regional LRFD procedure for drilled shafts in Iowa

    with preliminary resistance factors using a probability-based reliability theory. This was done by examining current design and

    construction practices used by the Iowa Department of Transportation (DOT) as well as recommendations given in the American

    Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications and the FHWA drilled shaft


    Various analytical methods were used to estimate side resistance and end bearing of drilled shafts in clay, sand, intermediate

    geomaterial (IGM), and rock. Since most of the load test results obtained from O-cell do not pass the 1-in. top displacement criterion

    used by the Iowa DOT and the 5% of shaft diameter for top displacement criterion recommended by AASHTO, three improved

    procedures are proposed to generate and extend equivalent top load-displacement curves that enable the quantification of measured

    resistances corresponding to the displacement criteria.

    Using the estimated and measured resistances, regional resistance factors were calibrated following the AASHTO LRFD framework and

    adjusted to resolve any anomalies observed among the factors. To illustrate the potential and successful use of drilled shafts in Iowa, the

    design procedures of drilled shaft foundations were demonstrated and the advantages of drilled shafts over driven piles were addressed

    in two case studies

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