Modification of LRFD Resistance Factors Based on Site Variability

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Modification of LRFD Resistance Factors Based on Site Variability

Author: McVay, Michael | Size: 2.76 MB | Format: PDF | Quality: Original preprint | Publisher: University of Florida, Gainesville | Year: 2009 | pages: 146

Current practice by the Florida Department of Transportation (FDOT), Federal Highway Administration (FHWA), and American Association of State Highway Transportation Officials (AASHTO) for deep foundation design is to use a constant load and resistance factored design (LRFD) Φ depending on redundancy, but independent of pile/shaft dimension. Unfortunately, soil/rock properties vary from point-to-point (CVq: coefficient of variation) and are typically spatially correlated. Since both the skin friction and end bearing involve spatial averaging of soil/rock properties over the shaft surface, the resulting total shaft resistance variability (CVR) will not share the same point variability (CVq). Moreover, the total shaft variability (CVR) will also vary with different degrees of spatial correlation, typically represented with a covariance function and a correlation length a. This work employs well established geostatistical principles to establish both the covariance function (e.g., variogram) and expected total pile/shaft variability (CVR) using borehole data. Consideration is given to the number of borings, the location of borings relative to each other, and to the design foundation (e.g., if the borings are within the footprint of the design pile/shaft). Since the resulting pile/shaft variability CVR is a function of pile/shaft dimensions, soil/rock variability CVq, and its spatial correlation, the resulting LRFD Φ is not constant for any site. To help the designer, four quadrant iterative design charts are developed for single and group pile/shaft layouts, which consider side and tip resistances, as well as layered systems. To better define the total pile/shaft variability CVR, the practice of load testing is also incorporated into the proposed approach. The work includes multiple design examples and data from existing FDOT bridge sites (e.g., 17th Street, Fuller Warren, and Jewfish Creek bridges).

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