Development of Variable LRFD Φ Factors for Deep Foundation Design Due to Site Variabi

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Development of Variable LRFD Φ Factors for Deep Foundation Design Due to Site Variabi

Author: McVay, Michael C Klammler, Harald Faraone, Michael A Dase, Krishnarao Jenneisch, Chris | Size: 3.68 MB | Format: PDF | Quality: Original preprint | Publisher: University of Florida, Gainesville | Year: 2012 | pages: 134

The current design guidelines of Load and Resistance Factor Design (LRFD) specifies constant resistance factors (Φ) values for deep foundation design, based on analytical method selected and degree of redundancy of the pier. However, investigation of multiple sites in Florida reveals significant variability of soil/rock properties from site to site (coastal conditions) suggesting the introduction of variable Φ values based on reliability-based design approach. Building on previous work (BD545-76) a geostatistical (variograms) approach was developed to quantify the spatial uncertainty for site specific conditions. As a result, Φ values are evaluated due to both a site’s measured spatial uncertainty and error associated with a particular analytical method. This report summarizes subsequent efforts to further expand the applicability of the reliability design to the analytical models currently available in the FB-DEEP software program. For the geostatistical analysis, a simple yet robust graphical user interface (GUI) was developed, which considers two design scenarios: 1) conditioning to nearby boring data, and 2) unconditional mean site data. For either scenario the GUI generates thousands of potential data sets, which are evaluated by FB-DEEP to assess mean pile/shaft resistance and spatial uncertainty at a pier location. Spatial uncertainty is then combined with the design method error associated with the selected FB-DEEP model to assess Φ. For demonstration of the application of the GUI, standard penetration test (SPT) and laboratory strength data were collected from seven FDOT projects and subsequent Φ values were evaluated. The Φ values ranged from 0.3 to 0.7 depending upon amount of subsurface data, measure summary statistics, and degree of spatial correlation. The report concludes with recommendations (in situ measurements, load testing, etc.) on improving the computed Φ on a site-by-site basis.

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