S - Kam Ng PhD Dissertation Final.pdf - Digital Repository of CCEE ...

2166.2. IntroductionAllowable Stress Design (ASD) philosophy has long been used for the design of pilefoundations for decades. Uncertainties associated with load (Q) and resistance (R) areaddressed in this approach through a factor of safety (SF), and the selection of factor ofsafety is highly dependent on the pile designers’ judgments and experiences. Thus, aconsistent and reliable performance of bridge foundations cannot be ensured. To overcomethis limitation, the Load and Resistance Factor Design (LRFD) philosophy has beenprogressively developed since the early 1990s with the primary objective of ensuring auniform reliability of bridge foundations in the United States. The LRFD framework isexpressed in Eq. (6.1) as noted by the American Association of State Highway andTransportation Officials (AASHTO) (2001) LRFD Bridge Design Specifications.∑γ φ (6.1)where,= applied load,R = nominal pile resistance,= load factor corresponding to load , andφ = resistance factor.The uncertainties associated with the resistance (R) principally originate from sitecharacterization, soil variability, design method, and construction practice. Theseuncertainties are significantly different from those that affect the applied load (Q)(Paikowsky et al., 2004). Thus, the suitable resistance factor (φ) and load factor (γ) areseparately multiplied to the resistance and applied load, respectively. As a result, theuncertainties of resistance and applied load can be individually accounted for in achieving aconsistent and reliable performance of bridge foundations.Recognizing the advantages of LRFD philosophy, the Federal HighwayAdministration (FHWA) mandated all new bridges initiated after October 1, 2007 shouldfollow the LRFD design approach. Unfortunately, the current AASHTO (2007) LRFD