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3 CHAPTER 1 Background 1.1 Research Objectives 1.2 Engineering Design Methodologies NCHRP Project 24-31, "LRFD Design Specifications for Shallow Foundations" was initiated with the objective to 1.2.1 Working Stress Design "develop recommended changes to Section 10 of the AASHTO The WSD method, also called ASD, has been used in civil LRFD Bridge Design Specifications for the strength limit engineering since the early 1800s. Under WSD, the design state design of shallow foundations." The current AASHTO loads (Q), which consist of the actual forces estimated to be specifications, as well as other existing codes employing applied to the structure (or a particular element of the struc- reliability-based design (RBD) principles, were calibrated ture), are compared to the nominal resistance, or strength using a combination of reliability theory, fitting to allow- (Rn) through a factor of safety (FS): able stress design (ASD) (also called working stress design [WSD]), and engineering judgment. The main challenges of Rn Qult Q Qall = = (1) the project were, therefore, the compilation of large, high- FS FS quality databases and the development of a procedural and data management framework that would enable load and resis- where tance factor design (LRFD) parameter evaluation and future Q = design load, updates. Meeting these challenges required the following Qall = allowable design load, specific objectives: Rn = nominal resistance of the element or the structure, and Qult = ultimate geotechnical foundation resistance. 1. Establish the state of practice in bridge shallow founda- The Standard Specifications for Highway Bridges (AASHTO, tions design and construction. 1997), based on common practice, presents the traditional fac- 2. Define the ultimate limit states (ULSs) for individual and tors of safety used in conjunction with different levels of control combined loading of shallow foundations under expected in analysis and construction. Although engineering experience bridge loading conditions. over a lengthy period of time resulted in adequate factors of 3. Build databases of shallow foundation performance under safety, their source, reliability, and performance had remained vertical, lateral, and moment loading conditions. mostly unknown. The factors of safety do not necessarily con- 4. Establish methods for the various limit state predictions sider the bias, in particular, the conservatism (i.e., under- and assess their uncertainty via databases, model analyses, prediction) of the analysis methods; hence, the validity of their parametric studies, and the probabilistic approach when assumed effect on the economics of design is questionable. required. 5. Develop a procedure for calibrating resistance factors for 1.2.2 Limit State Design the identified ULS. 6. Establish factors and procedures. Demand for more economical design and attempts to 7. Modify AASHTO's specifications based on the above improve structural safety have resulted in the re-examination findings. of the entire design process over the past 50 years. The design