Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 4


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 3
3 CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH 1.1 BACKGROUND tions. Statistical methods are used for the development of rec- ommendations for number of piles to be tested in quality assur- National Cooperative Highway Research Program Project ance. Chapter 4 presents the conclusions supported by the NCHRP 24-17, "LRFD Deep Foundations Design," was initi- study, suggestions for additional research, and a framework ated to provide (1) recommended revisions to the driven pile for LRFD for deep foundations that incorporates knowledge- and drilled shaft portions of section 10 of the AASHTO LRFD based design. Detailed data and analyses are provided in the Bridge Design Specifications (AASHTO, 2001) and (2) a appendices available on the accompanying CD. detailed procedure for calibrating deep foundation resistance factors. The current AASHTO specifications, as well as other 1.2 STRESS DESIGN METHODOLOGIES existing codes based on Load and Resistance Factor Design (LRFD) principles, were calibrated using a combination of 1.2.1 Working Stress Design reliability theory, fitting to Allowable Stress Design (ASD-- also called Working Stress Design, or WSD), and engineering The working Stress Design (WSD) method, also called judgment. The main challenges of the project were, therefore, Allowable Stress Design (ASD), has been used in Civil Engi- the compilation of large, high-quality databases and the devel- neering since the early 1800s. Under WSD, the design loads opment of a procedural and data management framework (Q), which consist of the actual forces estimated to be applied that would enable LRFD parameter evaluation and future to the structure (or a particular element of the structure), are updates. Meeting these challenges required (1) organizing compared to resistance, or strength (Rn ) through a factor of the resistance factors into a design-construction-quality- safety (FS): control sequence (i.e., independence in resistance factors according to the chronological stage and the evaluation pro- Rn Qult Q Qall = = (1) cedure) and (2) overcoming the generic difficulties of apply- FS FS ing the LRFD methodology to geotechnical applications, i.e., incorporation of indirect variability (e.g., site or parameters Where Q = design load; Qall = allowable design load; Rn = interpretation), judgment based on previous experience, and resistance of the element or the structure, and Qult = ultimate similar factors into the methodology. The project team, headed geotechnical pile resistance. by the author, was divided into three groups dealing respec- tively with static analyses (University of Florida), proba- Table 1, from Standard Specifications for Highway Bridges bilistic and structural analyses (University of Maryland), and (AASHTO, 1997), presents common practice, the traditional factors of safety used in conjunction with different levels of dynamic analyses (University of Massachusetts Lowell). control in analysis and construction. Presumably, when a This chapter provides a background for design methodolo- more reliable and consistent level of control is used, a smaller gies and LRFD principles and usage. In Chapter 2, following FS can be used, which leads to more economical design. a discussion of the major findings from a questionnaire and Practically, however, the factors of safety in Table 1 do not survey designed to discover the state of current practice, the necessarily consider the bias, in particular, the conservatism databases that were developed for the project are presented and (i.e., underprediction) of the methods listed; hence, the valid- analyzed. Selected design methods are described, followed by ity of their assumed effect on the economics of design is an in-depth evaluation of the dynamic methods for the evalu- questionable. (These traditional factors of safety are further ation of the capacity of driven piles and an examination of their discussed and evaluated in section 3.5.2) controlling parameters. The performance of different predic- tion methods, categorized according to the examined methods of analysis and controlling parameters, are also discussed in 1.2.2 Limit States Design Chapter 2. In Chapter 3, the results of these analyses are used for the development of the resistance factors recommended for In the 1950s, the demand for more economical design of the revision of the AASHTO LRFD Bridge Design Specifica- piles brought about the use of Limit States Design (LSD).