Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 3

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 2
2 from the NCHRP Oversight Panel, Task 11 was implemented. Factors to Consider This task involved (1) making further modifications to the The basic requirement for this Project is to ensure that fac- specifications, commentaries, and example problems; (2) ad- tored capacity exceeds factored load as defined by the following dressing the Oversight Panel's comments on the third Interim equation for various limit states (or acceptable performance): Report, and (3) and preparing a Final Report. This work was scheduled to be completed after 35 months but took approx- r Rn pi Qi (1-1) imately 39 months. The final work activity in Phase 2 on the Project, Task 12, where involved preparation of this Final Report and the revised spec- r = performance factor; ifications, commentaries, and example problems. This task Rn = nominal resistance; was finalized in November of 2007, approximately 44 months pi = load factor for load component I; and following initiation of the Working Plan in April of 2004. Fol- Qi = load effect due to load component i. lowing this submittal, an additional example problem was During the initial phase of work for this Project, the LRFD completed, specifications and commentaries were revised, methodology was not formerly introduced. Rather, the focus and the Final Report finalized in June 2008. of the work was on the identification and evaluation of a de- Throughout work on each task within the Project there was sign methodology without load or resistance factors. Once the a continuing effort to focus on the final product of the Project. methodologies were developed and approved, then an approach This product involved a methodology that could be used in for incorporating load and resistance factors was established areas that are both highly seismic and relatively aseismic; that relative to the recommended methodologies. could be implemented by staff from DOTs, vendors, and con- Although work on the initial phase of work did not present sulting firms using existing software without the need for ex- recommendations on load and resistance factors to use with tensive training; and that "made sense" relative to observed the proposed methodologies, consideration was given by the performance during past earthquakes. This theme was im- Project Team to how load and resistance factors might eventu- plemented throughout the Project, from start to finish. To the ally be used during seismic design. Ideally this approach would extent practical, this theme is followed in the presentation of build on the load and resistance factors used in the conven- each chapter of this Draft Final Report. tional static load case presented in the current version of the AASHTO LRFD Bridge Design Specifications. For the static design case the appropriate load and resist- 1.2 Project Background ance factors have been developed to yield a consistent margin of safety in the designed structure. This same logic needs to Work on the NCHRP 12-70 Project was initiated in April be followed for seismic loading to retaining walls, slopes and of 2004. The following three subsections provide background embankments, and buried structures. However, the approach information for the work that has been accomplished. This for defining a consistent margin of safety is more difficult to background information includes a summary of plans for define for the following reasons: implementing the overall LRFD design methodology and overviews of interim conclusions from the work performed The load factors and load cases (that is, on the right- on the Project. The overview of conclusions helps provide a hand-side of the above equation) had to be consistent with perspective for the development work that is being summarized those recommended by the NCHRP Project 20-07 Recom- in subsequent chapters. mended LRFD Guidelines for the Seismic Design of Highway Bridges (Imbsen, 2006). At the time the NCHRP 12-70 Proj- ect was initiated, the NCHRP 20-07 Project was establishing 1.2.1 Plans for Implementing the appropriate earthquake loading return period--subject the LRFD Design Methodology to the approval of the AASHTO Highway Subcommittee The work carried out for the NCHRP 12-70 Project must be on Bridges and Structures (HSCOBS T-3) and eventually consistent with the philosophy and format of the AASHTO the AASHTO voting members. These recommendations LRFD Bridge Design Specifications and the seismic provisions would result in larger loads associated with a seismic event for highway bridges. In this philosophy, "Bridges shall be de- at a specific site relative to the then current AASHTO re- signed for specified limit states to achieve the objectives of quirements, but the likelihood of the load occurring de- constructibility, safety, and serviceability, with due regard to creased and would be relatively infrequent. Under this sit- issues of inspectibility, economy, and aesthetics. . . ." In the uation use of a load factor on the seismic load was believed LRFD procedure, margins of safety are incorporated through to be overly conservative. (The NCHRP 20-07 Project was load (p) factors and performance (or resistance, r) factors. originally referred to as the NCHRP 12-49 Update Project.

OCR for page 2
3 The intent of the NCHRP 12-07 Project was to revise rec- The various limit states to be examined were categorized ommendations given in the NCHRP 12-49 Project (NCHRP into three areas. The first involved the evaluation of the global Report 472, 2003) for use in updating seismic provisions in stability of the overall site, which includes requirements for the AASHTO LRFD Bridge Design Specifications. One of the slope stability and similar mechanisms. The next dealt with key recommendations initially made by the NCHRP 20-07 the design of the foundation system for external stability (that Project was to increase the return period for seismic design is, sliding, overturning, and bearing) to ensure that the size of from the 500-year level in the then current (2006) LRFD the foundation and the implied geotechnical (that is, overall specifications to a 1,000-year return period. The probability soil) capacity was sufficient. The last involved the design for of occurrence for the 1,000-year event is approximately internal structural stability to ensure that structural compo- 7 percent in 75 years. This recommendation was approved by nents functioned properly under the increased dynamic load AASHTO in July of 2007, at the time that the NCHRP 12-70 from the earthquake. Depending on whether a design project Project report was being finalized.) involved a retaining wall, a slope or embankment, or a buried From a resistance factor standpoint, design could be per- structure, an assessment of one or more of these limit states may formed using either a limit equilibrium or displacement- not be required. For example, the limit state for seismic design based approach. The selection of resistance factors for these of slopes and embankments only involves global stability, two cases will differ. For example, use of a resistance factor while the buried structure only considers internal stability. less than 1.0 often will result in a conservative design using limit equilibrium methods, but could lead to an unconser- Relationship to Design Process vative design for a displacement-based approach. From past earthquake experience, most cases of observed While the starting point involved use of load and resistance or postulated failures relate to intolerable structural damage, factors equal to 1.0, in certain geographic areas and for certain as opposed to excessive overall movement, especially for categories of design, use of a resistance factor less than 1.0 retaining walls and buried structures. These structures are (that is, < 1.0) was considered for simplifying the design inherently more sensitive to movement relative to above- process. An example of this was for the evaluation of seismic ground structures. Also, most freestanding retaining walls stability of slopes. If a deformational approach is not taken (that is, other than bridge abutments) can undergo a signifi- and the owner wants to base the evaluation strictly on a com- cant degree of movement without adversely impacting their parison of soil capacity to seismic loads, the current approach intended functions. would be to confirm that the factor of safety is greater than Therefore, the most germane LRFD design issue was to as- 1.1 to 1.2 for an acceleration coefficient of 0.5 times the peak sure structural integrity, commonly referred to as designing for ground acceleration (PGA) at the ground surface. (Many ap- the internal stability of the earth retaining system. When de- plications in geotechnical engineering are based on factors of signing for structural integrity, the geotechnical engineer will safety--where the resistance of the soil is compared to the define the seismic loading criteria and conducts soil-structure forces causing failure. When using LRFD methods for the interaction analyses, as needed, for characterizing foundation same design, it is often more meaningful to refer to the ca- stiffness and damping parameters. The responsibility of actual pacity to demand (C/D) ratio rather than the factor of safety. design usually falls to the structural designer. The structural en- The use of C/D ratio also is consistent with terminology used gineer typically will bear the responsibility for conducting the by bridge engineers. Discussions in this report will refer to structural response analyses and will make use of the recom- C/D ratio and factor of safety interchangeably.) This same ap- mendations regarding seismic loading and foundation stiffness proach can be taken in the context of LRFD design, but in this in a global model. The structural designer would be the one case the resistance factor is defined by the reciprocal of the who actually goes through the LRFD design process in check- factor of safety used, assuming that the load factor is equal to ing the structural capacity versus demand, and eventually will 1.0 for the reasons stated above. sign the structural drawings. Requirements in other sections of With this in mind the thrust of the work was to formulate the AASHTO LRFD Bridge Design Specifications are followed the LRFD specifications in terms of the following three when conducting structural analyses and design checks. considerations: Note that this general approach is not always the case. For some wall types, such as the Mechanically Stabilize Earth 1. Identifying the limit states to be considered during the (MSE) or soil nail walls, the geotechnical engineer also may be earthquake load case. responsible for the internal stability as well. In this case the geo- 2. Defining the expected performance of the designed system technical engineer would select reinforcing or soil nail size, for each of the limit states defined in item (1) above. and confirm that the stresses imposed by seismic loading are 3. Outlining the design analysis procedure and capacity criteria. acceptable relative to LRFD requirements.