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52 CHAPTER 2 Research Approach 2.1 Scope and Structure tigation of their sources, (2) development of resistance factors and examination of them in design cases, (3) development of NCHRP Project 24-31 was structured under two major final resistance factors and the conditions for their implemen- units, each leading to a key requirement in the accomplish- tation, and (4) development of the specifications. ment of the final objective. This section describes the concep- Unit II was subdivided along the geotechnical challenges tual method of approach behind each of the units. Flow considering the design of shallow foundations on soil and charts merging the various activities are provided to elucidate rock. Unit II(a) addresses the effort required for the develop- the interrelations of the activities. ment of resistance factors for shallow foundations constructed on granular soils, outlined in Figure 43. A separation is made 2.1.1 Unit I between foundations subjected to centric vertical loads only and foundations subjected to inclined and/or eccentric loads. Unit I involved assembly and assessment of knowledge and This separation is associated with the nature of the databases, data with the final goal of establishing (1) databases, (2) design the parameters that can be obtained in each case, and the com- methods and alternative design methods, (3) typical struc- plexity of inclined/eccentric loading discussed in Section 1.6 tures and case histories, and (4) expected load ranges and their of this report. Unit II(b) addresses the effort required for the distributions. development of resistance factors for shallow foundations on Figure 41 provides a flow chart of Unit I(a) outlining the rock as outlined in Figure 44. research plan for establishing the state of practice in design and construction as well as case histories and loading. Figure 42 provides a flow chart of Unit I(b), addressing the establish- 2.1.3 Additional Topics ment of databases allowing for the statistical parameters re- The outlined method of approach addresses the conditions quired for the calibrations that are addressed in Unit II. The and difficulties associated with the prevailing design and con- material required for the statistical parameters for the calibra- struction practices of shallow foundations for bridges and their tion was assembled in Unit I. In the direct Resistance Factor systematic adaptation to LRFD. The presented scope reflects Approach (RFA) implemented in this research, the focus is budget restrictions and needs in addressing the most urgent is- on the uncertainty of the model (to be discussed further in the sues as directed by the research panel. Topics such as foundations following section); hence, the parameters required for cali- on cohesive soils or friction-cohesive soils (-c materials) ma- bration are obtained from analysis of databases of case histo- terials will require, therefore, additional effort. Other pertinent ries. The utilization of the data and knowledge assembled in conditions like foundation sliding, footings on slopes, and two- Unit I along the bearing capacity evaluation for the calibra- layer soil systems were addressed in various detail depending on tion of the design methods is addressed in Unit II. importance and the available information. 2.1.2 Unit II 2.2 Methodology The data and methods established in Unit I are analyzed in Section 1.4 reviewed the format for the design factors. The Unit II with the following goals: (1) establishment of the un- resistance factor approach (RFA) was adopted in this study certainty of the methods and parameters including the inves- following previous NCHRP deep foundation LRFD database

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53 Existing AASHTO Specifications and FHWA Manuals AASHTO (2006) FHWA Reference Manual, Munfakh et Available Questionnaires of Review Design Examination of al., 2001 Foundations Design Cases Used in Lateral Loads Data FHWA GEC No. 6, Kimmerling, 2002 Methods and Construction NCHRP Project 12-66 on Structures FHWA Spread Footings of Highway Practices Bridges, Gifford et al., 1987 NCHRP Report 507, FHWA Soils & Foundations Workshop Paikowsky et al., 2004 Manual, Cheney & Chassie, 1982 NCHRP Project 12-66, Paikowsky et al., 2005 Design Cases in Manuals NCHRP Project 24-31 FHWA GEC No. 6, Questionnaire Determination Kimmerling, 2002 of DOT Design Methods and FHWA Soils & Construction Practices of Foundation Workshop Examination of Load Shallow Foundations Manual, Cheney & Ranges and Statistics of UDE Institute of Soil Chassie, 1982 Horizontal and Vertical Mechanics & Foundation FHWA RD-86/185, Loading for the Typical Engineering Determination of Gifford et al., 1987 Design Examples and Alternative Design Methods Case Histories Gifu Univ., Japan Japan Geotech. Soc. International Society Established: of Soil Mechanics AASHTO/FHWA and DOTs' Design Methods and Foundation Complementary and/or Alternative Design Methods Engineering Typical Structures under Common Construction Practices Design Cases Load Ranges and their Distributions Figure 41. Flowchart outlining the research plan for Unit I(a) establishing design methods, construction practices, design cases, and loads. Literature Identifying Additional Shallow Foundation Load Tests 31 Data Cases Collected at Cornell Institute of Soil Existing UML/GTR (Prakoso, 2002) Mechanics & Shallow Foundation 39 Data Cases Foundation Database Collected at MIT Engineering UDE 329 Load Test Cases (Zhang and Einstein, Germany Load 1998) Testing Program Database III Database II Database I Loading of Shallow Vertical Inclined & Eccentric Vertical-Centric Loading Foundations on Loading of Shallow of Shallow Foundations Rock Foundations on Granular Soils on Granular Soils Data Solicitation from DOTs across the USA Figure 42. Flowchart outlining the research plan for Unit I(b) establishing databases for shallow foundation load tests.

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54 Database I Database II Uncertainty Reliability of Establish BC Reliability of in BC Conventional BC Models Based Conventional BC Factor N Design Methods on Unit I(a) Design Methods Examine Conditions for Resistance Factors Interaction Diagram Preferable Analysis for BC Design Under and/or Need for Vertical-Centric Alternative Loads Load Ranges & Resistance Factors for BC Design Under Inclined DistributionsUnit I(a) Examine Typical and/or Eccentric Loads SLS Structures/Case Histories Final Resistance Factors Examine Typical and Conditions for Structures/Case SLS Implementation Histories Notes: BC Bearing Capacity AASHTO Modified Target Reliability Specifications SLS Serviceability Limit State ULS Ultimate Limit State Figure 43. Flowchart outlining the research plan for Unit II(a) to develop LRFD parameters for the ULS design of shallow foundations on granular soils. calibrations (Paikowsky et al., 2004). Figures 45 and 46 illus- under the assumption of a homogenous cross-section, a hor- trate the sources of uncertainty and principal differences be- izontal symmetry line, and beam height, h, one can accurately tween probability-based design (PBD) application to the de- calculate moments (hence, stresses) and deflections in the sign of a structural element of the superstructure and to a beam. The major source of uncertainty is the loading (especially geotechnical design of a foundation in the substructure. If the live and extreme event loading on the bridge); the material one considers a bridge girder as a simple supported beam properties and physical dimensions present relatively less Load Ranges used in NCHRP and Project 24-17 Database III Distributions & Other Codes Unit I(a) Worldwide Establish BC Models Based Establish the on Unit I(a), e.g.: Resistance Uncertainty of Goodman (1989) Factors for BC the Models Carter and Kulhawy (1988) Final Resistance Examine Typical Factors and Structures/Case Conditions for Histories Implementation Notes: BC Bearing Capacity Target Reliability SLS Serviceability Limit State AASHTO Modified ULS Ultimate Limit State Specification Figure 44. Flow chart outlining the research plan for Unit II(b) to develop LRFD parameters for the ULS of shallow foundations on rock.

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55 q loading A B l Sources of Uncertainty ql Loading (q) A B shear 2 Dimensions/Geometry (l, h, I) Material Properties (E) moment Most Noticeable: 1. No uncertainty in the model--under given loading conditions the ql 2 uncertainty in the material properties M max dictates the uncertainty in strength 8 and deflection 2. Largest uncertainty in the loading, y deflection source, magnitude, and distribution (in case of bridges) 5 ql 4 5 max l 2 ymax 384 EI 24 E h (Assuming homogenous cross-section, horizontal symmetry line, and beam height, h) Figure 45. Simplified example of a beam design and associated sources of uncertainty. Soil sampling and testing for engineering material parameters Method of Approach Analysis model LOAD Use the load uncertainty from the structures (until better research is done) RESISTANCE Establish the uncertainty of the "complete" foundation resistance (capacity) analysis (including established Assumed Failure Pattern under procedures for parameters) by Foundations comparing a design procedure to measured resistance (failure) Uncertainty in the assumptions made Loading in the model development leaves unknown analysis versus actual performance Uncertainty due to site, material and testing variability, and estimation of parameters FOUNDATION Code of practice DESIGN Sources of Uncertainty Material properties and strength parameters Resistance model Traditional design, although developed over Loading many years and used as a benchmark, has undocumented, unknown uncertainty Figure 46. Components of foundation design and sources of uncertainty.