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Page 52
Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures. Washington, DC: The National Academies Press. doi: 10.17226/14381.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures. Washington, DC: The National Academies Press. doi: 10.17226/14381.
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Page 53
Page 54
Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures. Washington, DC: The National Academies Press. doi: 10.17226/14381.
×
Page 54
Page 55
Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures. Washington, DC: The National Academies Press. doi: 10.17226/14381.
×
Page 55
Page 56
Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures. Washington, DC: The National Academies Press. doi: 10.17226/14381.
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Page 56

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

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

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

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

56 uncertainty. Figure 46 (borrowing from the concept presented by Ovesen, 1989) demonstrates the higher degree of uncer- tainty associated with the design of a foundation. The material properties are based on subsurface investigation and direct or indirect parameter evaluation. The loading of the foundation and its distribution is mostly unknown as only limited infor- mation has ever been gathered on loading at the foundation level. Because of this, the loading uncertainty is assumed as that attributed to the design of the structural element. The main difficulty associated with the design of a foundation in comparison with the design of a structural element remains with the analysis model. While the calculation model in the structural element is explicit (although becoming extremely complex and less definite as the element evolves in geometry and composition and requires the interaction with other units), the analysis model for the evaluation of the soil resistance (i.e., bearing capacity) is extremely uncertain due to the as- sumptions made during its establishment and the empirical data on which it is based. As such, the uncertainty of the geo- technical resistance model controls the resistance evaluation of the foundation. The concept adopted in this research (similar to that adopted by Paikowsky et al., 2004, for deep foundations) focused, there- fore, on the calibration of selected bearing capacity (resistance) models as a complete unit while reducing other associated sources of uncertainty by following specific procedures, e.g., soil parameter establishment. This approach is discussed in Section 1.4, and demonstrated in the examples presented in Sections 1.4.4 and 1.4.5. The systematic analysis of many case histories via a selected resistance model and their comparison to mea- sured resistance provided the uncertainty of the model applica- tion, but also included in it the influence of the different sites from which the data were obtained as well as the uncertainty associated with the “measured” resistance. The assumption that the uncertainty obtained by the process discussed above represents the variability of the model appli- cation for a specific foundation analysis (i.e., the resistance variability as depicted in Figures 1 and 3) is reasonable and has proven successful although it may contain some conservatism, depending on the quality and reliability of the database cases. The calibration, referring to soil type, specific model, and pile type combination as applied previously to deep foundations, has proven extremely effective compared to arbitrary selection of parameters or WSD back-calculated values that defeat the PBD principles as demonstrated in Section 1.4.4. The present calibration is composed mostly of adopting the vertical load statistics established in NCHRP Project 24-17 (Paikowsky et al., 2004) and new development of horizontal load statistics and resistance for design methodologies based on the state of practice established as outlined above. The detailed calibration methodology and process are presented in Section 4.3. 2.3 Execution and Presentation 1. The execution of Unit I(a) (see Section 2.1, Figure 41) re- sulted in the selection of the bearing capacity equations to be analyzed, i.e., established the (calculated) limit state equations to be evaluated. Section 3.1 outlines the findings for establishing the state of practice in design and con- struction leading to Section 3.4 presenting the selected bearing capacity methodology for soils and Section 3.8 the bearing capacity methodology for foundations on rock. 2. The execution of Unit I(b) (see Section 2.1, Figure 42) re- sulted in the development of case history databases, pre- sented in Section 3.2. Examination and determination of the measured strength limit state in these database case histories are described in Sections 3.3, 3.6, and 3.7. Com- parison of the calculated strength limit state (defined in Item 1 above) to the measured strength limit state resulted in the statistical parameters of the resistance distribution functions. These are described in Sections 3.5, 3.6, and 3.7. The distribution functions of the loads are defined and es- tablished in Section 4.2. 3. Selection of target reliability is described in Section 4.3.2. 4. The development of resistance factors is described in Chap- ter 4 with summaries presented in Sections 4.10 and 4.13 for foundations in/on granular soil and rock, respectively.

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LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 651: LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures explores recommended changes to Section 10 of the American Association of State Highway and Transportation Officials’ Load Resistance Factor Design Bridge Design Specifications for the strength limit state design of shallow foundations.

Appendixes A through H for NCHRP Report 651 are available online.

Appendix A: Alternative Model Background

Appendix B: Findings—State of Practice, Serviceability and Databases

Appendix C: Questionnaire Summary

Appendix D: UML-GTR ShalFound07 Database

Appendix E: UML-GTR RockFound07 Database

Appendix F: Shallow Foundations Modes of Failure and Failure Criteria

Appendix G: Bias Calculation Examples

Appendix H: Design Examples

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