2012 Ohio Geotechnical Consultant Workshop Columbus, Ohio; May 8, 2012

Transcription

1 2012 Ohio Geotechnical Consultant Workshop Columbus, Ohio; May 8, 2012 Overview of New FHWA Course: NHI Implementation of LRFD Geotechnical Design for Bridge Foundation Naser Abu-Hejleh, Ph.D., P.E Geotechnical Engineering Specialist FHWA Resource Center Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan Slide 1

2 NHI Course: Implementation of LRFD Geotechnical Design for Bridge Foundations Summary..

3 Background Standard Specifications 17 th Edition, 2002 (Final Edition) LRFD Specifications 5 th Edition,

4 Status of LRFD Implementation for Foundations DOTs are at various stages of implementation Do you have guidance or a process for implementing LRFD? Continued LRFD requests from State DOT LRFD to FHWA NHI Course is not adequate 4

5 Goal of the New Course Assist State DOTs with successful development of LRFD design guidance for bridge foundations based on AASHTO LRFD Section 10 and their local experience State DOT LRFD Design Guidance for Bridge Foundations 5

6 Course Sessions and Lessons Session 1 Lessons: 2. LRFD Implementation plan 3. Changes in AASHTO Design from ASD to LRFD 4. Calibration Methods for Resistance Factors Session 2 Lessons: 5. Calibration Conditions/Assessment of Site Variability 6. Selection of LRFD Design Method 7. Development of LRFD Design Guidance 6

7 Lesson 2: Implementation Plan Step 1 Form LRFD Implementation Committee Step 2 Review Key LRFD Design References Step 3 Identify Changes to Transition to LRFD Step 4 Select LRFD Geotechnical Design Methods Step 5 Develop LRFD Design Specifications Step 6 Develop LRFD Design Delivery Processes Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan Slide 7

8 Step 3. Identify Changes to Transition to LRFD How? Compare ASD design specifications against AASHTO LRFD Section 10 design specifications The changes to LRFD can be either: In accordance with AASHTO LRFD Section 10 Exceptions from AASHTO LRFD Section 10 (deletions, additions, or significant modifications). Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan Slide 8

9 Lesson 3: Changes in AASHTO Design from ASD to LRFD Three principal changes: 1. Incorporation of limit state designs 2. Load and resistance factors to account for uncertainties 3. New and improved methods to determine foundation loads, displacements, and resistances

10 1 st Change: Incorporation of Limit State Designs All possible structural and geotechnical failure for foundations that could lead to bridge failure are grouped into three distinct limit states: Service Limit States Strength Limit States Extreme Events Limit States

11 LRFD Design Equations at all Limit States For all applicable geotechnical limit states Σ γ i Q i φ i R ni For all applicable structural limit states Σ γ i Q i φ i P ni Where is summation for a failure mode (e.g., bearing capacity) identified in the limit state

12 2 nd Change: Use of Load and Resistance Factors ASD ΣQ i R ni / F Si Safety Factor, FS LRFD Σ Q i φ i R ni γ, Load factors φ, Resistance factors β, reliability index Design or Service Load e.g., = DL+LL Factored Load e.g., = γ Dl DL+ γ LL LL Allowable capacity = R ni / FS i Factored Resistance = φ i R ni

13 Use of Load and Resistance Factors Service and extreme event limit states LRFD: φ= 1 for most resistances; γ=1 for most loads ASD: FS= 1 Conclusion: no major design changes Strength limit: Changes with LRFD are significant Resistance factors Five load combinations

14 Load Factors for the Strength Limit Why? To account for all possible loads that may act on the bridge during its entire design life

15 3 rd Change: New and Improved Methods to determine Foundation Loads, Displacements and Resistances ASD: ΣQ i R ni /Fs i vs. LRFD: Σ γ i Q i φ i R ni Design loads (Q) and nominal resistances (R n ) are used in both platforms, BUT AASHTO LRFD: continue to improve/update methods to compute Q & R n AASHTO Standard Specifications: final update in 2002

16 AASHTO LRFD Methods to Calculate Loads Increased live loads from trucks New: Downdrag (DD) loads= lost nominal side geotechnical resistance above the level contributing to DD γ i Q i At all limit states, total factored axial compressive load per a pile= Σγ i Q i + DDγ p

17 Types of AASHTO s Methods to Determine Foundation Resistances/Displacements 1. Field static load test: measure resistances/displacements 2. Analytical expressions: predict resistances/displacements Static analysis methods (design phase) based on soil and rock properties from subsurface exploration Field dynamic analysis methods for driven piles based on field driving information (e.g., blow count, hammer energy) EOD and BOR conditions

18 AASHTO LRFD Resistance/Displacement Determination Methods at all Limit States AASHTO Article 10.4: Soil and Rock Properties AASHTO Article : Tolerable Movements AASHTO Article 10.6: Spread Footings AASHTO Section 10.7: Driven Piles: major changes AASHTO Section 10.8: Drilled Shafts AASHTO Section 10.9: Micropiles Geotechnical resistance losses to foundations due to downdrag, scour, and liquefaction are discussed.

19 AASHTO Allows for Exceptions from AASTHO AASHTO approves development of local LRFD design methods if justified: Long-term successful experience Research, and Local issues not addressed in AASHTO AASHTO s φ were developed based on calibration by fitting to ASD and reliability analysis Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan Slide 19

20 Step 4: Select LRFD Geotechnical Design Methods State DOTs have three options: Adopt AASHTO s LRFD methods Develop local LRFD methods by fitting to ASD methods Develop local LRFD methods through reliability analysis of data at load test sites Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan Slide 20

21 Lesson 4: Calibration Methods for Geotechnical Resistance Factors Calibration by fitting to ASD methods Reliability Analysis of Data at Load Test Sites AASHTO s Calibration Methods Focus on: Strength 1 Limit Load combination Axial compression resistance

22 Calibration by Fitting to ASD Methods ASD: Q s R n / FS ; LRFD: Q f φ R n Information needed: FS of the method to be calibrated Average load factor, γ ave = Q f /Q s (around 1.4) Calibration rules: I. φ= γ ave /FS II. Factored Resistance= γ ave x Allowable Capacity

23 Reliability Analysis of Data at Load Test Sites Reliability Analysis Procedure Step 1. Compile Data at Load Test Sites Step 2. Statistical Analysis Step 3. Reliability Analysis to determine φ Applications of the Reliability Analysis Results

24 Step 1. Compile Data at Load Test Sites At load test sites, collect for test foundations: Measured resistances from load tests, R m, and all the conditions used to measure them Predicted resistances from the calibrated method, R n and all the conditions used to predict them The design and construction conditions for test and production foundations need to be similar

25 2. Statistical Analysis of Bias Resistances: R m /R n # of Data Location SPT-N for the Base Material Base Resistance (base area, A = 1 ft 2 ) Predicted Resistance from the Calibrated Design Method = N A Measured Resistance from Load Test (Bpf) (Kips) (Kips) Bias Resistance = Measured Resistance /Predicted Resistance 1 Colorado New York Florida California Egypt For Normal Distribution: Resistance Mean Bias (λ ) Standard Deviation 0.33 COV 0.30

26 φ is a function of λ and COV Resistance Mean Bias = λ. Measures the overall tendency of the calibrated method to underestimate or overestimate resistances λ = (R m /R n )/n Coefficient of Variation (COV). Measures the variability of the method in predicting the measured resistance from load tests.

27 Reliability Analysis: φ Function of λ and COV Figure from NCHRP Report 507

28 Economics of the Resistance Determination Method The economics of the method is function of its Efficiency = φ/λ not just φ The larger φ/λ of the method, the More economical is the method Smaller the pile length or # of piles

29 Example of Reliability Calibrated Results Design Method # of Cases λ COV φ Efficiency φ/λ Static Analysis Methods Nordlund Method: H-Piles, sand λ-method, Concrete Pile, Clay α-tomlinson α-api, Concrete Pile, Clay FHWA CPT, Concrete Pile, Mixed Soil Nordlund Method: H-Piles, sand Dynamic Analysis Methods Dynamic Load Test WEAP EOD BOR EOD BOR* * FHWA, Modified Gates, EOD

30 Topic 3. AASHTO s Calibration Methods (Key References)

31 AASHTO s Axial Compression Resistance Determination Methods of a Driven pile and a Drilled Shaft AASHTO LRFD. Based on NCHRP Report 507 reliability analysis and load test results 2010 AASHTO LRFD. Significant changes to reflect past ASD practices and the need for engineering judgment: φ for driven piles handling site variability Redundancy for driven piles

32 Lesson 5: Calibration Conditions and Assessment of Site Variability AASHTO s Conditions Conditions for Development of Local LRD Design Methods Assessment of Site Variability Adopt AASHTO LRFD s loads Adhere to AASHTO LRFD Article , #, location, and depth of borings

33 AASHTO s Conditions AASHTO LRFD Section 10.5 and NCHRP Report 507 Design Soil and rock properties Design methods for driven piles Construction Load Testing Statistical and Reliability Analyses

34 AASHTO s Compression Resistance Determination Methods for a for a Single Pile AASHTO Standards: finalize pile length in the field AASHTO LRFD: φ is calibrated for Field dynamic analysis methods, φ dyn at BOR or/and EOD conditions Static analysis methods, φ sta Static analysis methods can be used to finalize pile length in the design if site variability is addressed

35 Impact of Foundation Redundancy on φ No changes to φ when # of piles 5 # of shafts 2 Driven Piles: reduce φ by 20% for a small pile group Drilled Shafts: reduce φ by 20% for a single shaft

36 Conditions for Local Calibration by Fitting to ASD As those in the ASD geotechnical design methods For example: continue the use of the same ASD testing methods and practice to determine and select design soil and rock properties

37 Conditions for Local Reliability Calibration Three types of conditions are discussed: From AASHTO s reliability calibration Statistical and reliability Analyses Local design and construction conditions Load test data can be obtained from: New load test data on large projects Published load test data

38 Topic 3. Assessment of Site Variability Site Variability: Horizontal variation of subsurface material. Quantified through: COV of the measured design soil properties across the site from various borings Site inherent variability, COV inherent : acceptable level of site variability considered in the resistance factor Uniform Site OR Zone: Site OR Zone COV < COV inherent

39 Lesson 6: Selection of LRFD Geotechnical Design Methods Comparison of AASHTO LRFD and AASHTO Standards Comparison of AASHTO LRFD and Local ASD Design Methods Advantages of Local Reliability Calibration

40 AASHTO s Piles Field Design Methods Static load test: φ implied from AASHTO Standards is 0.7 φ in AASHTO LRFD ranges from 0.75 to 0.8 Dynamic testing with signal matching: φ implied from AASHTO Standards is 0.62 φ in AASHTO LRFD ranges from 0.65 to 0.75 AASHTO LRFD rewards use of better methods and increased level of quality control

41 Comparison of AASHTO LRFD and Local ASD Use AASHTO LRFD Loads in both Platforms Reliability Economics. Compare: Results of ASD and LRFD on actual projects Factored geotechnical resistance from ASD and LRFD methods

42 Advantages of Local Reliability Calibration Advantages over LRFD methods developed from calibration by fitting AASHTO s LRFD methods

43 Lesson 7: Development of LRFD Design Guidance Development of LRFD design specifications Materials needed for development Roles and responsibilities Contents Development of LRFD design delivery processes Roles and responsibilities

44 Questions?