“Implementation of LRFD Geotechnical Design for Bridge Foundation”

2012 Ohio Geotechnical Consultant Workshop Columbus, Ohio; May 8, 2012 Overview of New FHWA Course: NHI-132083

“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

NHI-132083 Course: “Implementation of LRFD Geotechnical Design for Bridge Foundations” Summary…..

Background Standard Specifications 17th Edition, 2002 (Final Edition)

3

LRFD Specifications 5th Edition, 2010

Status of LRFD Implementation for Foundations  DOTs are at various stages of implementation

 Continued LRFD requests from State DOT LRFD to FHWA  NHI Course 130082 is not adequate

4

Do you have guidance or a process for implementing LRFD?

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

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

Lesson 2: Implementation Plan Step 1

Step 2

Step 3

Step 4

• Form LRFD Implementation Committee • Review Key LRFD Design References • Identify Changes to Transition to LRFD • 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

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

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

1st 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

LRFD Design Equations at all Limit States 

For all applicable geotechnical limit states Σ γi Qi ≤ ∑φi Rni



For all applicable structural limit states Σ γi Qi ≤ ∑φi Pni

Where ∑ is summation for a failure mode (e.g., bearing capacity) identified in the limit state

2nd Change: Use of Load and Resistance Factors ASD ΣQi ≤ ∑Rni/ FSi Safety Factor, FS

Design or Service Load e.g., = DL+LL Allowable capacity = ∑Rni/ FSi

LRFD Σ Qi ≤ ∑φi Rni  γ, Load factors  φ, Resistance factors  β, reliability index Factored Load e.g., = γDl DL+ γLL LL Factored Resistance = ∑φi Rni

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

Load Factors for the Strength Limit

Why? To account for all possible loads that may act on the bridge during its entire design life

3rd Change: New and Improved Methods to determine Foundation Loads, Displacements and Resistances ASD: ΣQi ≤ ∑Rni/Fsi vs. LRFD: Σ γi Qi ≤ ∑φi Rni

Design loads (Q) and nominal resistances (Rn) are used in both platforms, BUT • AASHTO LRFD: continue to improve/update methods to compute Q & Rn • AASHTO Standard Specifications: final update in 2002

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  At all limit states, total factored axial compressive load per a pile= Σγi Qi + DDγp

∑γ i Qi

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

AASHTO LRFD Resistance/Displacement Determination Methods at all Limit States      

AASHTO Article 10.4: Soil and Rock Properties AASHTO Article 10.5.2.2: 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.

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

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

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

Calibration by Fitting to ASD Methods ASD: Qs ≤ Rn/ FS

;

LRFD:

Qf ≤ φ R n

 Information needed:  FS of the method to be calibrated  Average load factor, γave = Qf/Qs (around 1.4)  Calibration rules: I.

φ= γave/FS

II. Factored Resistance= γave x Allowable Capacity

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

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

2. Statistical Analysis of Bias Resistances: Rm/Rn

# of Data 1 2 3 4 5 .

. . . .

Location

SPT-N for the Base Material

Colorado New York Florida California Egypt

(Bpf) 5 22.5 15 16.5 10

. . . . .

. . . . .

Base Resistance 2 (base area, A = 1 ft ) Bias Predicted Resistance = Resistance from Measured the Calibrated Resistance Measured Design Method = Resistance from /Predicted NA Resistance Load Test (Kips) (Kips) 4.5 5 0.90 20 22.5 0.89 15 12 0.80 16.5 23.5 1.42 10 15 1.50 For Normal Distribution: Resistance Mean Bias (λ ) Standard Deviation

1.10 0.33

COV

0.30

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 = ∑(Rm/Rn)/n

 Coefficient of Variation (COV). Measures the variability of the method in predicting the measured resistance from load tests.

Reliability Analysis: φ Function of λ and COV

Figure from NCHRP Report 507

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

Example of Reliability Calibrated Results # of Cases

λ

COV

φ

Efficiency φ/λ

Nordlund Method: H-Piles, sand

19

0.94

0.4

0.46

0.49

λ-Method, Concrete Pile, Clay

8

0.81

0.51

0.32

0.39

α-Tomlinson

18

0.87

0.48

0.36

0.41

α-API, Concrete Pile, Clay

17

0.81

0.26

0.54

0.67

FHWA CPT, Concrete Pile, Mixed Soil

30

0.84

0.31

0.51

0.6

Nordlund Method: H-Piles, sand

19

0.94

0.4

0.46

0.49

EOD

125

1.63

0.49

0.64

0.4

BOR

162

1.16

0.34

0.65

0.56

EOD

99

1.66

0.72

0.39

0.24

BOR*

99

0.94

0.42

0.43

0.46*

FHWA, Modified Gates, EOD

135

1.07

0.53

0.38

0.36

Design Method Static Analysis Methods

Dynamic Analysis Methods Dynamic Load Test WEAP

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

AASHTO’s Axial Compression Resistance Determination Methods of a Driven pile and a Drilled Shaft  2006-2009 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

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 10.4.2, #, location, and depth of borings

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

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, φdyn at  BOR or/and EOD conditions  Static analysis methods, φsta Static analysis methods can be used to finalize pile length in the design if site variability is addressed

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

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

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

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, COVinherent: acceptable level of site variability considered in the resistance factor  Uniform Site OR Zone: Site OR Zone COV < COVinherent

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

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

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

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

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

Questions?