Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan – Slide 1

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

NHI-132083 Course:

“Implementation of LRFD

Geotechnical Design for

Bridge Foundations”

Summary…..

3

Background

Standard Specifications 17th Edition, 2002

(Final Edition)

LRFD Specifications 5th Edition, 2010

4

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

Do you have guidance or a

process for implementing LRFD?

5

Assist State DOTs with successful development of LRFD

design guidance for bridge foundations based on

AASHTO LRFD Section 10 and their local experience

Goal of the New Course

State DOT LRFD Design Guidance for Bridge Foundations

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

Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan – Slide 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 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).

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

For all applicable structural limit states

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

Σ γi Qi ≤ ∑φi Rni

Σ γi Qi ≤ ∑φi Pni

2nd Change: Use of Load and Resistance Factors

ASD ΣQi ≤ ∑Rni/ FSi

LRFD Σ Qi ≤ ∑φi Rni

Safety Factor, FS γ, Load factors φ, Resistance factors β, reliability index

Design or Service Load e.g., = DL+LL

Factored Load e.g., = γDl DL+ γLL LL

Allowable capacity = ∑Rni/ FSi

Factored Resistance = ∑φi Rni

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

Use of Load and Resistance Factors

Load Factors for the Strength Limit

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

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

3rd Change: New and Improved Methods to determine Foundation Loads, Displacements and Resistances

ASD: ΣQi ≤ ∑Rni/Fsi vs. LRFD: Σ γi Qi ≤ ∑φi Rni

AASHTO LRFD Methods to Calculate Loads

Increased live loads from trucks

∑γi Qi 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

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 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 LRFD Resistance/Displacement Determination Methods at all Limit States

Implementation of LRFD Geotechnical Design for Bridge Foundations Lesson 2: Implementation Plan – Slide 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 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

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

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

Calibration by Fitting to ASD Methods

ASD: Qs ≤ Rn/ FS ; LRFD: Qf ≤ φ Rn

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 Location

SPT-N for the Base Material

Base Resistance (base area, A = 1 ft2) Bias

Resistance = Measured Resistance /Predicted Resistance

Predicted Resistance from

the Calibrated Design Method =

N A

Measured Resistance from

Load Test (Bpf) (Kips) (Kips)

1 Colorado 5 5 4.5 0.90 2 New York 22.5 22.5 20 0.89 3 Florida 15 15 12 0.80 4 California 16.5 16.5 23.5 1.42 5 Egypt 10 10 15 1.50 . . . . .

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For Normal Distribution: Resistance Mean Bias (λ ) 1.10

Standard Deviation 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

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

λ = ∑(Rm/Rn)/n

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

Design Method # of Cases λ COV φ Efficiency φ/λ

Static Analysis Methods

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

Dynamic Analysis Methods

Dynamic Load Test

EOD 125 1.63 0.49 0.64 0.4

BOR 162 1.16 0.34 0.65 0.56

WEAP 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

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

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

AASHTO’s Axial Compression Resistance Determination

Methods of a Driven pile and a Drilled Shaft

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

Reliability

Economics. Compare: Results of ASD and LRFD on actual

projects

Factored geotechnical resistance from ASD and LRFD methods

Comparison of AASHTO LRFD and Local ASD Use AASHTO LRFD Loads in both Platforms

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?