2012 ohio geotechnical consultant workshop … · driven piles: reduce φ by 20% for a small pile...
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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?