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LRFD Calibration of Axially-Loaded Concrete Piles Driven into Louisiana
SoilsLouisiana Transportation Conference
February 10, 2009
Sungmin “Sean” Yoon, Ph. D., P.E. (Presenter)Murad Abu-Farsakh , Ph. D., P.E.
Problem statement Different design methods Statistical concept Methods used in LADOTD for driven piles LRFD calibration Conclusion
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Problem Statement and Research Objectives
Working Stress Design (WSD) versus LRFD
Bridge super structures vs. Foundation
Federal Highway Administration and ASSHTO set a transition date of October 1, 2007
Resistance Factor (Φ) reflecting Louisiana soil and DOTD design process
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Stress Design Methodologies vs. LRFD
Working Stress Design (WSD) –also called Allowable Stress Design (ASD), since early 1800s.
where, Q=design load; Qall= allowable design load; and Rn= ultimate resistance of the structure
nall
RQ QFS
≤ = =
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Stress Design Methodologies vs. LRFD
Limit State Design (LSD), 1950s
Ultimate Limit Stress (ULS)
Service Limit Stress (SLS)
Factored resistance ≥ Factored load effects
Deformation ≤ Tolerable deformation to remain serviceable
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Stress Design Methodologies vs. LRFD
Load and Resistance Factor Design (LRFD)
where, Φ=resistance factor, Rn=ultimate resistance; γD=load factor for dead load; γL=load factor for live load; γi=corresponding load factor, and Qi=summation of load
�β1 Short Large High Large�β2 Large Small Low Small
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Reliability Index, β
β2β1
β Distance Overlapped area
Probability of failure
Design �φ
Pile Size
�β1 Short Large High Large Small�β2 Large Small Low Small Big
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How to Treat Uncertainty
1 2
Graph Variability Overlapped area
Probability of failure
Calculated φ
Pile Size
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How to Treat Uncertainty
1 2
Graph Variability Overlapped area
Probability of failure
Calculated φ
Pile Size
1 Low2 High
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How to Treat Uncertainty
1 2
Graph Variability Overlapped area
Probability of failure
Calculated φ
Pile Size
1 Low Small2 High Large
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How to Treat Uncertainty
1 2
Graph Variability Overlapped area
Probability of failure
Calculated φ
Pile Size
1 Low Small Low2 High Large High
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How to Treat Uncertainty
1 2
Graph Variability Overlapped area
Probability of failure
Calculated φ
Pile Size
1 Low Small Low Large2 High Large High Small
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How to Treat Uncertainty
1 2
Graph Variability Overlapped area
Probability of failure
Calculated φ
Pile Size
1 Low Small Low Large Small2 High Large High Small Big
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Benefits of LRFD
Improved reliability
More rational and rigorous treatment of uncertainties in design
Improved design and construction process (sub and super structures)
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First Order Second Moment (FOSM)
Load and Resistance Factor Design (LRFD)
(1)
where, Φ=resistance factor, Rn=ultimate resistance; γD=load factor for dead load; γL=load factor for live load; γi=corresponding load factor, and Qi=summation of load
Comparison of Resistance Factors, φ (βT=2.33) using FOSM, FORM and M-C
Design MethodResistance Factor, φ
FOSM FORM M-C
Static Method α-Tomlinson method and Nordlund method 0.56 0.63 0.63
Direct CPTMethod
Schmertmann 0.48 0.54 0.53
LCPC/LCP 0.56 0.63 0.62
De Ruiter and Beringen 0.68 0.77 0.75
Dynamicmeasurement
CAPWAP (EOD) 1.31 1.41 N/A
CAPWAP (14 days BOR) 0.58 0.63 0.63
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Conclusions
Preliminary resistance factors (φ) for Louisiana soil were evaluated for different driven pile design methods
Statistical analyses comparing the predicted and measured pile resistances were conducted to evaluate the performance of the different pile design methods.
LRFD in deep foundation can improve its reliability due to more balanced design.
More statistical data is needed for more rational resistance factor.
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Issues
Load sharing and overall redundancy Reduced φ to reflect increased β
Site variability Based on the filed and laboratory testing Resistance factor and number of static load test
needed
Scour
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Acknowledgement
The project is financially supported by the Louisiana Transportation Research Center and Louisiana Department of Transportation and Development (LA DOTD).