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Fundamental Principles of Foundation Design
Anthony M. DiGioia Jr., PhD, PE, PresidentDiGioia, Gray and Associates, LLC
IEEE TP&C Subcommittee Meeting
Orlando, Florida
January 8 – 11, 2007
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Foundation Design Requires a Blending of
Soil/Foundation Interaction Modelingand
Engineering Judgment
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Foundation Design Process
INPUT DATA
FOUNDATION DESIGN
CONTRACT DRAWINGS AND SPECIFICATIONS
CONSTRUCTION MONITORING
AND AS-BUILT INFORMATION
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SUBSURFACE
INVESTIGATION
Input Data
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PAST SUBSURFACE INVESTIGATIONS
GEOLOGY STUDY
SUBSURFACE
INVESTIGATION
Input Data
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SUBSURFACE INVESTIGATION
Input DataGEOLOGY
STUDYPAST SUBSURFACE
INVESTIGATIONS
STRUCTURE TYPES(Lattice Towers, Poles, H-Frames, Etc.)
AND ROLE(Tangent, Runing Angle, Dead End, Etc.)
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SUBSURFACE INVESTIGATION
Input DataGEOLOGY
STUDYPAST SUBSURFACE
INVESTIGATIONS
STRUCTURE TYPES(Lattice Towers, Poles, H-Frames, Etc.)
AND ROLE(Tangent, Runing Angle, Dead End, Etc.)
MODE OF FOUNDATION LOADS(Moment, Horizontal, Uplift, Compression
and Combinations)
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Input Data
SINGLE POLE STRUCTURES
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Input Data
LATTICE TOWER (FOUR-LEGGED) STRUCTURES
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Input Data
H-FRAME STRUCTURES
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SUBSURFACE INVESTIGATION
Input DataGEOLOGY
STUDYPAST SUBSURFACE
INVESTIGATIONS
STRUCTURE TYPES(Lattice Towers, Poles, H-Frames, Etc.)
AND ROLE(Tangent, Runing Angle, Dead End, Etc.)
MODE OF FOUNDATION LOADS
(Moment, Horizontal, Uplift, Compression and Combinations)
NUMBER, TYPE AND LOCATION
OF BORINGS
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SUBSURFACE INVESTIGATION
Input DataGEOLOGY
STUDYPAST SUBSURFACE
INVESTIGATIONS
STRUCTURE TYPES(Lattice Towers, Poles, H-Frames, Etc.)
AND ROLE(Tangent, Runing Angle, Dead End, Etc.)
MODE OF FOUNDATION LOADS
(Moment, Horizontal, Uplift, Compression and Combinations)
NUMBER, TYPE AND LOCATION
OF BORINGS
LINE DESIGN CRITERIA
Line Reliability Issues Component Reliability
Issues
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SUBSURFACE INVESTIGATION
Input DataGEOLOGY
STUDYPAST SUBSURFACE
INVESTIGATIONS
STRUCTURE TYPES(Lattice Towers, Poles, H-Frames, Etc.)
AND ROLE(Tangent, Runing Angle, Dead End, Etc.)
MODE OF FOUNDATION LOADS
(Moment, Horizontal, Uplift, Compression and Combinations)
NUMBER, TYPE AND LOCATION
OF BORINGS
LINE DESIGN CRITERIA
Line Reliability Issues Component Reliability
Issues
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PI1
PI3 PI2
PI4
PI5
How should you approach obtaining subsurface information and geotechnical design parameters?
• SOIL TYPES
• ROCK TYPES
• DEPTH TO GROUND WATER
• DEPTH TO ROCK
GEOTECHNICAL ISSUES
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PI1
PI3 PI2
PI4
PI5
How should you approach obtaining subsurface information and geotechnical design parameters?
• STRUCTURE TYPES
• STRUCTURE ROLES
• PERFORMANCE CRITERIA
• RELIABILITY LEVEL
STRUCTURE ISSUES
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Ws Ws
GWL
U
H Wc
β
• What geotechnical data do we need?- All cohesive soil- All cohesionless soil- Layered soil conditions
• Free-Standing Lattice Tower; Concrete Spread Footings;Frustrum Design Method
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Ws Ws
GWL
U
H Wc
τ τ
• What geotechnical data do we need?- All cohesive soil- All cohesionless soil- Layered soil conditions
• Free-Standing Lattice Tower; Concrete Spread Footings;Side Friction Design Method
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H
GWL
U
Wcττ
• What geotechnical data do we need?- All cohesive soil- All cohesionless soil- Layered soil conditions
• Free-Standing Lattice Tower; Drilled Shaft;Cylindrical Shear Design Method
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• What geotechnical data do we need?- All cohesive soil- All cohesionless soil- Layered soil conditions
• Tubular Steel Pole; Drilled Shaft;Hansen Design Method
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Foundation Design Process
INPUT DATA
FOUNDATION DESIGN
CONTRACT DRAWINGS AND SPECIFICATIONS
CONSTRUCTION MONITORING
AND AS-BUILT INFORMATION
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Foundation Design Process
INPUT DATA
FOUNDATION DESIGN
CONTRACT DRAWINGS AND SPECIFICATIONS
CONSTRUCTION MONITORING
AND AS-BUILT INFORMATION
ASCE MANUAL 74 RBD METHOD
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• Consider the variability of loadings.• Consider the variability of component
strength.• Vary reliability levels between lines.• Vary reliability levels between line
components.
The ASCE Method allows the designer to:
ASCE Manual 74 Reliability-Based Design (RBD) Method
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The Load Resistance Factor Design (LRFD) equation presented in Section 1 of Manual 74 for weather-related (reliability based) loads is as follows:
ΦCRe > effect of [DL + γ Q50] (1)
in which:
ΦC = strength (resistance) factor which can be selected to adjust the reliability of the component;
Re = the e-th % design strength for the component;DL = dead load effect in the component;γ = load factor applied to the live load effect Q50;
Q50 = load effect produced by combinations of wind velocity, ice thickness, and/or temperature, which has a 50-year return period
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Load Factors to Adjust Line Reliabilityby Factor LRF
40020010050Load Return Period - RP (years)
1.41.31.151.0Load Factor, γ
8421Line Reliability Factor (LRF)
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Strength Factors to AdjustComponent Reliability by Factor CRF forStrength Exclusion Limit, e of 5 to 10%
0.750.901.11CRF, ΦC, for COVR = 50%0.770.881.09CRF, ΦC, for COVR = 40%0.760.871.05CRF, ΦC, for COVR = 30%0.730.851.00CRF, ΦC, for COVR = 10-20%
421Component Reliability Factor
(CRF)
NOTE: COVR = Coefficient of Variation of Resistance
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Foundation Design Process
INPUT DATA
FOUNDATION DESIGN
CONTRACT DRAWINGS AND SPECIFICATIONS
CONSTRUCTION MONITORING
AND AS-BUILT INFORMATION
ASCE MANUAL 74 RBD METHOD
FOUNDATION DESIGN MODEL SELECTION & CALIBRATION
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Hansen Design Model
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0
1000
2000
3000
4000
5000
6000
0 1000 2000 3000 4000 5000
Predicted Nominal Strength - X (kip-ft)
Test
Str
engt
h - Y
(kip
-ft)
Constant COVm
least squares f ity = mm Xy = 1.242 X
Calibration of Hansen Design ModelTP3 Revised, 4-in criteria – Drilled Shafts Under Drained Moment – 20 Tests – Lognormal PDF
RT5 = 0.75 Rnφ5 = 0.75
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MFAD Design ModelThe Schematic Four-Spring Model in MFAD
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0
1000
2000
3000
4000
5000
6000
7000
0 1000 2000 3000 4000 5000 6000 7000
Predicted Nominal Strength - X (kip-ft)
Test
Str
engt
h - Y
(kip
-ft)
Constant COVm
least squares fity = mm Xy = 0.929 X
Calibration of MFAD Design ModelTP3 Revised, 4-in criteria – Drilled Shafts Under Drained Moment – 20 Tests – Lognormal PDF
RT5 = 0.60 Rnφ5 = 0.60
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CAISSON Design Model
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0
1000
2000
3000
4000
5000
6000
0 1000 2000 3000 4000 5000 6000
Predicted Nominal Strength - X (kip-ft)
Test
Str
engt
h - Y
(kip
-ft)
Constant COVm
least squares f ity = mm Xy = 1.017 X
TP3 Revised, 4-in criteria – Drilled Shafts Under Drained Moment – 20 Tests – Lognormal PDF
Calibration of CAISSON Design Model
RT5 = 0.42 Rnφ5 = 0.42
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0.4249.91.0220CAISSON
0.6024.90.9320MFAD
0.7528.91.2420Hansen
φ5(Lognormal
PDF)COVm(%)mmnDesign Model
Summary of Calibration Statistics and Strength Factor Data
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Foundation Design Process
INPUT DATA
FOUNDATION DESIGN
CONTRACT DRAWINGS AND SPECIFICATIONS
CONSTRUCTION MONITORING
AND AS-BUILT INFORMATION
ASCE MANUAL 74 RBD METHOD
FOUNDATION DESIGN MODEL SELECTION & CALIBRATION
LABORATORY TESTING PROGRAM
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Laboratory Testing ProgramCOHESIVE SOILS
•Total Density
•Moisture Content
•Undrained Shear Strength
•Modulus of Deformation
COHESIONLESS SOILS•Total Density
•Moisture Content
•Angle of Internal Friction
•Compaction Characteristics
•Modulus of Deformation
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Engineering Property Correlations –Cohesionless Soils
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Engineering Property Correlations –Cohesive Soils
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Foundation Design Process
INPUT DATA
FOUNDATION DESIGN
CONTRACT DRAWINGS AND SPECIFICATIONS
CONSTRUCTION MONITORING
AND AS-BUILT INFORMATION
ASCE MANUAL 74 RBD METHOD
FOUNDATION DESIGN MODEL SELECTION & CALIBRATION
LABORATORY TESTING PROGRAM
SELECTION OF GEOTECHNICAL DESIGN PARAMETERS
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Selection of Geotechnical Design Parameters
+ =BORING
LOG
SUMMARY of
LAB TEST RESULTS
SUMMARY of
SELECTED GEOTECHNICAL
DESIGN PARAMETERS
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Selection of Geotechnical Design Parameters
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Foundation Design ProcessINPUT DATA
FOUNDATION DESIGN
CONTRACT DRAWINGS AND SPECIFICATIONS
CONSTRUCTION MONITORING
AND AS-BUILT INFORMATION
ASCE MANUAL 74 RBD METHOD
FOUNDATION DESIGN MODEL SELECTION & CALIBRATION
LABORATORY TESTING PROGRAM
SELECTION OF GEOTECHNICAL DESIGN PARAMETER
FOUNDATION DESIGN
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Using the reliability-based design approach, determine D for the Hansen, MFAD and CAISSON design methods.
M50 =1000 kip-ft
D = ?
Groundwater level at surface
Stiff to very stiff clayγT =120 pcfsu = 2.0 ksf
ll = ll = ll ll = ll
Laterally Loaded Drilled ShaftFoundation Design Process
5 FT
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Laterally Loaded Drilled ShaftLognormal PDF
23810.42CAISSON16670.60MFAD13330.75Hansen
The Required Nominal Design Moment
Capacity (1)Φ5Design Model
(1)The nominal design capacity moment required = M50/φ5
Foundation Design Process
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Nominal Moment Capacity (Mn)Versus Embedment Depth
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0
Embedment Depth (ft)
Nom
inal
Mom
ent C
apac
ity (k
ip-ft
) CAISSON
HANSEN
MFAD
Mn =2381 k-ft
Mn =1667 k-ft
Mn =1333 k-ft
Foundation Design Process
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100200300400500600700800900
10001100120013001400150016001700180019002000
10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0
Embedment Depth (ft)
Desig
n Mom
ent C
apac
ity φ
5Mn (
kip-ft)
CAISSON
MFADHANSEN
Design Moment Capacity (Φ5Mn)Versus Embedment Depth
Foundation Design Process
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Summary•FOUNDATION DESIGN REQUIRES A BLENDING
•A WELL-PLANNED SUBSURFACE INVESTIGATION IS CRITICAL
•IMPLEMENTATION OF THE RBD METHOD IN ASCE MANUAL 74 IS RECOMMENDED
•CALIBRATING FOUNDATION DESIGN METHODS PROVIDES A RATIONAL DESIGN FRAMEWORK FOR DEVELOPING STRENGTH FACTORS
•FIELD INSPECTION OF CONSTRUCTION AND GEOTECHNICAL PARAMETER CONFIRMATION ARE CRITICAL
SOIL/FOUNDATION INTERACTION MODELING
ENGINEERING JUDGEMENT
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