LRFD FOUNDATION DESIGN - Louisiana Transportation … Ching.pdf · LRFD FOUNDATION DESIGN...
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LRFD FOUNDATION DESIGN Ching-Nien Tsai, P.E. LADOTD Pavement and Geotechnical Services
Transcript of LRFD FOUNDATION DESIGN - Louisiana Transportation … Ching.pdf · LRFD FOUNDATION DESIGN...
LRFD FOUNDATION DESIGN
Ching-Nien Tsai, P.E.
LADOTDPavement and Geotechnical Services
Why Change?
WHAT IS LRFD?• Load and
Resistance Factor Design– Reliability
based design
– Not a new concept
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
-2 -1 0 1 2 3 4 5 6
x
f(x)
Load
Resistance
Zone of
Failure
FS
LRFD vs. ASD• Load Factors & Resistance Factors
– Combined to determine the reliability of the design instead of one FS for ASD
– Separate component risk levels vs. one lumped FS
– Calibrated resistance factors vs. empirical FS
• Deep Foundations• Shallow Foundations• Retaining Wall and Abutment
RESISTANCE FACTORS• Current Resistance Factors
– Deep foundation static calculation – from calibration– Others – use resistance factors that fit the current
practices• Stability – 0.7 (FS=1.5)• Load test – 0.7 (FS=2.0 with LF=1.5)• Dynamic load test – 0.65 (FS=2.25 with LF=1.5)
– Site variability – not calibrated• Future Resistance Factors
– Site variability– Calibration
• Field methods– Static load test– Statnamic/Fundex Load Test– Dynamic load test (PDA)– Driving Formulae
• Static analysis methods
Determining Geotechnical Resistance of Piles
Geotechnical Resistance Factors for Piles
Method Site Variability φ
Static Load Test
Low 0.8 – 0.9Medium 0.7 – 0.9
High 0.55 – 0.8
AASHTO Table 10.5.5.2.2-2
Site Variability Defined in NCHRP Report 507
Range of Values of Resistance Factors Depends on Number of Static Load Tests
Geotechnical Resistance Factors for Piles
Method φ(LADOTD)
φ
Wave Equation only 0.4FHWA-Modified Gates (EOD) 0.8 0.4ENR 0.1
AASHTO Table 10.5.5.2.2-1
Geotechnical Resistance Factors for Piles
Method φ (LADOTD) φDynamic Test w/Signal Matching (e.g., PDA + CAPWAP)
0.53 (>14 days) 0.65
AASHTO Table 10.5.5.2.2-1 & 3
Test 1% to 50% of Production Piles, Depending on Site Variability and Number of Piles Driven
Site Variability Defined in NCHRP Report 507
Geotechnical Resistance FactorsPile Static Analysis Methods
Method φComp φTen
α - Method 0.4/0.53 0.3β - Method 0.35 0.25λ - Method 0.4 0.3Nordlund-Thurman 0.45/0.45SPT 0.3 0.25CPT 0.45/0.4-0.6 0.35Group 0.6 0.5
Table 10.5.5.2.2-1 Resistance Factors for Driven Piles
CONDITION/RESISTANCE DETERMINATION METHODRESISTANCE
FACTOR
Nominal Resistance of Single Pile in Axial Compression –Dynamic Analysis and Static Load Test Methods, ϕdyn
Driving criteria established by static load test(s); qualitycontrol by dynamic testing and/or calibrated wave equation,or minimum driving resistance combined with minimumdelivered hammer energy from the load test(s). For thelast case, the hammer used for the test pile(s) shall beused for the production piles.
Values in Table 2
Driving criteria established by dynamic test with signalmatching at beginning of redrive conditions only of at leastone production pile per pier, but no less than the numberof tests per site provided in Table 3. Quality control ofremaining piles by calibrated wave equation and/ordynamic testing.
0.65
Wave equation analysis, without pile dynamicmeasurements or load test, at end of drive conditions only 0.40
FHWA-modified Gates dynamic pile formula (End Of Drivecondition only) 0.40
Engineering News Record (as defined in Article 10.7.3.7.4)dynamic pile formula (End Of Drive condition only) 0.10
Side Resistance
Tip Resistance
Total Resistance
AB
CD
RP
RS
RR = φRn = φqpRp + φqsRs
Displacement
Resi
stan
ce
Drilled Shaft Resistance
External Failure Mechanisms
Sliding Failure Overturning Failure
Bearing FailureDeep-Seated Sliding Failure
Shallow Foundation Geotechnical Resistances
• ASD Failure Modes– Overall Stability– Bearing Capacity– Settlement– Sliding– Overturning
Nominal Shallow Foundation Geotechnical Resistances
• LRFD Service Limit State– Overall Stability– Vertical (Settlement) and Horizontal
Movements• LRFD Strength Limit State
– Bearing Resistance– Sliding– Eccentricity Limits (Overturning)
Stabilize Destabilize
Service Limit State
Global Stability
ASD Factors of Safety
Soil Parameters and Ground Water Conditions Based On:
Slope Supports Abutment or
Other Structure?Yes No
In-situ or Laboratory Tests and Measurements 1.5 1.3
No Site-specific Tests 1.8 1.5
Resistance Factors
LRFD
Stability Wrap-Up• Unfactored loads
– Service Limit State• Applied stress must be limited
– Footings supported in a slope– φ ≤ 0.65 (FS ≥ 1.5)
• Stress criteria for stability can control footing design
Service Limit State Design –Settlement• Cohesive Soils
– Evaluate Using Consolidation Theory• Cohesionless Soils
– Evaluate Using Empirical or Other Conventional Methods
– Hough Method
Use of Eccentricity and Effective Footing Dimensions
• Service Limit State– Nominal Bearing Resistance Limited by
Settlement• Strength Limit State
– Nominal Bearing Resistance Limited by Bearing Resistance
• Prevent Overturning– All Applicable Limit States
ML MB
LB
eB e L
B’ L’
P
q
Applied Stress Beneath Effective Footing Area
Stress Applied to SoilStrip Footing
METHOD/SOIL/CONDITIONRESISTANCE
FACTORBearingResistance
φ All methods, soil and rock 0.45
Plate Load Test 0.55
Sliding φτ Precast concrete placed on sand 0.90
Cast-in-Place Concrete on sand 0.80
Clay 0.85Soil on soil 0.90
φep Passive earth pressure component of sliding resistance
0.50
Strength Limit State Resistance Factors
LRFD vs. ASD• All modes are expressly checked at a limit
state in LRFD• Eccentricity limits (0.25B) replace the
overturning Factor of Safety
Settlement vs. Bearing Resistance
00
1212
N=30N=30
B, ftB, ft
qq aa, k
sf, k
sf N=25N=25
N=5N=5
N=20N=20
N=15N=15
N=10N=10
22 44 66 1414101088 1212
22
00
44
66
88
1010
00
1212
N=30N=30
B, ftB, ft
qq aa, k
sf, k
sf N=25N=25
N=5N=5
N=20N=20
N=15N=15
N=10N=10
22 44 66 1414101088 1212
22
00
44
66
88
1010
1.25
DC
β β
0.90
DC
1.00
WA V
1.00
WA V
β+δ β+δ1.50
EHcos(β+δ)1.50
EHcos(β+δ)
1.50 EH 1.50 EH1.
35 E
V
1.00
EV
1.50 EHsin(β+δ) 1.50 EHsin(β+δ)
1.00 WAH 1.00 WAH
Load Factors for Bearing Resistance
Load Factors for Sliding and Eccentricity
Load Factors for Conventional Walls
Conventional Walls - Summary
• Use resistance factors for spread footings or deep foundations, as appropriate (Section 10.5)
• Eccentricity limited to:– e/B < 0.25 for soil (compare to ASD 0.167)– e/B < 0.375 for rock (compare to ASD 0.25)
Resistance Factors
Bearing ResistancePassive ResistanceFlexural Resistance
Section 10.51.000.90
• Code allows increase in Resistance Factors for temporary walls but specific guidance is not provided
Non-gravity Cantilevered Walls
• Below excavation line, multiply by 3b on passive side of wall and 1b on active side of wall for discrete elements
• Look at forces separately below excavation line on passive side and active side (because different load factors)
Pressure Diagrams – Discrete Elements
ASD
LRFD
• Factor embedment by 1.2 for continuous wall elements
• Do not factor embedment for discrete wall elements (conservatism of 3b assumption)
Non-gravity Cantilevered Walls
Recommended AEP for SandsH
H1 H1
Hn+
1
p p
2 /3
H1
2 /3
H1
2 /3
Hn+
1
2 /3
(H-H
1)1 /
3H
Th1
Th1
Th2
Thn
H2
Hn
R R
(a) Walls with one levelof ground anchors
(b) Walls with multiplelevels of ground anchors
HKHLOAD TOTALp A
32
γ≈=1n3
113
1 HH-HLOAD TOTALp
+−=
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