Transcript LRFD WORKSHOP - Pile Driving Contractors Association

Driven Pile Design
George Goble
Basic LRFD Requirement
ηk Σ γij Qij ≤ φg Rng
ηk – factor for effect of redundancy, ductility and importance
γij – Load factor for the ith load type in the jth load combination
Qij – The ith load type in the jth load combination
φg – The resistance factor for the ath failure mode
Rng - The nominal strength for the ath failure mode
Definition of Loads
N – Axial load
FT – Load transverse to the
bridge centerline
FL – Load parallel to the
bridge centerline
MT – Moment about the
transverse axis
WL – Wind on Live Load
– Wind Load on Structure
DC – Structural Dead Load
LL – Vehicular Live Load
IM – Vehicular Dynamic Load
ML – Moment about the
longitudinal axis
BR – Vehicular Braking WS
Force
Note: Two different wind loads are specified – winds greater
than 55 miles per hour and winds less than 55 miles per hour.
At greater than 55 miles per hour no traffic loads are included
Force Effects
Load Set 1, Maximum axial effect with overturning effect
All units are kips and feet
• LOAD
•
•
•
•
•
•
DC
LL
WS (>55)
WS (<55)
WL
BR
N
5564
894
-254
-142
0
0
FT
0 0
0 0
182
107
20 -4.2
24.2
FL
MT
0
0
145
66
-125
-54.5
0
3742
4334
1961
600
-1636
ML
5454
3226
727
Force Effects
Load Set 2, Maximum overturning effect with axial effect
All units are kips and feet
LOA
DC
LL
WS (>55)
WS (<55)
WL
BR
N
5564
662
-254
-142
0
0
FT
0
0
182
107
20
17.9
FL
MT
ML
0
0
145
66
-4.2
-40.0
0
0
4334
1961
-125
-1208
0
12552
5454
3226
600
537
AASHTO Load Combinations
• STR I MAX = 1.25 DC + 1.75 (LL + IM + BR)
• STR I MIN = 0.9 DC + 1.75 (LL + IM + BR)
• STR III = 0.9 DC + 1.4 WS
• STR IV = 1.5 DC
• STR V MAX = 1.25 DC + 1.35 (LL + IM + BR) + 0.4 WS + 1.0 WL
• STR V MIN = 0.9 DC + 1.35 (LL + IM + BR) + 0.4 WS + 1.0 WL
Table 2
Factored Loads
LOAD
N
FT FL
z
MT
x
ML
y
Mx
My
STR I MAX
8520
42
-95
-2863
7821
STR I MIN
6166
31
-71
-2114
22906
STR III
4652
255
203
6068
7635
STR IV
8346
0
0
0
0
STR V MAX
8105
95
-51
-1549
7924
STR V MIN
5845
87
-32
-971
19561
Soil
Boring
TRY
• 18 inch Square Prestressed Concrete
pile
• Use 7000 psi Concrete
• Structural Axial Strength
– Pn = 0.80 [ 0.85f’c Ag–(fpe- 85.5) Ag ]
– Pn = 1360 kips
Wave Equation Results
•
•
•
•
•
•
D-36-32 Hammer
3 inches plywood !!
Capacity 1100 kips
Blow Count 10 Blows per inch
Maximum Compression Stress 3.6 ksi
Allowable Driving Stress
– φ(0.85f’c - fpe), - φ = 1.0
– For 7.0 ksi Concrete, Allowable Stress = 5.1 ksi
Wave
Equation
Bearing
Graph
Concrete Stress-Strain Curve
Trial No. 1
• 1100 kips Pile Capacity
• 16, 18 inch Square Piles 4 x 4 Group
• FB-Pier Input
–
–
–
–
Structural Elements and Material Properties
Soil Properties
Structural Geometry
Loads
• Lateral – O’Neil Sand Model
• DRIVEN Axial Model
– Increase Axial Capacity by a Factor of 2.0
• Effective Prestress – 800 psi
• Linear Analysis – No P-Δ – But Non-Linear Soil
Results
• Several Tries - 4 x 4 Group Doesn’t Work –
Pile Top Structural Failure
• Change to 20 Inch Square Pile – 4 x 4 Group
• Very Safe
• Try 3 x 4, 20 Inch Pile Group
• Successful After Several Trials
Final
Design
Results
Bi-Axial Interaction Diagram Pile 4, Load Case 2
Critical Conditions
Load Case
Max. Pile
Load, Pile No.
Kips
Max. Uplift
Demand/Capacity
Load, Pile No.
Ratio, Pile No.
Kips
Str I Max
847, 9
0.700
Str I Min
791
Str III
561
1.000, 4
Str IV
691
0.570
Str V Max
783
0.673
Str V Min
712
0.649
68, 4
0.654
Required Axial Capacity
Rn = Un-Factored Capacity/φ
Rn = 847/0.80
Rn = 1060 kips
Wave Equation Analysis
Final Requirements
• 12, 20 Inch Square Piles
• Estimated Length – 85 Feet – (Bottom of
Cap, -10 Feet)
• Required Blow Count – 80 Blows per Foot
• Maximum Compression Stress – 3.3 ksi
• Maximum Tension – 1.5 ksi – Excessive,
Throttle Back