Transcript AASHTO Section 10 Revisions - Virginia Department of
AASHTO LRFD Section 11 Abutments, Piers, and Walls
AASHTO Section 11
Design specifications for:
Conventional gravity/semigravity walls
Non-gravity cantilevered walls
Anchored walls
Mechanically Stabilized Earth (MSE) walls
Prefabricated modular walls
Common Load Groups for Walls Group
g
DC
g
EV
g
EH (Active)
g
ES
g
LS Strength Ia 0.90
1.00
1.50
1.50
1.75
Strength Ib 1.25
1.35
1.50
1.50
1.75
Service I 1.00
1.00
1.00
1.00
1.00
Load Definitions
DC – dead load of structural components and attachments
EV – vertical pressure from dead load of earth fill
EH – horizontal earth pressure load
ES – earth surcharge load
LS – live load surcharge (transient load)
Surcharge Loads
Earth surcharge AASHTO Section 3.11.6.1 and 3.11.6.2
Live load surcharge AASHTO 3.11.6.4
Conventional Retaining Walls
Strength Limit States
Sliding
Bearing resistance
Eccentricity
Service Limit States
Vertical settlement
Lateral wall movement
Overall stability
External Failure Mechanisms
Sliding Failure Bearing Failure Overturning Failure Deep-Seated Sliding Failure
Load Factors for Conventional Walls
b b 1.50 EHsin( b+d ) 1.50 EH b+d 1.50 EHcos( b+d ) 1.50 EHsin( b+d ) 1.50 EH b+d 1.50 EHcos( b+d ) Load Factors for Bearing Resistance 1.00 WA H Load Factors for Sliding and Eccentricity 1.00 WA H
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)
Non-gravity Cantilevered Walls
Strength Limit States
Bearing resistance of embedded portion of wall
Passive resistance of embedded portion of wall
Flexural resistance of wall/facing elements
Service Limit States
Vertical wall movement
Lateral wall movement
Overall stability
Resistance Factors
Bearing Resistance Passive Resistance Flexural Resistance Section 10.5
1.00
0.90
Code allows increase in Resistance Factors for temporary walls but specific guidance is not provided
Pressure Diagrams – Discrete Elements ASD LRFD
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)
Non-gravity Cantilevered Walls
Factor embedment by 1.2 for continuous wall elements
Do not factor embedment for discrete wall elements (conservatism of 3b assumption)
Example
Cantilevered sheet pile wall retaining a 10-ft deep cut in granular soils
Assume 36 ksi yield stress for sheet pile
Compare required embedment depth and structural section for ASD and LRFD
Load Factor of 1.5 used for EH (active)
Example Geometry
10' g = K a = g p = 125 pcf 0.33
1.5
L P p L p K p = j p = 3 1 A Factored P a = Factored P p = g p * 0.5 * (L+10) 2 * K a * g j p * 0.5 * L 2 * K p * g L a P a
Method ASD LRFD
Example Results
M max (k-ft) 15.4
29.2
Embedment (ft) 12.2
12.2
Section Modulus (in 3 /ft) 9.23 (S) (elastic) 10.83 (Z) (plastic) Since Z is about 1.15 to 1.20 times S, similar section would be acceptable
Anchored Walls
Strength Limit States
Bearing resistance of embedded portion of wall
Passive resistance of embedded portion of wall
Flexural resistance of wall/facing elements
Ground anchor pullout
Tensile resistance of anchor tendon
Service Limit States
Same as non-gravity cantilevered wall
Apparent Earth Pressure Diagrams
Based on FHWA-sponsored research
Builds upon well-known Terzaghi-Peck envelopes
Appropriate for walls built in competent ground where maximum wall height is critical design case
Same diagram shape for single or multi-leveled anchored walls
Recommended AEP for Sands
T h1 p T h1 T h2 T hn p R
p
=
TOTAL LOAD 2 3 H
K A
γ
H
(a) Walls with one level of ground anchors R
p
=
H TOTAL LOAD 1 3 H 1
1 3 H n
+
1
(b) Walls with multiple levels of ground anchors
LRFD Check on Tensile Breakage
Guaranteed Ultimate Tensile Strength (GUTS) GUTS
T n
Select tendon with: GUTS
Σ
γ
i Q i
Resistance Factors for Ground Anchors – Tensile Rupture Mild Steel High Strength Steel 0.90
0.80
Resistance factors are applied to maximum proof test load
For high strength steel, apply resistance factor to GUTS
Comparison to ASD – Tensile Rupture
ASD
0.8 GUTS > 1.33 Design Load (DL = EH + LS)
0.8 GUTS > 1.33 EH + 1.33 LS
LRFD
GUTS >
g
p EH + 1.75 LS
0.8 GUTS > 1.5 EH + 1.75 LS
Maximum proof test load must be at least equal to the factored load
Anchor Bond Length
L b(min)
=
T n Q a
L b = anchor bond length T n = factored anchor load Q a = nominal anchor pullout resistance
Nominal Anchor Pullout Resistance Q a
=
d
a
L b
Q a = nominal anchor pullout capacity d = anchor hole diameter
a = nominal anchor bond stress L b = anchor bond length
Preliminary Evaluation Only
Bond stress values in AASHTO should be used for FEASIBILITY evaluation
AASHTO values for cohesionless and cohesive soil and rock
Presumptive Nominal Bond Stress in Cohesionless Soils
Anchor/Soil Type (Grout Pressure) Soil Compactness or SPT Resistance Gravity Grouted Anchors (<50 psi) Sand or Sand-Gravel Mixtures Medium Dense to Dense 11-50 Pressure Grouted Anchors (50 to 400 psi) Fine to Medium Sand Medium to Coarse Sand w/Gravel Medium Dense to Dense 11-50 Medium Dense 11-30 Dense to Very Dense 30-50 Silty Sands Sandy Gravel Glacial Till ---- Medium Dense to Dense 11-40 Dense to Very Dense 40-50+ Dense 31-50 Presumptive Ultimate Bond Stress, n (ksf) 1.5 to 2.9
1.7 to 7.9
2.3 to 14 5.2 to 20 3.5 to 8.5
4.4 to 29 5.8 to 29 6.3 to 11
Resistance Factors – Anchor Pullout Cohesionless (Granular) Soils 0.65
(1) Cohesive Soils Rock 0.70
(1) 0.50
(1) Where Proof Tests Preformed 1.00
(2) 1) 2) Using presumptive values for preliminary design only Where proof tests conducted to at least 1.0 times the factored anchor load
Comparison to ASD – Anchor Pullout 1.1
1.05
1.0
0.95
0.9
Rock (FS = 3.0, = 0.50) Sand (FS = 2.5, = 0.65)
0.85
0.8
0 5 L b(min) (ASD)
=
EH LS
+
1
FS
Clay (FS = 2.5, = 0.70)
10
Dead Load / Live Load
15 20 L b(min) (LRFD)
=
1.5
EH LS
+
1.75
Final Anchor Design
Section 11.9.4.2 Anchor Pullout Capacity
“For final design, the contract documents shall require that verification tests or pullout tests on sacrificial anchors in each soil unit be conducted …”
Different than current ASD practice, but intent is not to require, in general, pullout testing
Bearing Resistance of Wall Element
Assume all vertical loads carried by portion of wall below excavation level Code refers designer to section on spread or deep foundations for analysis methods Resistance factors used are for static capacity evaluation of piles or shafts (i.e.,
0.3 to 0.5
FS ~ 3.0 to 4.5) = Resistance factors should be modified to correlate to FS = 2.0 to 2.5 for bearing resistance evaluation
MSE Walls
Strength Limit States
Same external stability checks as for conventional gravity walls Tensile resistance of reinforcement Pullout resistance of reinforcement Structural resistance of face elements and face element connection
Service Limits States
Same as for conventional gravity walls
MSE Walls – External Stability
MSE Walls – Internal Stability
Check pullout and tensile resistance at each reinforcement level and compare to maximum factored load, T max
Maximum Factored Load
Apply factored load to the reinforcements T max
=
σ H S v
s
H = factored horizontal soil stress at reinforcement (ksf)
S v = vertical spacing of reinforcement
AASHTO 11.10.6.2.1-2
Factored Horizontal Stresses
Factored Horizontal Stress σ H
g
P
= g
P
σ V k r
+
Δσ H
= load factor (=1.35 for EV)
k r
s
V = pressure coefficient = pressure due to resultant of gravity forces from soil self weight
Ds
H = horizontal stress
AASHTO 11.10.6.2.1-1
Reinforcement Tensile Resistance T max
T al R c
T al = Nominal long-term reinforcement design strength
= Resistance factor for tensile resistance
AASHTO 11.10.6.4.1-1
Resistance Factors for Tensile Resistance
Metallic Reinforcement • Strip Reinforcement Static loading • Combined static/earthquake loading Grid Reinforcement • • Static loading Combined static/earthquake loading 0.75
1.00
0.65
0.85
Geosynthetic Reinforcement • • Static loading Combined static/earthquake loading 0.90
1.20
ASD/LRFD Tensile Breakage
Example of Steel Strip Reinforcement ASD
T max = s h S v T max = ( s v k r + Ds h ) S v T al = (0.55 F y A c ) / b T al / T max = 0.55 / 1 = 0.55
LRFD
T max = g p s h S v T max = 1.35 ( s v k r + Ds h ) S v T al = ( F y A c ) / b with = 0.75 T al / T max = 0.75 / 1.35 = 0.55
Other Developments
LRFD for Soil Nails – NCHRP 24-21
Draft LRFD Design and Construction Specification for Micropiles
?
The End