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

γγγγDC γγγγEV γγγγEH

(Active)

γγγγES γγγγLS

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 Overturning Failure

Bearing Failure Deep-Seated Sliding Failure

1.25 D

C

ββββ ββββ

0.90 D

C

1.00 W

AV

1.00 W

AV

β+δβ+δβ+δβ+δ β+δβ+δβ+δβ+δ 1.50

EHcos(β+δ) 1.50

EHcos(β+δ)

1.50 EH 1.50 EH 1.35 EV

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)

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)

� 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

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)

γ = 125 pcf

Ka = 0.33

γp = 1.5

Kp = 3

ϕp = 1

Factored Pa = γp * 0.5 * (L+10)2 * Ka * γ

Factored Pp = ϕp * 0.5 * L2 * Kp * γ

Pa

LpLa

L

A

10'

Pp

Example Geometry

Example Results

Method Mmax

(k-ft)

Embedment

(ft)

Section

Modulus

(in3/ft)

ASD 15.4 12.2 9.23 (S)

(elastic)

LRFD 29.2 12.2 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 H

H1

H1

Hn+1

p p

2/ 3 H

1

2/ 3 H

1

2/ 3 H

n+1

2/ 3 (H-H

1)

1/ 3 H

Th1

Th1

Th2

Thn

H2

Hn

R R

(a) Walls with one level of ground anchors

(b) Walls with multiple levels of ground anchors

HKH

LOADTOTALp A

32

γ≈=1n3

113

1 HH-H

LOADTOTALp

+−=

� Guaranteed Ultimate Tensile

Strength (GUTS)

� Select tendon with:

φ≥ nT

GUTS

φ> iiQΣ

GUTSγ

LRFD Check on Tensile Breakage

Resistance Factors for Ground

Anchors – Tensile Rupture

� Resistance factors are applied to

maximum proof test load

� For high strength steel, apply

resistance factor to GUTS

Mild Steel 0.90

High Strength Steel 0.80

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 � φφφφ GUTS > γγγγp 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

a

nb(min)

Q

TL

×φ=

� Lb = anchor bond length

� Tn = factored anchor load

� Qa = nominal anchor pullout resistance

Nominal Anchor Pullout

Resistance

baa LdQ ×τ××π=

� Qa = nominal anchor pullout capacity

� d = anchor hole diameter

� ττττa = nominal anchor bond stress

� Lb = 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

Anchor/Soil Type

(Grout Pressure)

Soil Compactness or SPT

Resistance

Presumptive

Ultimate Bond

Stress, τn (ksf)

Gravity Grouted Anchors

(<50 psi)

Sand or Sand-Gravel Mixtures

Medium Dense to Dense 11-50

1.5 to 2.9

Pressure Grouted Anchors

(50 to 400 psi)

Fine to Medium Sand

Medium to Coarse Sand w/Gravel

Silty Sands

Sandy Gravel

Glacial Till

Medium Dense to Dense 11-50

Medium Dense 11-30

Dense to Very Dense 30-50

-----

Medium Dense to Dense 11-40

Dense to Very Dense 40-50+

Dense 31-50

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

Presumptive Nominal Bond Stress

in Cohesionless Soils

Resistance Factors –

Anchor Pullout

1) Using presumptive values for preliminary design only

2) Where proof tests conducted to at least 1.0 times the factored anchor load

Cohesionless (Granular)

Soils 0.65(1)

Cohesive Soils 0.70(1)

Rock 0.50(1)

Where Proof Tests Preformed 1.00(2)

FS1LS

EH(ASD)Lb(min)

+=φ

+

=1.75

LS

EH1.5

(LRFD)Lb(min)

LRFD/A

SD

1.05

1.1

1.0

0.95

0.9

0.85

0.8 0 5 10 15 20

Dead Load / Live Load

Rock (FS = 3.0, φ = 0.50)

Sand (FS = 2.5, φ = 0.65)

Clay (FS = 2.5, φ = 0.70)

Comparison to ASD –

Anchor Pullout

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., φφφφ = 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, Tmax

� Apply factored load to the

reinforcements

� σσσσH = factored horizontal soil stress

at reinforcement (ksf)

� Sv = vertical spacing of

reinforcement

vHmax SσT =

AASHTO 11.10.6.2.1-2

Maximum Factored Load

Factored Horizontal Stresses

� Factored Horizontal Stress

� γγγγP = load factor (=1.35 for EV)

� kr = pressure coefficient

� σσσσV = pressure due to resultant of

gravity forces from soil self weight

� ∆σ∆σ∆σ∆σH = horizontal stress

( )HrVPH ∆σkσσ +γ=

AASHTO 11.10.6.2.1-1

� Tal = Nominal long-term

reinforcement design strength

� φφφφ = Resistance factor for tensile

resistance

calmax RTT φ≤

AASHTO 11.10.6.4.1-1

Reinforcement Tensile

Resistance

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 LRFD

Tmax = σhSv

Tmax = (σvkr + ∆σh) Sv

Tal = (0.55 Fy Ac) / b Tal / Tmax = 0.55 / 1 = 0.55

Tmax = γpσhSv

Tmax = 1.35 (σvkr + ∆σh) Sv

φTal = (φ Fy Ac) / b with φ = 0.75

Tal / Tmax = 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