aashto lrfd section 11 abutments, piers, and...
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AASHTO LRFD Section 11 AASHTO LRFD Section 11 Abutments, Piers, and WallsAbutments, Piers, and Walls
AASHTO Section 11AASHTO Section 11
Design specifications for:Conventional gravity/semigravity wallsNon-gravity cantilevered wallsAnchored wallsMechanically Stabilized Earth (MSE) wallsPrefabricated modular walls
Common Load Groups for WallsCommon Load Groups for Walls
GroupGroupγγDCDC γγEVEV γγEHEH
(Active)(Active)γγESES γγLSLS
Strength IStrength I 0.900.90 1.001.00 1.501.50 1.501.50 1.751.75
Strength IStrength I 1.251.25 1.351.35 1.501.50 1.501.50 1.751.75
Service IService I 1.001.00 1.001.00 1.001.00 1.001.00 1.001.00
Load DefinitionsLoad Definitions
DC DC –– dead load of structural components dead load of structural components and attachmentsand attachmentsEH EH –– horizontal earth pressure loadhorizontal earth pressure loadEV EV –– vertical pressure from dead load of vertical pressure from dead load of earth fillearth fillES ES –– earth surcharge loadearth surcharge loadLS LS –– live load surcharge (transient load)live load surcharge (transient load)
Surcharge LoadsSurcharge Loads
Earth surcharge Earth surcharge AASHTOAASHTO Section Section 3.11.6.1 and 3.11.6.23.11.6.1 and 3.11.6.2Live load surcharge Live load surcharge AASHTOAASHTO3.11.6.43.11.6.4
Conventional Retaining WallsConventional Retaining Walls
Strength Limit StatesStrength Limit StatesSlidingSlidingBearing resistanceBearing resistanceEccentricityEccentricity
Service Limit StatesService Limit StatesVertical settlementVertical settlementLateral wall movementLateral wall movementOverall stabilityOverall stability
External Failure MechanismsExternal Failure Mechanisms
Sliding FailureSliding Failure Overturning FailureOverturning Failure
Bearing FailureBearing FailureDeepDeep--Seated Seated Sliding FailureSliding Failure
1.25
DC
1.25
DC
ββ ββ
0.90
DC
0.90
DC
1.00
WA
1.00
WA VV
1.00
WA
1.00
WA VV
β+δβ+δ β+δβ+δ1.50 1.50
EHcosEHcos((β+δβ+δ))1.50 1.50
EHcosEHcos((β+δβ+δ))
1.50 EH1.50 EH 1.50 EH1.50 EH1.
35 E
V1.
35 E
V
1.00
EV
1.00
EV
1.50 1.50 EHsinEHsin((β+δβ+δ)) 1.50 1.50 EHsinEHsin((β+δβ+δ))
1.00 WA1.00 WAHH 1.00 WA1.00 WAHH
Load Factors for Load Factors for Bearing ResistanceBearing Resistance
Load Factors for Load Factors for Sliding and EccentricitySliding and Eccentricity
Load Factors for Conventional WallsLoad Factors for Conventional Walls
Conventional Walls Conventional Walls -- SummarySummary
Use resistance factors for spread footings Use resistance factors for spread footings or deep foundations, as appropriate or deep foundations, as appropriate (Section 10.5)(Section 10.5)
Eccentricity limited to:Eccentricity limited to:e/B < 0.25 for soil (compare to ASD 0.167)e/B < 0.25 for soil (compare to ASD 0.167)e/B < 0.375 for rock (compare to ASD 0.25)e/B < 0.375 for rock (compare to ASD 0.25)
NonNon--gravity Cantilevered Wallsgravity Cantilevered Walls
Strength Limit StatesStrength Limit StatesBearing resistance of embedded portion of wallBearing resistance of embedded portion of wallPassive resistance of embedded portion of wallPassive resistance of embedded portion of wallFlexural resistance of wall/facing elementsFlexural resistance of wall/facing elements
Service Limit StatesService Limit StatesVertical wall movementVertical wall movementLateral wall movementLateral wall movementOverall stabilityOverall stability
Resistance FactorsResistance Factors
Bearing ResistanceBearing ResistancePassive ResistancePassive ResistanceFlexural ResistanceFlexural Resistance
Section 10.5Section 10.51.001.000.900.90
Code allows increase in Resistance Factors Code allows increase in Resistance Factors for temporary walls but specific guidance is for temporary walls but specific guidance is not provided not provided
Pressure Diagrams Pressure Diagrams –– Discrete ElementsDiscrete Elements
ASDASD
LRFDLRFD
NonNon--gravity Cantilevered Wallsgravity Cantilevered Walls
Below excavation line, multiply by 3b Below excavation line, multiply by 3b on passive side of wall and 1b on active on passive side of wall and 1b on active side of wall for discrete elementsside of wall for discrete elements
Look at forces separately below Look at forces separately below excavation line on passive side and excavation line on passive side and active side (because different load active side (because different load factors)factors)
Factor embedment by 1.2 for Factor embedment by 1.2 for continuous wall elementscontinuous wall elements
Do not factor embedment for discrete Do not factor embedment for discrete wall elements (conservatism of 3b wall elements (conservatism of 3b assumption)assumption)
NonNon--gravity Cantilevered Wallsgravity Cantilevered Walls
ExampleExample
Cantilevered sheet pile wall retaining a Cantilevered sheet pile wall retaining a 1010--ft deep cut in granular soilsft deep cut in granular soilsAssume 36 Assume 36 ksiksi yield stress for sheet yield stress for sheet pilepileCompare required embedment depth Compare required embedment depth and structural section for ASD and and structural section for ASD and LRFDLRFDLoad Factor of 1.5 used for EH (active)Load Factor of 1.5 used for EH (active)
γ = 125 pcfKa = 0.33γp = 1.5
Kp = 3ϕp = 1
Factored Pa = γp * 0.5 * (L+10)2 * Ka * γFactored Pp = ϕp * 0.5 * L2 * Kp * γ
Pa
LpLa
L
A
10'
Pp
Example GeometryExample Geometry
Example ResultsExample ResultsMethodMethod MMmaxmax
(k(k--ft)ft)
EmbedmentEmbedment
(ft)(ft)
Section Section ModulusModulus
(in(in33/ft)/ft)
ASDASD 15.415.4 12.212.2 9.23 (S)9.23 (S)(elastic)(elastic)
LRFDLRFD 29.229.2 12.212.2 10.83 (Z)10.83 (Z)(plastic)(plastic)
Since Z is about 1.15 to 1.20 times S, similar section would be acceptable
Anchored WallsAnchored Walls
Strength Limit StatesStrength Limit StatesBearing resistance of embedded portion of wallBearing resistance of embedded portion of wallPassive resistance of embedded portion of wallPassive resistance of embedded portion of wallFlexural resistance of wall/facing elementsFlexural resistance of wall/facing elementsGround anchor pulloutGround anchor pulloutTensile resistance of anchor tendonTensile resistance of anchor tendon
Service Limit StatesService Limit StatesSame as nonSame as non--gravity cantilevered wallgravity cantilevered wall
Apparent Earth Pressure Apparent Earth Pressure DiagramsDiagrams
Based on FHWABased on FHWA--sponsored researchsponsored researchBuilds upon wellBuilds upon well--known known TerzaghiTerzaghi--Peck Peck envelopesenvelopesAppropriate for walls built in competent Appropriate for walls built in competent ground where maximum wall height is ground where maximum wall height is critical design casecritical design caseSame diagram shape for single or Same diagram shape for single or multimulti--leveled anchored walls leveled anchored walls
Recommended AEP for SandsRecommended AEP for SandsHH
HH11 HH11
HHn+
1n+
1
pp pp
22 //33
HH11
22 //33
HH11
22 //33
HHn+
1n+
1
22 //33
(H(H-- HH
11))11 //
33HH
TTh1h1
TTh1h1
TTh2h2
TThnhn
HH22
HHnn
RR RR
(a) Walls with one level(a) Walls with one levelof ground anchorsof ground anchors
(b) Walls with multiple(b) Walls with multiplelevels of ground anchorslevels of ground anchors
HKHLOADTOTALp A32
γ≈=1n3
113
1 HH-HLOADTOTALp
+−=
Guaranteed Ultimate Tensile Guaranteed Ultimate Tensile Strength (GUTS) Strength (GUTS)
Select tendon with:Select tendon with:
φ≥ nTGUTS
φ> iiQΣGUTS γ
LRFD Check on Tensile BreakageLRFD Check on Tensile Breakage
Resistance Factors for Ground Resistance Factors for Ground Anchors Anchors –– Tensile RuptureTensile Rupture
Resistance factors are applied to Resistance factors are applied to maximum proof test loadmaximum proof test loadFor high strength steel, apply For high strength steel, apply resistance factor to GUTSresistance factor to GUTS
Mild SteelMild Steel 0.900.90High Strength SteelHigh Strength Steel 0.800.80
Comparison to ASD Comparison to ASD ––Tensile RuptureTensile Rupture
ASDASD0.8 GUTS > 1.33 Design Load 0.8 GUTS > 1.33 Design Load (DL = EH + LS)(DL = EH + LS)0.8 GUTS > 1.33 EH + 1.33 LS0.8 GUTS > 1.33 EH + 1.33 LS
LRFDLRFDφφ GUTS > GUTS > γγpp EH + 1.75 LSEH + 1.75 LS0.8 GUTS > 1.5 EH + 1.75 LS0.8 GUTS > 1.5 EH + 1.75 LS
Maximum proof test load must be at least Maximum proof test load must be at least equal to the factored loadequal to the factored load
Anchor Bond LengthAnchor Bond Length
a
nb(min) Q
TL×φ
=
LLbb = anchor bond length = anchor bond length TTnn = factored anchor load= factored anchor loadQQaa = nominal anchor pullout resistance= nominal anchor pullout resistance
Nominal Anchor Pullout Nominal Anchor Pullout ResistanceResistance
baa LdQ ×τ××π=
QQaa = nominal anchor pullout capacity= nominal anchor pullout capacityd = anchor hole diameterd = anchor hole diameterττaa = nominal anchor bond stress= nominal anchor bond stressLLbb = anchor bond length= anchor bond length
Preliminary Evaluation Only Preliminary Evaluation Only
Bond stress values in AASHTO Bond stress values in AASHTO should be used for FEASIBILITY should be used for FEASIBILITY evaluationevaluation
AASHTO values for cohesionless AASHTO values for cohesionless and cohesive soil and rockand cohesive soil and rock
Anchor/Soil TypeAnchor/Soil Type(Grout Pressure)(Grout Pressure)
Soil Compactness or SPT Soil Compactness or SPT ResistanceResistance
Presumptive Presumptive Ultimate Bond Ultimate Bond Stress, Stress, ττnn ((ksfksf))
Gravity Grouted AnchorsGravity Grouted Anchors(<50 (<50 psipsi))Sand or SandSand or Sand--Gravel MixturesGravel Mixtures Medium Dense to Dense 11Medium Dense to Dense 11--5050 1.5 to 2.91.5 to 2.9
Pressure Grouted AnchorsPressure Grouted Anchors(50 to 400 (50 to 400 psipsi))Fine to Medium SandFine to Medium SandMedium to Coarse Sand w/GravelMedium to Coarse Sand w/Gravel
SiltySilty SandsSands
Sandy GravelSandy Gravel
Glacial TillGlacial Till
Medium Dense to Dense 11Medium Dense to Dense 11--5050Medium Dense 11Medium Dense 11--3030Dense to Very Dense 30Dense to Very Dense 30--5050
----------
Medium Dense to Dense 11Medium Dense to Dense 11--4040Dense to Very Dense 40Dense to Very Dense 40--50+50+Dense 31Dense 31--5050
1.7 to 7.91.7 to 7.92.3 to 142.3 to 145.2 to 205.2 to 20
3.5 to 8.53.5 to 8.5
4.4 to 29 4.4 to 29 5.8 to 295.8 to 296.3 to 116.3 to 11
Presumptive Nominal Bond Stress Presumptive Nominal Bond Stress in Cohesionless Soilsin Cohesionless Soils
Resistance Factors Resistance Factors ––Anchor PulloutAnchor Pullout
1)1) Using presumptive values for preliminary Using presumptive values for preliminary design onlydesign only
2)2) Where proof tests conducted to at least 1.0 Where proof tests conducted to at least 1.0 times the factored anchor loadtimes the factored anchor load
CohesionlessCohesionless (Granular) (Granular) SoilsSoils 0.650.65(1)(1)
Cohesive SoilsCohesive Soils 0.700.70(1)(1)
RockRock 0.500.50(1)(1)
Where Proof Tests Where Proof Tests PreformedPreformed 1.001.00(2)(2)
Final Anchor DesignFinal Anchor Design
Section 11.9.4.2 Anchor Pullout Section 11.9.4.2 Anchor Pullout CapacityCapacity
““For final design, the contract documents For final design, the contract documents shall require that verification tests or shall require that verification tests or pullout tests on sacrificial anchors in each pullout tests on sacrificial anchors in each soil unit be conducted soil unit be conducted …”…”Different than current ASD practice, but Different than current ASD practice, but intent is not to require, in general, pullout intent is not to require, in general, pullout testingtesting
Bearing Resistance of Wall Bearing Resistance of Wall ElementElement
Assume all vertical loads carried by portion Assume all vertical loads carried by portion of wall below excavation levelof wall below excavation levelCode refers designer to section on spread or Code refers designer to section on spread or deep foundations for analysis methodsdeep foundations for analysis methodsResistance factors used are for static Resistance factors used are for static capacity evaluation of piles or shafts (i.e., capacity evaluation of piles or shafts (i.e., φφ = = 0.3 to 0.5 0.3 to 0.5 ≅≅ FS ~ 3.0 to 4.5)FS ~ 3.0 to 4.5)Resistance factors should be modified to Resistance factors should be modified to correlate to FS = 2.0 to 2.5 for bearing correlate to FS = 2.0 to 2.5 for bearing resistance evaluation resistance evaluation
MSE WallsMSE Walls
Strength Limit StatesStrength Limit StatesSame external stability checks as for Same external stability checks as for conventional gravity wallsconventional gravity wallsTensile resistance of reinforcementTensile resistance of reinforcementPullout resistance of reinforcementPullout resistance of reinforcementStructural resistance of face elements and face Structural resistance of face elements and face element connectionelement connection
Service Limits StatesService Limits StatesSame as for conventional gravity wallsSame as for conventional gravity walls
MSE Walls MSE Walls –– External StabilityExternal Stability
MSE Walls MSE Walls –– Internal StabilityInternal Stability
Check pullout and tensile Check pullout and tensile resistance at each reinforcement resistance at each reinforcement level and compare to maximum level and compare to maximum factored load, factored load, TTmaxmax
Apply factored load to the Apply factored load to the reinforcements reinforcements
σσHH = factored horizontal soil stress = factored horizontal soil stress at reinforcement (at reinforcement (ksfksf))SSvv = vertical spacing of = vertical spacing of reinforcement reinforcement
vHmax SσT =
AASHTO 11.10.6.2.1-2
Maximum Factored LoadMaximum Factored Load
Factored Horizontal StressesFactored Horizontal Stresses
Factored Horizontal StressFactored Horizontal Stress
γγPP = load factor (=1.35 for EV)= load factor (=1.35 for EV)kkrr = pressure coefficient= pressure coefficientσσVV = pressure due to resultant of = pressure due to resultant of gravity forces from soil self weight gravity forces from soil self weight ΔσΔσHH = horizontal stress= horizontal stress
( )HrVPH Δσkσσ +γ=
AASHTO 11.10.6.2.1-1
TTalal = Nominal long= Nominal long--term term reinforcement design strengthreinforcement design strengthφφ = Resistance factor for tensile = Resistance factor for tensile resistanceresistance
calmax RTT φ≤
AASHTO 11.10.6.4.1-1
Reinforcement Tensile Reinforcement Tensile ResistanceResistance
Resistance Factors for Tensile Resistance Factors for Tensile ResistanceResistance
Metallic Metallic ReinforcementReinforcement
Strip ReinforcementStrip Reinforcement•• Static loadingStatic loading•• Combined static/earthquake loadingCombined static/earthquake loadingGrid ReinforcementGrid Reinforcement•• Static loadingStatic loading•• Combined static/earthquake loadingCombined static/earthquake loading
0.750.751.001.00
0.650.650.850.85
GeosyntheticGeosyntheticReinforcementReinforcement
•• Static loadingStatic loading•• Combined static/earthquake Combined static/earthquake
loadingloading
0.900.901.201.20
ASD/LRFD Tensile BreakageASD/LRFD Tensile Breakage
Example of Steel Strip ReinforcementExample of Steel Strip ReinforcementASD LRFD
Tmax = σhSv
Tmax = (σvkr + Δσh) Sv
Tal = (0.55 Fy Ac) / b Tal / Tmax = 0.55 / 1 = 0.55
Tmax = γpσhSv
Tmax = 1.35 (σvkr + Δσh) Sv
φTal = (φ Fy Ac) / b with φ = 0.75 Tal / Tmax = 0.75 / 1.35 = 0.55
Other DevelopmentsOther Developments
LRFD for Soil Nails LRFD for Soil Nails –– NCHRP 24NCHRP 24--2121
? TheEnd