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AASHTO’s LRFD Specifications for AASHTO’s LRFD Specifications for Foundation and Earth Retaining Foundation and Earth Retaining
Structure DesignStructure Design
(Through 2006 Interims and Beyond)(Through 2006 Interims and Beyond)
Jerry A. DiMaggio, P.E.Jerry A. DiMaggio, P.E.
Principal Bridge/Geotechnical EngineerPrincipal Bridge/Geotechnical Engineer
FHWA, Washington D. C. FHWA, Washington D. C.
Existing SpecificationsExisting Specifications
StandardStandard
1717thth Edition Edition
LRFDLRFD
33rdrd Edition Edition
““AASHTO and FHWA AASHTO and FHWA have agreed that all have agreed that all state DOT’s will use state DOT’s will use LRFD for design of LRFD for design of NEW structures by NEW structures by
2007.”2007.”
0-2-2NE60%
MO
TN NC
VA
WV80%
PA100%
NY 50%
ME100%
IA 5%
IL5%
KY
FL100%
GATX
13%
OK 100%
KS50%
OH
WA100%
OR100%
WI
CA
CO 90%
SC50%
NJ
MA
CT
DEMD
VT 5%
MN 40%
MI
INUT
75%
ND
SD 10%
ID 100% WY
NMAZ
NV
MT 35%
AR 5%
LA
MS AL
NH
RI
0- 24 -10
0-2-2NE
MO
TN NC
VAWV
PA
NY
ME
IA
IL
KY
FL
GATX
OK
KSOH
WA
OR WI
CA
AASHTO LRFD SurveyAASHTO LRFD SurveyMay 2005May 2005
CO
SC
VT
MN
MI
INUT
ND
SD ID WY
NMAZ
NV
MT
AR
LAMS AL
AK 95%AK
HI
PRFull Implementation50-90% Partial Implementation
1-10% Partial ImplementationNo Implementation
26-50% Partial Implementation11-25% Partial Implementation
Superstructure: LRFD
Substructure: LRFD/ASD
Foundations: ASD
Earthwork and walls: ASD
Reasons for Not AdoptingReasons for Not Adopting
• Human nature.Human nature.• No perceived benefits.No perceived benefits.• Unfamiliarity with LRFD methods.Unfamiliarity with LRFD methods.• Lack of confidence in the computed Lack of confidence in the computed
results. results. • Perceived errors and inconsistencies. Perceived errors and inconsistencies. • A specification that did not reflect A specification that did not reflect
current design practices.current design practices.
What is FHWA doing?What is FHWA doing?
• Bridge Design examples.Bridge Design examples.• NHI LRFD Training Courses.NHI LRFD Training Courses.• FHWA Technical Assistance.FHWA Technical Assistance.• FHWA/ NCHRP Calibration efforts.FHWA/ NCHRP Calibration efforts.• AASHTO Section 11 and 10 AASHTO Section 11 and 10
Revisions. Revisions.
Bridge Design ExamplesBridge Design Examples
http://www.fhwa.dot.gov/bridge/lrfd/examples.htmhttp://www.fhwa.dot.gov/bridge/lrfd/examples.htm
ConcreteConcrete SteelSteel
NHI LRFD Training CoursesNHI LRFD Training Courses
Course 130082ACourse 130082A
LRFD for Highway LRFD for Highway Bridge Substructures Bridge Substructures and Earth Retaining and Earth Retaining
Structures Structures
FHWA/ NCHRP ActivitiesFHWA/ NCHRP Activities
• NCHRP Project 12-66, Specifications for Serviceability in the Design of Bridge Foundations
• NCHRP Report 507, Load and Resistance Factor Design (LRFD) for Deep Foundations
FHWA/ NCHRP ActivitiesFHWA/ NCHRP Activities
• Publication No. FHWA-NHI-05-052, Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation Strength Limit State Design
Revisions to Section 10Revisions to Section 10• Compiled by a Technical Expert PanelCompiled by a Technical Expert Panel• Review and input from A Technical Review and input from A Technical
Working Group (TWG)Working Group (TWG)• Accepted by AASHTO Subcommittee Accepted by AASHTO Subcommittee
T-15 in June 2005 in Newport, Rhode T-15 in June 2005 in Newport, Rhode IslandIsland
• To be published in 2006 InterimTo be published in 2006 Interim
http://bridges.transportation.org/?siteid=34&c=downloads
•Attachments to Agenda Item 39 Section 3 revisions
•Attachments to Agenda item 40 Section 10 revisions
Topics IncludedTopics Included• Subsurface Subsurface
investigationsinvestigations• Soil and rock Soil and rock
propertiesproperties• Shallow foundationsShallow foundations• Driven pilesDriven piles• Drilled shaftsDrilled shafts• Rigid and flexible Rigid and flexible
culvertsculverts• AbutmentsAbutments• Walls (All types)Walls (All types)
• Integral Integral abutmentsabutments
• MicropilesMicropiles• Augercast pilesAugercast piles• Soil nailsSoil nails• Reinforced slopesReinforced slopes• All soil and rock All soil and rock
earthwork earthwork features. features.
Topics NOT IncludedTopics NOT Included
Section 10 ContentsSection 10 Contents10.1 SCOPE 10.1 SCOPE
10.2 DEFINITIONS10.2 DEFINITIONS
10.3 NOTATION10.3 NOTATION
10.4 SOIL AND ROCK PROPERTIES10.4 SOIL AND ROCK PROPERTIES
10.5 LIMIT STATES AND RESISTANCE FACTORS10.5 LIMIT STATES AND RESISTANCE FACTORS
10.6 SPREAD FOOTINGS10.6 SPREAD FOOTINGS
10.7 DRIVEN PILES10.7 DRIVEN PILES
10.8 DRILLED SHAFTS10.8 DRILLED SHAFTS
PROPERTY INFOPROPERTY INFO
NO SIGNIFICANT CHANGENO SIGNIFICANT CHANGE
UPDATEDUPDATED
UPDATED, CONSISTANTUPDATED, CONSISTANT
REORGANIZED,REORGANIZED,
NEW CONTENTNEW CONTENTNEW CONTENTNEW CONTENT
Section 10.4 Soil and Rock PropertiesSection 10.4 Soil and Rock Properties
GEC 5GEC 5
Sabatini, 2002Sabatini, 2002
Subsurface Subsurface InvestigationsInvestigations
Mayne, 2002Mayne, 2002
Section 10.4 Soil and Rock PropertiesSection 10.4 Soil and Rock Properties
• Soil Strength Soil Strength • Soil Deformation Soil Deformation • Rock Mass Strength Rock Mass Strength • Rock Mass Deformation Rock Mass Deformation • Erodibility of rockErodibility of rock
10.4.6 SELECTION OF DESIGN PROPERTIES10.4.6 SELECTION OF DESIGN PROPERTIES
NEW!
NEW!
Section 10.5 Limit States and Section 10.5 Limit States and Resistance FactorsResistance Factors
• Resistance factors revised Resistance factors revised • Additional discussion on the basis for Additional discussion on the basis for
resistance factorsresistance factors• Additional discussion of extreme event Additional discussion of extreme event
considerationsconsiderations
Articles 3.4.1 and 3.11.8Articles 3.4.1 and 3.11.8
MaximumMaximum MinimumMinimum
Piles, Piles, -method-method 1.41.4 0.250.25
Piles, Piles, -method-method 1.051.05 0.300.30
Drilled Shafts, O’neill Drilled Shafts, O’neill and Reese (1999)and Reese (1999)
1.251.25 0.350.35
DowndragDowndrag• Methods for computingMethods for computing• Load FactorsLoad Factors• Use of minimum load factors clarifiedUse of minimum load factors clarified
Section 10.6 Spread FootingsSection 10.6 Spread FootingsEccentricity provisions clarifiedEccentricity provisions clarified
BB′′ = B – 2e = B – 2eBB
LL′′ = L – 2e = L – 2eLL
Q = P/(B’ L’)Q = P/(B’ L’)ML MB
P
q
Applies to Applies to geotechnical design geotechnical design for settlement and for settlement and bearing resistancebearing resistance
Section 10.6 Spread FootingsSection 10.6 Spread Footings
Hough methodHough method
Elastic Settlement of Elastic Settlement of cohesionless soilscohesionless soils
vo
vvoc σ'
Δσσ'log
C'
1HΔH
Section 10.6 Spread FootingsSection 10.6 Spread Footings
qqnn = c N = c Ncmcm + + D Df f NNqm qm CCwqwq + 0.5 + 0.5 B NB Nm m CCw w
NOMINAL RESISTANCENOMINAL RESISTANCE
NNc c ssc c iicc NNq q ssq q ddq q iiqq N N s s i i
Shape Correction FactorsShape Correction Factors
COHESIONCOHESIONUNIT WEIGHTUNIT WEIGHT
DEPTHDEPTH WIDTHWIDTH
Bearing Capacity FactorsBearing Capacity FactorsInclination FactorsInclination FactorsShear through overburden Shear through overburden correction factorcorrection factor
Water table correctionWater table correction
Settlement correction factors removedSettlement correction factors removed
Section 10.7 Driven PilesSection 10.7 Driven PilesSettlement of pile groupsSettlement of pile groups
4 new diagrams4 new diagrams
From:From:
Hannigan (2005)Hannigan (2005)
Section 10.7 Driven PilesSection 10.7 Driven Piles
HHtt
QQtt
MMtt
PP
yy
The P-y method specified The P-y method specified for horizontal deflectionfor horizontal deflection
Section 10.7 Driven PilesSection 10.7 Driven Piles
Original c
urve
Original c
urvePP
yyModified curve
Modified curve
PPmm * P * P
PP
Spacing (S)Spacing (S) Row 1Row 1 Row 2Row 2 Row 3Row 3
3D3D 0.70.7 0.50.5 0.350.35
5D5D 1.01.0 0.850.85 0.70.7
P-multiplier (PP-multiplier (Pmm))DD
SS
Section 10.7 Driven PilesSection 10.7 Driven PilesField determination of nominal resistanceField determination of nominal resistance
Static load testStatic load test Dynamic load testDynamic load test
Section 10.7 Driven PilesSection 10.7 Driven Piles
Static analysis methodsStatic analysis methods
• Nordlund – Nordlund – Thurman method Thurman method addedadded
Section 10.7 Driven PilesSection 10.7 Driven Piles
Static analysis methodsStatic analysis methods
• Primary use is for pile length estimation Primary use is for pile length estimation for contract drawingsfor contract drawings
• Secondary use for estimation of downdrag, Secondary use for estimation of downdrag, uplift resistance and scour effectsuplift resistance and scour effects
• Should rarely be used as sole means of Should rarely be used as sole means of determining pile resistancedetermining pile resistance
Section 10.7 Driven PilesSection 10.7 Driven PilesComp Str
ksi
30
20
10
Ult Cap
200
400
600
800
kips
0 160 320 480 Blows/ft
4.0
8.0
12.0
16.0ft
Stroke
Tens Str
ksi
Requirements for Requirements for driveability analysis driveability analysis have been added and have been added and clarifiedclarified
Section 10.7 Driven PilesSection 10.7 Driven Piles
NEW!
10.7.3.2 10.7.3.2 PILE LENGTH ESTIMATES FOR PILE LENGTH ESTIMATES FOR CONTRACT DOCUMENTSCONTRACT DOCUMENTS
10.7.6 10.7.6 Determination of minimum pile Determination of minimum pile penetrationpenetration
NEW!
Section 10.8 Drilled shaftsSection 10.8 Drilled shaftsRefers to driven piles section where possibleRefers to driven piles section where possible• DowndragDowndrag• Group settlementGroup settlement• Horizontal displacement (single and group)Horizontal displacement (single and group)• Lateral squeezeLateral squeeze• Water table and buoyancyWater table and buoyancy• ScourScour• Group resistance (cohesive soil only)Group resistance (cohesive soil only)• Uplift (group and load test sections)Uplift (group and load test sections)• BucklingBuckling• Extreme event limit stateExtreme event limit state
Section 10.8 Drilled shaftsSection 10.8 Drilled shafts
• Static analysis methods Static analysis methods for soil and rock have for soil and rock have been updatedbeen updated
• Consideration of both Consideration of both base and side resistance base and side resistance in rock is now includedin rock is now included
O’Neill and Reese (1999)O’Neill and Reese (1999)
Section 10.8 Drilled shaftsSection 10.8 Drilled shafts
A + BA + B
QQPP
QQSS
DisplacementDisplacement
Res
ista
nce
Res
ista
nce
Side ResistanceSide Resistance
Tip ResistanceTip Resistance
Total ResistanceTotal Resistance
BBCCDD
AA
A + DA + D
B + CB + C
ConclusionConclusion
Future EnhancementsFuture EnhancementsOverall stabilityOverall stability• Weight is both a load and a resistanceWeight is both a load and a resistance• Service limit state (should be strength Service limit state (should be strength
limit state)limit state) +
WWTTWWTT
WWTT WWTT
NNNN
TTTT
TT TTll ll
ccll
ccllN tan N tan
N tan N tan
Future EnhancementsFuture Enhancements
Inclination FactorsInclination Factors• Ignored by many practicing engineersIgnored by many practicing engineers• Based on small scale tests and theoryBased on small scale tests and theory• Effect of embedment (DEffect of embedment (Dff))
• Resistance factors are for vertical loadResistance factors are for vertical load
Q
Df
Future EnhancementsFuture Enhancements
Nominal bearing resistance of rockNominal bearing resistance of rock• Very little guidance availableVery little guidance available• CSIR Rock Mass Rating System proposedCSIR Rock Mass Rating System proposed• CSIR developed for tunnel designCSIR developed for tunnel design• Includes life safety considerations and Includes life safety considerations and
therefore, margin of safetytherefore, margin of safety• May be conservativeMay be conservative
Future EnhancementsFuture Enhancements
Pile head fixityPile head fixity• Connection detailsConnection details• Effects of axial loadsEffects of axial loads
H H
V
Future EnhancementsFuture Enhancements
Serviceability limitsServiceability limits
NCHRP 12-66 NCHRP 12-66
Due April 2006Due April 2006
xx
zz
What Should I Know and Do?What Should I Know and Do?
• Become familiar with BOTH Become familiar with BOTH the AASHTO standard the AASHTO standard specifications and LRFD specifications and LRFD specs.specs.
• Develop an understanding Develop an understanding of your agency’s current of your agency’s current design practice design practice
What Should I Know and Do?What Should I Know and Do?
• Develop and compare results Develop and compare results for SEVERAL example for SEVERAL example problems with LRFD and problems with LRFD and YOUR standard design YOUR standard design practicepractice
• Translate your current Translate your current practice to an LRFD formatpractice to an LRFD format
What Should I Know and Do?What Should I Know and Do?
• Communicate your findings to Communicate your findings to AASHTO’s SubCommitteee AASHTO’s SubCommitteee membersmembers
AASHTO Section 11AASHTO Section 11
• Design specifications for:• Conventional gravity/semigravity
walls• Non-gravity cantilevered walls• Anchored walls• Mechanically Stabilized Earth (MSE)
walls• Prefabricated modular walls
LRFD Specifications for LRFD Specifications for Foundation/ Earth Retaining Foundation/ Earth Retaining
Structure DesignStructure Design
Questions? Questions?