AASHTO AASHTO ’ ’ s LRFD Specifications for s LRFD Specifications for Foundation and Earth Retaining Foundation and Earth Retaining Structure Design Structure Design (Through 2007 Interims and Beyond) (Through 2007 Interims and Beyond) Jerry A. DiMaggio, P.E. Jerry A. DiMaggio, P.E. Principal Bridge/Geotechnical Engineer Principal Bridge/Geotechnical Engineer FHWA, Washington D. C. FHWA, Washington D. C.
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AASHTOAASHTO’’s LRFD Specifications for s LRFD Specifications for Foundation and Earth Retaining Foundation and Earth Retaining
Structure DesignStructure Design
(Through 2007 Interims and Beyond)(Through 2007 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
StandardStandard1717thth EditionEdition
LRFD LRFD now thenow the
““44””thth EditionEdition
““AASHTO and FHWA AASHTO and FHWA have agreed that all have agreed that all state DOTstate DOT’’s will use s will use LRFD for design of LRFD for design of NEW structures by NEW structures by
•• 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 AASHTO/FHWA doing?What is AASHTO/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. •• AASHTO TAASHTO T--15 Continues to refine 15 Continues to refine
and develop the spec. details.and develop the spec. details.
for Serviceability in the Design of Bridge Foundations
• NCHRP Report 507, Load and Resistance Factor Design (LRFD) for Deep Foundations
• NCHRP 20-7 activities.
FHWA/ NCHRP ActivitiesFHWA/ NCHRP Activities
• Publication No. FHWA-NHI-05-052, Development of Geotechnical Resistance Factors and DowndragLoad 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
TT--15 in June 2005 in Newport, Rhode 15 in June 2005 in Newport, Rhode IslandIsland
•• First published in 2006 InterimFirst published in 2006 Interimhttp://bridges.transportation.org
investigationsinvestigations•• Soil and rock propertiesSoil and rock properties•• Shallow foundationsShallow foundations•• Driven pilesDriven piles•• Drilled shaftsDrilled shafts•• Rigid and flexible Rigid and flexible
•• Integral abutmentsIntegral abutments•• Micropiles (added 2007)Micropiles (added 2007)•• Augercast pilesAugercast piles•• Soil nailsSoil nails•• Reinforced slopesReinforced slopes•• All soil and rock All soil and rock
earthwork features. earthwork features. ==========================================•• There are also AASHTO There are also AASHTO
LRFD CONSTRUCTION LRFD CONSTRUCTION SPECS. (driven, drilled SPECS. (driven, drilled and and micropilesmicropiles).).
Topics NOT IncludedTopics NOT Included
Section 10 ContentsSection 10 Contents10.1 SCOPE 10.1 SCOPE 10.2 DEFINITIONS10.2 DEFINITIONS10.3 NOTATION10.3 NOTATION10.4 SOIL AND ROCK PROPERTIES10.4 SOIL AND ROCK PROPERTIES10.5 LIMIT STATES AND RESISTANCE FACTORS10.5 LIMIT STATES AND RESISTANCE FACTORS10.6 SPREAD FOOTINGS10.6 SPREAD FOOTINGS10.7 DRIVEN PILES10.7 DRIVEN PILES10.8 DRILLED SHAFTS10.8 DRILLED SHAFTS
PROPERTY INFOPROPERTY INFO
NO SIGNIFICANT CHANGENO SIGNIFICANT CHANGEUPDATEDUPDATEDUPDATED, CONSISTANTUPDATED, CONSISTANT
REORGANIZED,REORGANIZED,NEW CONTENTNEW CONTENT
NEW CONTENTNEW CONTENT
Section 10.4 Soil and Rock PropertiesSection 10.4 Soil and Rock Properties
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
DowndragDowndrag•• Methods for computingMethods for computing•• Load FactorsLoad Factors•• Use of minimum load factors clarifiedUse of minimum load factors clarified
The PThe P--y method specified y method specified for horizontal deflection for horizontal deflection and wedge strain for and wedge strain for ““shortshort”” pilespiles
•• Primary use is for pile length estimation Primary use is for pile length estimation for contract drawings and feasibility.for contract drawings and feasibility.
•• Secondary use for estimation of Secondary use for estimation of downdragdowndrag, , 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 resistance. ONLY IN determining pile resistance. ONLY IN SPECIAL SITUATIONS!SPECIAL SITUATIONS!
NEW!10.7.3.2 10.7.3.2 PILE LENGTH ESTIMATES FOR PILE LENGTH ESTIMATES FOR
CONTRACT DOCUMENTSCONTRACT DOCUMENTS10.7.6 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
for soil and rock have for soil and rock have been updatedbeen updated
•• Consideration of both Consideration of both base and side base and side resistance in rock is resistance in rock is now includednow included
OO’’Neill and Reese (1999)Neill and Reese (1999)Currently being updated by Currently being updated by
FHWA.FHWA.
Section 10.8 Drilled shaftsSection 10.8 Drilled shaftsA + 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
NN NNTT
TT
TT TTll ll
ccll ccllN tan N tan φφ
N tan N tan φφ
Future EnhancementsFuture EnhancementsInclination 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 (Effect of embedment (DDff))•• 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 conservative depending on what May be conservative depending on what
your existing practice has been.your existing practice has been.
Future EnhancementsFuture Enhancements
Pile head fixityPile head fixity•• Connection detailsConnection details•• Effects of axial loadsEffects of axial loads
H HV
Future EnhancementsFuture Enhancements
Serviceability limitsServiceability limitsNCHRP 12NCHRP 12--66 66 Due April 2006 very late. Due April 2006 very late.
Nearing completion June Nearing completion June 2007.2007.
ΔΔxx
ΔΔzz
What Should I Know and Do?What Should I Know and Do?
•• Become familiar with BOTH the Become familiar with BOTH the AASHTO standard specifications AASHTO standard specifications and LRFD specs.and LRFD specs.
•• Develop an understanding of your Develop an understanding of your agencyagency’’s current design practice s current design practice
What Should I Know and Do?What Should I Know and Do?
•• Develop and compare results for Develop and compare results for SEVERAL example problems with SEVERAL example problems with LRFD and YOUR standard design LRFD and YOUR standard design practicepractice
•• Translate your current practice to an Translate your current practice to an LRFD formatLRFD format
What Should I Know and Do?What Should I Know and Do?•• Whenever possible perform some less Whenever possible perform some less
of comparative designsof comparative designs•• Communicate your findings to Communicate your findings to
AASHTOAASHTO’’s Subcommittee Ts Subcommittee T--15 Chair 15 Chair or members, certainly to your client.or members, certainly to your client.