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

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

CTDEMD

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 -100-2-2NE

MO

TN NCVA

WV

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/ASDFoundations: 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 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.

Bridge Design ExamplesBridge Design Examples

http://http://www.fhwa.dot.gov/bridge/lrfd/examples.htmwww.fhwa.dot.gov/bridge/lrfd/examples.htm

ConcreteConcrete SteelSteel

NHI LRFD Training CoursesNHI LRFD Training Courses

Course 130082Course 130082LRFD 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

• 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

Topics IncludedTopics Included•• Subsurface Subsurface

investigationsinvestigations•• Soil and rock propertiesSoil and rock properties•• Shallow foundationsShallow foundations•• Driven pilesDriven piles•• Drilled shaftsDrilled shafts•• Rigid and flexible Rigid and flexible

culvertsculverts•• AbutmentsAbutments•• Walls (most types)Walls (most types)

•• 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

GEC 5GEC 5Sabatini, 2002Sabatini, 2002

Subsurface Subsurface InvestigationsInvestigationsMayne, 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 MinimumMinimumPiles, Piles, αα--methodmethod 1.41.4 0.250.25Piles, Piles, λλ--methodmethod 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 = B –– 2e2eBB

LL′′ = L = L –– 2e2eLL

Q = P/(BQ = P/(B’’ LL’’)) ML MB

LB

eB e L

B’ L’

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 cohesionlesscohesionless soilssoils

⎟⎟⎠

⎞⎜⎜⎝

⎛ +=

vo

vvoc σ'

Δσσ'logC'1HΔH

Section 10.6 Spread FootingsSection 10.6 Spread Footings

qqnn = c = c NNcmcm + + γγ DDff NNqmqm CCwqwq + 0.5 + 0.5 γγ B B NNγγmm CCww γγ

NOMINAL RESISTANCENOMINAL RESISTANCE

NNcc ssc c iicc NNqq ssq q ddqq iiqq N N γγ s s γγ i i γγ

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 correctionSettlement 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:HanniganHannigan (2005)(2005)

Section 10.7 Driven PilesSection 10.7 Driven Piles

HHtt

QQtt

MMtt

PP

yy

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

Section 10.7 Driven PilesSection 10.7 Driven Piles

Original cu

rve

Original cu

rvePP

yyModified curve

Modified curve

PPmm * P* P

PP

Spacing (S)Spacing (S) Row 1Row 1 Row 2Row 2 Row 3Row 33D3D 0.70.7 0.50.5 0.350.355D5D 1.01.0 0.850.85 0.70.7

PP--multiplier (Pmultiplier (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 PilesStatic analysis methodsStatic analysis methods

•• NordlundNordlund ––Thurman method Thurman method addedadded

Section 10.7 Driven PilesSection 10.7 Driven PilesStatic analysis methodsStatic analysis methods

•• 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!

Section 10.7 Driven PilesSection 10.7 Driven Piles

Requirements for Requirements for driveabilitydriveability analysis 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 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

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 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.

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?