Newsletter Shallow Foundation

7/27/2019 Newsletter Shallow Foundation

1/47

Finite Element Anal sis of Elastic Settlementof Spreadfootings Founded in Soil

JaeH.Chung,Ph.D.

r ge o ware ns u eUniversityofFlorida,Gainesville,FL,USA

UNIVERSITY OF

FLORIDA EngineeringCivil & Coastal


2/47

ContentContent

1. Background

2. FBMultiPierFEAmodeldevelopment

3. TheoreticalbasisofFBMultiPierFEAmodel

3.1Computationofsoilresistance (Newmarks solutionofBoussinesqs equation)

3.2Constitutiverelationship

3.3Homogenezation ofheterogeity

3.4Soilstiffness

3.5Averagingmethods

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Strengthvs.Serviceability(1/3)Strengthvs.Serviceability(1/3)

Foundationsshouldbeproportionedtowithstandall

anticipatedloadssafelyincludingthepermanentloadsofthe

s ruc urean rans en oa s.

Mostdesigncodesspecifythetypesofloadsandloadcombinationstobeconsideredinfoundationdesi n e. .

AASHTO.

Theseload

combinations

can

be

used

to

identify

the

limit

s a es or e oun a on ypes e ngcons ere . m s a e

isreachedwhenthestructurenolongerfulfillsitsperformance

requirements.

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Anultimatelimitstate(ULS)correspondstothemaximum

loadcarryingcapacity (eitherstructuralorgeotechnical

a ure o e oun a on. eu ma es a e sa soca e e

strengthlimit

state.

bearingcapacityofsoilexceeded,

excessivelossofcontact,i.e.,eccentricity,

slidingatthebaseoffooting,

lossof

overall

stability,

i.e.,,

global

stability,

or

exceedanceofstructuralcapacity 1997UBCorACI318

Aserviceabilitylimitstate(SLS)correspondstolossof

excessivedifferentialortotalfoundationsettlements,

excessivelateraldisplacements,or

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.


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Allrelevantlimitstatesmustbeconsideredinfoundation

designtoensureanadequatedegreeofsafetyand

serv cea y. oun a on es gn nprac ce sgeare

towardsaddressing

the

ULS

and

the

SLS.

Existin desi nmethodolo includes:

theAllowableStressDesign(ASD)

theUltimateStrengthDesign(USD)

theLoadandResistanceFactorDesi n LRFD

FocusismadeonASDforgeotechnicaldesign.

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Designconceptandprocedure(1/4)

Thegeotechnicaldesignofaspreadfootingisatwopart

process.

. st matet ea owa eso ear ngcapac tytoensuresta tyo t e

foundationand

determine

if

the

proposed

structural

loads

can

be

supportedonareasonablysizedfoundation.

. re ctanamounto sett ement uetot eactua structura oa san

thetimeofoccurrenceestimated.Experiencehasshownthat

settlementis

usually

the

controlling

factor

in

design.

s sno surpr s ngs nces ruc ura cons era onsusua y m o era e

settlementstovaluesthatcanbeachievedonlyoncompetentsoilsnot

pronetoabearingcapacityfailure.

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Geotechnicaldesignconceptandprocedure(1/2)

Theallowablebearingcapacityofaspreadfootingisthelesserof

Theappliedstressthatresultsinashearfailuredividedbyasuitablefactorof

safety(FS);thisisacriterionbasedonULS;or

eapp e stresst atresu ts naspec e amounto sett ement;t s sa

criterionbased

on

SLS.

FactorofSafety(FS)=MeanvalueofResistance(MaterialStrength)/DesignLoad

FSranges from2.5to3

Theconceptofdecreasingallowablebearingcapacitywithincreasingfootingwidth

forthe

settlement

controlled

cases

is

an

important

concept

to

understand.

Asthefootingwidthincreases,thestressincreasefeltbythesoilmaydecrease

buttheeffectoftheappliedstresswillextendmoredeeplybelowthefootingbase.

Settlementsmayincreaseasthewidthincreasesdependingonthetypeofsoils

.

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Geotechnicaldesignconceptandprocedure(2/2)

Insuchcases,theonlywaytolimitthesettlementstoacertaindesiredvalueisby

reducingtheappliedstress;theallowablebearingcapacityisthevalueofthe

appliedstressatthefootingbasethatwillresultinagivensettlement.

tothe

footing

which

in

turn

means

that

the

allowable

bearing

capacity

is

correspondinglyreduced.Inallowablebearingcapacityestimation,atotalsafety

factorof2.5~3.0ismostlyused.

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Allowable

bearing

capacity

(shear

failure

vs.

settlement

)(source:GeotechnicalEngineering:ShallowFoundationsby Zhou,Y.,FHWANHI06089


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Structuraldesignconceptandprocedure

Foundationdesignprocedurestypicallyprovidesoilbearing

pressuresonanallowablestressdesign(ASD)basiswhile

se sm c orces n e ,an nmos concre e es gn

underACI

318,

are

on

an

ultimate

strength

design

(USD)

basis.

Thedesi nermakesatransitionfromtheASD rocedureto

determineasizeofthefootingtotheUSDproceduresto

design

the

footing.

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ExampleofacombinedfootingExampleofacombinedfooting

They are used primarily when the column spacing is nonuniform (Bowles, 1996) or

when isolated spreadfootings become so closely spaced that a combination footing is

simpler to form and construct.

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Spillthrough


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Theoryvs.Reality(1/2)Theoryvs.Reality(1/2)

There arevarioustheoriestopredict(1)shearfailureand(2)loaddeformationbehaviorofsoil. Why:semiempiricalnatureanduncertainty

Generalshearfailure

Elasticsettlementanal sismethods:

Variationoffrictionalshearstrengthfactor Localshearfailure

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Newmark,Griffiths,Janbu etal.,Mayne andPoulos

Schmertmann,Meyerhof,

Burland and

Burbidge


12/47


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NecessityofanumericalanalysistoolNecessityofanumericalanalysistool

Forpracticalapplicationindesignpractice,areliablestandardized

procedurehastobeacombinationofthesetwomethods.

predicting

settlements

for

practical

design

where

a

computationally

efficientnumericalproceduretoestimaterepresentativesoilproperties

basedoninsitutestsanswerstotheallim ortant uestionofselectin a

soilstiffness(modulus)foruseintheseapproximatedresults.

Goal:simulatethesoilstructureinteractioninthefieldconditionswherea

proposedmodel

would

allow

the

engineer

to

easily

calibrate

and

modify

themodelingparametersbycapturingthesoilnonlinearitywithina

reasonablemarginofconservativeerrors.

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Winklermodel

Overviewofmodelcomponents Eulerbeamtheory

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Schematicdiagramofprimarymodelcomponents ApplicationofEulerBeamTheory


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LinearFiniteElementAnalysisLinearFiniteElementAnalysisofWinklerproblemofWinklerproblem

Discreteelementapproach

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Conceptofdiscretization


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CoefficientofCoefficientofsubgradesubgrade reactionreaction

If a flexible foundation is to be analyzed, then it is recommended that Subgrade

Modulus, i.e., the coefficient of subgrade reaction, be selected in consideration of

geometry (B or L) and embedment depth (Terzaghi 1955 and Vesic 1961,

respec ve y . s s ecause e va ue o su gra e mo u us s no cons an or a

given soil but depends onlength, width, andembedment depthof the foundationunder consideration.

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FEAmodelofFEAmodelofacombinedfootingacombinedfooting

A combined footing that supports a threecolumn faade is subjected to service loads.

Dimensions of the column is 1 m x 1m. The bearing soil is a 10 m thick mediumdense

sand. Compute an immediate settlement due to service loads only.

LinearFEAmodelofWinklerproblem

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Aschematicsketchofacombinedfooting


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FBFBMultiPierNonlinearFiniteElementAnalysisMultiPierNonlinearFiniteElementAnalysis

Shallowfoundationsystemismodeledusingfiniteshellelements

andsoilsprings.

AschematicsketchofFEAmodelofspreadfooting Springstiffness

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FBFBMultiPierNonlinearFEAvs.linear(Winkler)FEAMultiPierNonlinearFEAvs.linear(Winkler)FEA

Soilnonlinearbehaviorsignificantlyaffectstheloaddisplacementbehaviorof

thestripfooting.Qu isabout38150kN forafrictionangleof30deg.

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Variationofelasticsettlementprediction


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FBFBMultiPierNonlinearFEAvs.LinearFEAMultiPierNonlinearFEAvs.LinearFEA

Whatifafactorofsafetyof3isusedindesign,i.e.,amaximumserviceloadis

limitedtoQu/3=12716kN. Thatsaboutatotalappliedload.Butsettlement

isstilltooexcessive(linearanalysispredictsabout12insettlementwhereas

non inearFEApre icts18in.sett ementat1 3o Qu an notaccepta e.

UNIVERSITY OF

FLORIDA EngineeringCivil & CoastalConcept

of

safety

factor


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FBFBMultiPierNonlinearFEAvs.linearFEAMultiPierNonlinearFEAvs.linearFEA

Evenwithafactorofsafetyof3,thepredictedsettlementistooexcessive.

Ifthesettlementislimitedto2in.,anallowableloadis2086kN (459kips).

.

UNIVERSITY OF

FLORIDA EngineeringCivil & CoastalApplication

of

serviceability

limit


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InterpretationofthenumericalresultsInterpretationofthenumericalresults

Limitationoflinearelasticanalysis

Stiffnessofsoilisconstantandindependentofappliedloads

Thefootingbehavesalmostlikearigidbody(relativelyspeaking)

Deflectionof

the

footing

is

predicted

to

be

0.33

m

(13

in.)

Thecurrentfootingdesign(size)isinadequateforserviceability

Consideringanexcessivedeformation,reliabilityoftheresultsisinquestion

Soilcouldbeinnearfailurestages;whatwouldbetheultimatebearingcapacity

ofthe

footing?

Membranestiffnessoffootingmaybeacontributingfactortoflexureand,thus,

deflection.

FBMultiPier

solution

captures

nonlinear

soil

structure

interactionphenomena.

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ParametricstudyParametricstudy(weak(weaksoil)soil)

Loosesandwithk=1000kN/m^3and

Internalfrictionangle=30deg.

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Deformationofastripfootingfoundedonsoftsoil


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Parametricstudy(stiffsoil)Parametricstudy(stiffsoil)

Densesandwithk=12000kN/m^3and

Internalfrictionangle=38deg.

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Deformationofastripfootingfoundedonstiffsoil


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Parametricstudy(stiffsoil)Parametricstudy(stiffsoil)

FBFBMultiPiernonlinearFEAresultsMultiPiernonlinearFEAresults

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2.FB2.FBMultiPierFEAmodeldevelopmentMultiPierFEAmodeldevelopment

FBMultiPieris

a

3

D

nonlinear

FEA

software

program

for

use

in

bridge

pier

application.

Withaprovenrecordofthevalidity,FBMultiPieriswidelyusedinanalysisofbridge

subfoundationsbothinU.S.andworldwide.AstepbystepprocedureofFBMultiPier

shallow foundation model develo ment is rovided in the followin .

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prea oo ng

(fromFHWANHI06089,Dec./2006)


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ExampleofFBExampleofFBMultiPierFEAmodeldevelopmentMultiPierFEAmodeldevelopment

A single, rigid square foundation (118 in X 118 in) is to be

constructed to support a column load. Assume that the

suppor ng so s a me um ense san w ose ang e o n erna

friction, total unit weight, and subgrade modulus are equal to32 degrees,109 pcf, and 150 pci, respectively.

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Step1:SelectaproblemtypeStep1:Selectaproblemtype

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Step2:SelectglobalparametersStep2:Selectglobalparameters

Go to Analysis page and specify a maximum number of iteration and the tolerance

for a degree of accuracy of numerical solution. User must be familiar with the

numerical solution procedure of FBMultiPier in nonlinear analysis and choose an

appropriate tolerance for problem of interest.

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Step3:DiscretizationoffootingStep3:Discretizationoffooting

Change a pilecap grid spacing in Pile Edit and locate a pile at the center of the pile

cap. NOTE: At least one pile must be assigned. In Step 8, it will be explained how to

make the axial resistance of this pseudo pile negligible and thus, its contribution to

the bearing capacity of the foundation can be minimal.

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Step3:DiscretizationoffootingStep3:Discretizationoffooting

Change a pilecap grid spacing in Pile Edit and locate a pile at the center of the pile

cap. NOTE: At least one pile must be assigned. In Step 8, it will be explained how to

make the axial resistance of this pseudo pile negligible and thus, its contribution to

the bearing capacity of the foundation can be minimal.

GridspacingtableMeshgeneration

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Step4:SpecifyafootingelevationStep4:Specifyafootingelevation

Change the cap (footing) elevation accordingly to the ground surface. NOTE: The pile

cap is modeled using finite thin shell elements. The centerline elevation of the cap

(the shell elements) must be properly located to include the half of the cap

thickness. The length of a pseudo pile have to be at least equal to the pile cap

thickness: by default, the program expects to have at least one pile embedded in thesoil.

Footingelevation

Footin dimensions

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Step5:ChooseBearingResistanceoption

tep

:

pec ymater a

propert es

o

oot ng

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BearingResistanceoptionBearingResistanceoption

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Step7:SpecifysoilmaterialpropertiesStep7:Specifysoilmaterialproperties

Selectalateralpy modelofmediumdensesandofinterest.

Thebearingstiffnessinthesoilcapinteractioniscontrolledbythelateralpy modelof

theFBMultiPierprogram;internalfrictionangleandsubgrade modulusarethekey

parametersinordertocomputethebearingstiffnessofcohesionlesssoilwhereas

undrainedshear

strength

(=

cohesion)

is

used

as

to

compute

the

bearing

stiffness

for

cohesivesoil.

Soilpage

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Soilproperties


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Step8:Step8:ModifytheaxialsoilmodelsModifytheaxialsoilmodels

Apile

length

of

5

ft

is

chosen

in

the

model.

Setvaluesforultimateskinfrictionofthesoilaxialmodel(tz curve)andaxialbearing

failureloadofthesoiltipmodel(qz curve)equaltoasmallmagnitude,e.g.,0.001psf

. . , .

minimum,onepilemustbeincludedinthemodel.

Tipproperties(Qzcurve)

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Axialproperties(Tzcurve)


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Step9:Step9:ValidationoftheresultsValidationoftheresults

Forthis

particular

example,

FB

MultiPier

solution

process

can

converge

with

a

tolerance

of0.01kipofwhichthefoundationissubjectedtoanaxialloadof200kipsandresulting

displacementis0.1076in.Withanappliedloadgreaterthan1000kips,convergence

.

foundationiscomputedas0.0720ksi,whichisequivalenttoanappliedloadof1002.5

kipsto

a

square

shallow

foundation

with

a

size

of

118

in

X

118

in.

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Step9:Step9:ValidationoftheresultsValidationoftheresults

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Step10:Step10:InterpretationofthedataInterpretationofthedata

Thedata

from

all

shell

elements

consist

of

shear

and

moment.

It

is

important

to

note

thatthemomentsandshearresultsareperunitlengthofplate.Forexample,unitof

momentiskNmperm(orkipinperininUScustomaryunit)andunitofsheariskN per

.

Moment(My)contour

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FBMultiPier3Dresultswindow


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3.TheoreticalbasisofFB3.TheoreticalbasisofFBMultiPierFEAmodelMultiPierFEAmodel

TheoryandImplementationofnonlinearsoilstructureinteraction3.1Computationofsoilresistance:Newmarks solutionofBoussinesqs equation

.

3.3Homogenezation ofheterogeity:Averagingmethods

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stressinelasticmedium


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3.1Computationofsoilresistance(1/3)3.1Computationofsoilresistance(1/3)

Newmarks loadcharacterization:

stressunderarectangularareaof

un ormcon ac pressure y

Boussinesq (1885)

UNIVERSITY OFFLORIDA Engineering

Civil & Coastal

//


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Stresssuperpositionmethodisappliedtoanalyticalsolution

ofBoussinesqs equationbyNewman(1935)

Superpositiontechnique Variationoftheinfluencefactors

beneaththecorner(red)andthecenter(blue)


Civil & Coastal

f l ( / )f l ( / )


43/47


Variationofinducedverticalcompressivestresses

beneatharectangularshallowfoundation


Civil & Coastal

Threedimensionalviewofvariationofstressinelasticmedium

3 2 C i i l i hi h b li d l3 2 C i i l i hi h b li d l


44/47

3.2Constitutiverelationship:hyperbolicmodel3.2Constitutiverelationship:hyperbolicmodel

Kondners originalmodel(1963)

Hyperbolic

stressstrain

relationship

DuncanandChangmodel(1970)


Civil & Coastal

3 2 C tit ti l ti hi3 2 C tit ti l ti hi


45/47

3.2Constitutiverelationship3.2Constitutiverelationship

DuncanandChang(1970)modelisconceptually simpleand

computationallyrobust

etwoparameterso t snon nearstressstra nre at ons pcan e

directly

obtained

from

drained

triaxial compression

test

of

both

cohesive

andcohesionlesssoilwhereasthemodelparametersarealsoabundantly

Limitation

Numericalinstabilitymayoccurwhenstressapproachesshearfailure

Novo umec ange uetos earstressisconsi ere ,i.e.s ear i atancy

Inputparametersmustbeselectedappropriatelyforsoilconditions;whatif

Nonhomogeneoussoilconditionexists


Civil & Coastal

3 33 3 H tiH ti f h t itf h t it


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3.33.3HomogenezationHomogenezation ofheterogeneityofheterogeneity

Averagingtechniquesoftheelastic modulusofthesoilisevaluatedoveradepthoftheshallowfoundation

Bowlesweightedaveraging

method(shown

in

the

right)

E uivalentthicknessmethod

(predictsverycomparable

resultstoTexasA&Mloadtest)

(greatforstraininfluence

method;compatibleforstrain


Civil & Coastal

SchematicsketchofsoillayersandtheSchematicsketchofsoillayersandthe

influencedepth(influencedepth(H)H)

OnOn going efforts at BSIgoing efforts at BSI


47/47

OnOngoingeffortsatBSIgoingeffortsatBSI

Goal

,

engineers

Objectives

c emertmann sc emertmann s n uence nemet o s e ng mp emente nn uence nemet o s e ng mp emente n u t eru t er

expectedreleasedate:expectedreleasedate:May/2012May/2012

Makebothstressandstraininfluencemethodsavailableforengineersto

Provideengineerswithawelldocumentedvalidationusingrelevantinsituand

loadtestresults,i.e.,adegreeofthevalidity ofthenumericalmodel

Refinethetoolbasedonfeedbackfromtheen ineers continualefforts.


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