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CivilPark:ShortArticlesforDesignersSeriesModeling,AnalysisandDesignofMatonPileFoundations

Using2DPlate/ShellFiniteElementBasedModels

MainIssueAddressedinThisArticleCanwerelyon2D(Planar)FEmodel(suchasadoptedbyCSISAFEsoftware)for

findingthedesignreactionsonpilesanddesignactions(M,V)anddeflectionsofMat

onpile?

WhyThisArticle?Inlast56yearswehaveencounteredthisquestionmanytimes,practically,every

casethatwewereinvolvedinthedesignofbuildingprojectsthatincludepilematfoundationandthe

questionsrevolvedaroundtheoneshownonthefirstparagraph(above).

ShortAnswerBasedonourknowledgeofFiniteElementAnalysisanddesignexperiencewehavefound:

(1) 2DPlanarFEmodelforPileMatcantcapturethetrueshortandlongtermdistributionofsuperstructure load on pile through mat, though such analysis apparently maintains the

overallequilibriumofthesystem.Oneverycommoncasetojustifythispointisthat,with35

m thick mat on large bored piles (11.5m dia) @ 56m center to center, one easily gets

reactions (asanalysisoutput)onfouradjacentpiles intheorderof4,000;2,000,1,000and

500ton.

(2)As

the

vertical

reactions

on

piles

are

not

correct,

same

will

be

the

case

with

the

moment

andshearforcesproducedonthemat(modeledasplate/shell).Wecaneasilyunderstandthis

pointjustbycomparingtheonestripofmatasmultispanbeamusingthesimplelogicthat

Moments&ShearscantbecorrectunlessReactionsarecorrect(EquilibriumLaw).

ShortExplanations(1) Quick reviewof the fundamental formulationof FiniteElements, theplate/shell in thiscase, one can easily identify that these elements are not developed or meant to model

thick/deepstructureslikeTransferGirder,PileCaps,MatFoundationsandso.

(2)Bydefinition theplate/shellelementsaresuitable tomodel that situationwhere the inplanedimensions are significantly larger (say in theorderof 4or above) than the vertical

dimension.Incurrentcase,thehorizontaldimensionsarenotthefullsizeofthematbutthe

piletopiledistancesandtheverticaldimensionisthematthickness.

SolutionsSolid element based model of mat, with proper accountability of superstructure contribution, non

conventionalmodelingofpiles(springs)andincrementalloading(allowingforloadredistribution)isabetter

solution.Wehavetriedthisapproachinsomeofourprojectsinthepastandfoundtoperformwell&feasible

(practical).OneofearlyapplicationofsuchapproachisFullertonBuilding(37Story,Sukhumvit67,Bangkok).

Yourquestions,comments,experiencesharing,feedbackaremostwelcome:Suda@civilpark.comorBuddhi@civilpark.com

BuddhiS.

SudaT.