structural analysis-Chapter1 - upload.pdf

Related to the Structures I

1. 1
Lecturer: Dr. Mohammad Ali Sahraei
Faculty of Engineering
Structures I

2. 2
•Introduction
•Fundamental of Structural Theory
•Classification
•Loads
•Structural Design
Structures I

3. 3
Onceapreliminarydesignofastructureisproposed,
thestructuremustthenbeanalyzedtoensurethatithasits
requiredstiffnessandstrength.
Toanalyzeastructureproperly,certainidealizations
mustbemadeastohowthemembersaresupportedand
connectedtogether.
Theloadingsaredeterminedfromcodesandlocal
specifications,andtheforcesinthemembersandtheir
displacementsarefoundusingthetheoryofstructural
analysis,whichisthesubjectmatterofthistext.
Structures I

4. 4
Itisimportantforastructuralengineertorecognizethe
varioustypesofelementscomposingastructureandtobe
abletoclassifystructuresastotheirformandfunction.We
willintroducesomeoftheseaspectsnowandexpandon
thematappropriatepointsthroughoutthetext.
Classification of Structures
StructuralElements.Someofthe
morecommonelementsfromwhich
structuresarecomposedareasfollows.
TieRods:Structuralmembers
subjectedtoatensileforceareoften
referredtoastierodsorbracingstruts.
Duetothenatureofthisload,these
membersareratherslender,andare
oftenchosenfromrods,bars,angles,or
channels,Fig.1–1.

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Beams:Beamsareusuallystraighthorizontalmembers
usedprimarilytocarryverticalloads.Quiteoftentheyare
classifiedaccordingtothewaytheyaresupported,as
indicatedinFig.1–2.Inparticular,whenthecrosssection
variesthebeamisreferredtoastaperedorhaunched.Beam
crosssectionsmayalsobe“builtup”byaddingplatesto
theirtopandbottom.
Classification of Structures

6. 6
Classification of Structures
Tapered Beam

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Whenthematerialusedforabeamisametalsuchas
steeloraluminum,thecrosssectionismostefficientwhenit
isshapedasshowninFig.1–3.
Heretheforcesdevelopedinthetopandbottomflanges
ofthebeamformthenecessarycoupleusedtoresistthe
appliedmomentM,whereasthewebiseffectiveinresisting
theappliedshearV.
Classification of Structures

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Classification of Structures

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Columns:Membersthatare
generallyverticalandresist
axialcompressiveloadsare
referredtoascolumns,Fig.1–4.
Tubesandwide-flangecross
sectionsareoftenusedformetal
columns,andcircularand
squarecrosssectionswith
reinforcingrodsareusedfor
thosemadeofconcrete.
Occasionally,columnsare
subjectedtobothanaxialload
andabendingmomentasshown
inthefigure.Thesemembers
arereferredtoasbeamcolumns.
Classification of Structures

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Thecombinationofstructuralelementsandthematerials
fromwhichtheyarecomposedisreferredtoasastructural
system.Eachsystemisconstructedofoneormoreoffour
basictypesofstructures.Rankedinorderofcomplexityof
theirforceanalysis,theyareasfollows.
Trusses:Whenthespanofastructureisrequiredtobe
largeanditsdepthisnotanimportantcriterionfordesign,a
trussmaybeselected.Trussesconsistofslenderelements,
usuallyarrangedintriangularfashion.Planartrussesare
composedofmembersthatlieinthesameplaneandare
frequentlyusedforbridgeandroofsupport,whereasspace
trusseshavemembersextendinginthreedimensionsandare
suitableforderricksandtowers.
Types of Structures

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Types of Structures
Derrick

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CablesandArches:Twootherformsofstructuresused
tospanlongdistancesarethecableandthearch.Cablesare
usuallyflexibleandcarrytheirloadsintension.Theyare
commonlyusedtosupportbridges,(Figure),andbuilding
roofs.Whenusedforthesepurposes,thecablehasan
advantageoverthebeamandthetruss,especiallyforspans
thataregreaterthan150ft(46m).
Types of Structures

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Thearchachievesitsstrengthincompression,sinceit
hasareversecurvaturetothatofthecable.Thearchmustbe
rigid,however,inordertomaintainitsshape,andthisresults
insecondaryloadingsinvolvingshearandmoment,which
mustbeconsideredinitsdesign.Archesarefrequentlyused
inbridgestructures,(Figure),domeroofs,andforopeningsin
masonrywalls.
Types of Structures

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Frames:Framesareoftenused
inbuildingsandarecomposedof
beamsandcolumnsthatareeither
pinorfixedconnected,Fig.1–7.
Liketrusses,framesextendintwo
orthreedimensions.Theloadingon
aframecausesbendingofits
members,andifithasrigidjoint
connections,thisstructureis
generally“indeterminate”froma
standpointofanalysis.Thestrength
ofsuchaframeisderivedfromthe
momentinteractionsbetweenthe
beamsandthecolumnsattherigid
joints.
Types of Structures

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SurfaceStructures:Asurfacestructureismadefroma
materialhavingaverysmallthicknesscomparedtoitsother
dimensions.Sometimesthismaterialisveryflexibleandcan
taketheformofatentorair-inflatedstructure.Inbothcases
thematerialactsasamembranethatissubjectedtopure
tension.
Surfacestructuresmayalsobemadeofrigidmaterialsuch
asreinforcedconcrete.Assuchtheymaybeshapedasfolded
plates,cylinders,orhyperbolicparaboloids,andarereferredto
asthinplatesorshells.
Types of Structures

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Classification of Structures

17. 17
Classification of Structures

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Structure
•“a system of connected parts used to support a load”
Design of Structures
•Safety
•Esthetics
•Serviceability
•Environment
•Economy
Analysis of Structures
•Strength
•Rigidity
Idealization of Structures
•Physical Model
•Mathematical Model
Fundamental of Structural Theory

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Idealization
•Physical Model
•Members
•Connections
•Supports
•Loads
•Free-body Diagrams
•System
•Member
Mathematical Model
•Equilibrium Conditions
•Compatibility Conditions
•Constitutive Relationship
Fundamental of Structural Theory

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Equilibrium conditions
Mathematical Model
Compatibility conditions
•Continuity condition
•Boundary condition
Constitutive relationship of stress and strain

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Loads
Dead Loads
Live Loads
•Building Loads
•Bridge Loads
Wind Loads
Snow Loads
Earthquake Loads
Hydrostatic and Soil Pressure
Other Natural Loads

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Codes
General Building Codes
•MinimumDesignLoadsforBuildingandother
Structures,ANSI/ASCE7-95,AmericanSociety
ofCivilEngineers
•BasicBuildingCode,BuildingOfficialsandCode
AdministratorsInternational(BOCA)
•StandardBuildingCode,SouthernBuildingCode
CongressInternational
•UniformBuildingCode,InternationalConference
ofBuildingOfficials(UBC)

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Codes
Design Codes
•BuildingCodeRequirementsforReinforcedConcrete,Am.
Conc.Inst.(ACI)
•ManualofSteelConstruction,AmericanInstituteofSteel
Construction(AISC)
•StandardSpecificationsforHighwayBridges,American
AssociationofState
•HighwayandTransportationOfficials(AASHTO)
•NationalDesignSpecification,AmericanInstituteofTimber
Construction(ATTC)
•ManualforRailwayEngineering,AmericanRailway
EngineeringAssociation(AREA)

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Structural Design
Reinforced Concrete Structures
•1.) 1.4D + 1.7L
•2.) 0.75 [1.4D + 1.7L + 1.7W]
•3.) 0.9D + 1.3W
•4.) 1.4D + 1.7L + 1.7(soil pressure)
•5.) 0.75 [1.4D + 1.7(temperature load) + 1.7L]
•6.) 1.4(D + temperature load)
Steel Structures
•1.) 1.4D
•2.) 1.2D + 1.6L + 0.5(roof live load)
•3.) 1.2D + 0.5L (or 0.8W) + 1.6(roof live load)
•4.) 1.2D + 0.5L + 0.5(roof live load) + 1.3W
•5.) 1.2D + 0.5L + 1.5E
•6.) 0.9D -1.3W (or 1.5E)
Allowable Stress Design (ASD)
•1.) D + L + [roof live load]
•2.) D + L + [W or E]
Where, D = Dead load, L = Live load , W = Wind load , E =
Earthquake load

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Several Supports

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Several Supports