1 introduction
DESCRIPTION
Introduction to Design of structures to BS 8110TRANSCRIPT
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Lectures on Reinforced Concrete Design: Lecture 1
INTRODUCTION
BYBYDr.A.W.HAGO
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Reinforced ConcreteReinforcedConcrete Reinforcedconcreteisastrongdurablebuildingmaterialthatcanbeformedinto
many varied shapes and sizes (flexible in shaping)manyvariedshapesandsizes(flexibleinshaping). Itsutilityandversatilityareachievedbycombiningthebestfeaturesofconcrete
andsteel.C i th diff i ti f th t t i l Comparingthedifferingpropertiesofthesetwomaterials:
Property Concrete Steel
strength in tension poor goodstrengthintension poor good
strengthincompression good good,butslenderbarswillbuckle
strengthinshear fair goodg g
Durability good corrodesifunprotected
fireresistance good poor suffersrapidlossofstrengthathightemperatures
It can be seen from this list that the materials are complementary. when they are combined, the steel is able to provide the tensile strength and y gprobably some of the shear strength, while the concrete, strong in compression, protects the steel to give durability and fire resistance.
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CompositeAction1. The tensile strength of concrete is about 10 %of its compressive strength.2. For this reason, reinforced concrete structures are designed assuming
concrete does not resist any tensile forces.3. Reinforcing steel bars are designed to carry tensile forces, which are
transferred by bond between the two materials (composite action).4. Inadequate bond causes reinforcing bars to slip (no composite action).5. Bond improves by compacting concrete around the the reinforcement, and
some bars are ribbed or twisted so that there is an extra mechanical grip.6. with perfect bond, the strain in the reinforcement is identical to the strain in the
dj t tadjacent concrete.7. when tension occurs, cracking of concrete will take place. Reinforcement will
restrain the cracking of concrete8 When compressive or shearing forces exceed the strength of the8. When compressive or shearing forces exceed the strength of the
concrete, steel reinforcement must also be provided to supplement the load-carrying capacity concrete.
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Stress Strain RelationsStressStrainRelations
Stress
f67.0
Stress
cuf67.0
Steel
002.0 Strain
ConcreteConcrete
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IdealizedStressstrainCurveforConcrete Fordesignpurposes,thetypicalstressstraincurveforconcreteisgiven
below
5.1mod'
factorsafetyMaterialulussYoungE
strengtheCompressivf
m
cu
ff ym
EcEco=
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Stress strain relations for SteelStressstrainrelationsforSteelS l h l i i i d i Steelhasequalpropertiesintensionandcompression
YoungsmodulusEs=200kN/mm2
Becauseofitsgoodductility,itisassumedfordesignpurposestohavenolimitingstrain.
Gradesofsteelare:460N/mm2 and250N/mm2
Es
051mod'
f tf tM t i lulussYoungE
strengthYieldf y
05.1 factorsafetyMaterialm
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MaterialsSpecificationforDesign:Concrete Theselectionofthetypeofconcreteisgovernedbythestrengthrequired. Theconcretestrengthisassessedbymeasuringthecrushingstrengthofcubesor
cylinders of concrete made from the mix These are usually cured and testedcylindersofconcretemadefromthemix.Theseareusuallycured,andtestedaftertwentyeightdaysaccordingtostandardprocedures.
Concreteofagivenstrengthisidentifiedbyits'grade'.AgradeC25 concretehasa characteristic cube crushing strength of 25 N/mm2acharacteristiccubecrushingstrengthof25N/mm2
TheTablebelowshowsalistofcommonlyusedgradesandalsothelowestgradeappropriateforvarioustypesofconstruction.
Exposureconditionsanddurabilitycanalsoaffectthechoiceofthemixdesignandthegradeofconcrete,e.gastructuresubjecttocorrosiveconditionsinachemicalplantwouldrequireadenserandhighergradeofconcretethantheinterior
b f h l ffi bl kmembersofaschoolorofficeblock.Grade Lowestgradeforuseasspecified
C7.5,C10 Plainconcrete
C15,C20 Reinforced concretewithlightweightaggregate
C25,C30 Reinforced concretewithdenseaggregate
C35 Concretewithposttensionedtendons
C40,C50,C60 Concretewithpretensionedtendons
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MaterialsSpecificationforDesign:Steel1. Reinforcingsteelissuppliedinformofbars.Thesecanbeclassified
accordingtotheirmanufacturingprocessinto:) Mild l b (H ll d) i h i ld h f 250 N/ 2a) Mildsteelbars(Hotrolled)withyieldstrengthfy =250N/mm2
b) Highyieldbars(Hotrolled&coldworked)withyieldstrengthfy =460N/mm2/
2. Thenominalsizeofabaristhediameterofanequivalentcirculararea.3. Hotrolledmildsteelbarsusuallyhaveasmoothsurface.4. Highyieldbarsaremanufacturedeitherwitharibbedsurface(Type2)or
intheformofatwistedsquare(Type1).Squaretwistedbarshaveinferiorbondcharacteristicsandaremoreorlessobsolete.
5. Floorslabs,walls,shellsandroadsmaybereinforcedwithaweldedfabricofreinforcement,suppliedinrollsandhavingasquareorrectangular meshrectangularmesh.
6. Thebartypesarecommonlyidentifiedindesignsbythefollowingcodes:R=mildsteel;Y=highyielddeformedsteel,typeI;
T=highyielddeformedsteel,type2;
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Objectives oftheDesignofReinforcedConcreteStructures
Everystructurehasitsform,functionandaesthetics.y f , fNormallythearchitects willtakecareofthemandthestructuralengineers willbesolelyresponsibleforthestrengthandsafetyofthestructure.However,therolesofarchitectsandstructuralengineersareverymuchinteractiveto produce a good designtoproduceagooddesign.
The objectives of the design are as follows:Theobjectivesofthedesignareasfollows:
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Objectives of the Design of R.C. Structures
1 The structures so designed should have an acceptable probability of1. Thestructuressodesignedshouldhaveanacceptableprobabilityofperformingsatisfactorilyduringtheirintendedlife.
2. Thedesignedstructureshouldsustainallloadsanddeformwithinlimitsforconstructionanduse.Adequatestrengthsandlimiteddeformationsarethetworequirementsofthedesignedstructure.Thestructuresshouldgivesufficientwarningtotheoccupantsandmustnotfailsuddenly.
3. Thedesignedstructuresshouldbedurable.Thematerialsofreinforcedconcretestructuresgetaffectedbytheenvironmentalconditions.Thedesigned structures therefore must be checked for durabilitydesignedstructures,therefore,mustbecheckedfordurability.
4. Thedesignedstructuresshouldadequatelyresisttotheeffectsofmisuseandfire.Properlydesignedstructuresshouldallowsufficienttimeandsaferouteforthepersonsinsidetovacatethestructuresbeforetheyactuallycollapse.
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Methods of DesignMethodsofDesign Designobjectivescanbefulfilledbyunderstandingthestrengthanddeformation
characteristics of the materials used in the design as also their deteriorationcharacteristicsofthematerialsusedinthedesignasalsotheirdeteriorationunderhostileexposure.
Thenecessaryinformationregardingpropertiesandstrengthofthesematerialsare available in the standard codes of practices It is necessary to follow theseareavailableinthestandardcodesofpractices.Itisnecessarytofollowtheseclearlydefinedstandardsformaterials,production,workmanshipandmaintenance,andtheperformanceofstructuresinservice.Thecodeofpracticeused in this course is BS8110 The structural use of concreteusedinthiscourseisBS8110Thestructuraluseofconcrete.
Theknownmethodsofdesignofreinforcedconcretestructuresare:1. Thepermissiblestressmethodinwhichultimatestrengthsofthematerials
are divided by a factor of safety to provide design stresses which are usuallyaredividedbyafactorofsafety toprovidedesignstresseswhichareusuallywithintheelasticrange.
2. Theloadfactormethodinwhichtheworkingloadsaremultipliedbyafactorf f tofsafety.
3. Thelimitstatemethodwhichmultipliestheworkingloadsbypartialfactorsofsafetyandalsodividesthematerials'ultimatestrengthsbyfurtherpartialf f ffactorsofsafety
BS8110isbasedontheLimitstatemethod.
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Design StepsDesignStepsInanymethodofdesign,thefollowingarethecommonstepsto
b f ll dbefollowed:1. Toassessthedeadloadsandotherexternalloadsand
f lik l t b li d th t tforceslikelytobeappliedonthestructure,2. Todeterminethedesignloadsfromdifferent
combinations of loads Initial member sizing is neededcombinationsofloads.Initialmembersizingisneededforthisstep,
3 To estimate structural responses (bending moment shear3. Toestimatestructuralresponses(bendingmoment,shearforce,axialthrustetc.)duetothedesignloads(Analysis),
4. Todeterminethecrosssectionalareasofconcretesectionsandamountsofreinforcementneeded.
5. Preparationofdrawings showingthedetailsofthedesignedstructure
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LOADSANDFORCESThe following are the different types of loads and forces acting on the structure. Their values
can be assessed based on earlier data and experiences (provided by Codes of Practice):1 Dead loads: These are the self weight of the structure to be designed The1. Dead loads: These are the self weight of the structure to be designed. The
dimensions of the cross section are to be assumed initially which enable to estimate the dead loads from the known unit weights of the materials of the structure. The values of unit weights of the materials are provided by the Code of Practicevalues of unit weights of the materials are provided by the Code of Practice.
2. Imposed loads: They are also known as live loads and consist of all loads other than the dead loads of the structure. The values of the imposed loads depend on the functional requirement of the structure Residential buildings will have comparativelyfunctional requirement of the structure. Residential buildings will have comparatively lower values of the imposed loads than those of school or office buildings.
3. Wind loads: These loads depend on the velocity of the wind at the location of the structure permeability of the structure height of the structure etcstructure, permeability of the structure, height of the structure etc.
4. Snow loads: These are important loads for structures located in areas having snow fall, which gets accumulated in different parts of the structure depending on projections height slope etc of the structureprojections, height, slope etc. of the structure.
5. Earthquake forces: Earthquake generates waves which move from the origin of its location (epicenter) with velocities depending on the intensity and magnitude of the
th k Th i t f th k t t d d th tiff f thearthquake. The impact of earthquake on structures depends on the stiffness of the structure, stiffness of the soil media, height and location of the structure etc.
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Analysis of StructuresAnalysisofStructures
Structureswhensubjectedtoexternalloads(actions)haveinternalreactionsintheformofbendingmoment,shearforce axial thrust and torsion in individual membersforce,axialthrustandtorsion inindividualmembers.
Thestructuredevelopsinternalstressesandundergodeformations.deformations.
Essentially,weanalysethestructureelastically replacingeachmemberbyaline(withElvalues)andthendesignthesectiony ( ) gusingconceptsoflimitstateofcollapse.
Theexternalloadstobeappliedonthestructuresarethedesignloadsandtheanalysesofstructuresarebasedonlinearelastictheory.
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LimitStateMethodofDesign
What are limit states?Limit states are the acceptable limits for the safety andserviceability requirements of the structure before failureoccurs.The design of structures by this method will thus ensurethat they will not reach limit states and will not becomeunfit for the use for which they are intendedunfit for the use for which they are intended.
It is worth mentioning that structures will not just fail orIt is worth mentioning that structures will not just fail orcollapse by violating (exceeding) the limit states. Failure,therefore, implies that clearly defined limit states ofstructural usefulness has been exceededstructural usefulness has been exceeded.
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There are two main limit states:Therearetwomainlimitstates:(i)limitstateofcollapse (Ultimatelimitstate)and(ii)limitstateofserviceability.( ) y
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MainLimitStates
1. Limit state of collapse deals with the strength andstability of structures subjected to the maximum designloads out of the possible combinations of several types ofloads. Therefore, this limit state ensures that neither any, ypart nor the whole structure should collapse or becomeunstable under any combination of expected overloads.
2. Limit state of serviceability deals with deflection andcracking of structures under service loads, durabilityg yunder working environment during their anticipatedexposure conditions during service and stability of thestructure as a whole.structure as a whole.
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Other Limit StatesOtherLimitStates1. Excessivevibrationswhichmaycausediscomfortoralarmor
ddamage2. Fatigue: mustbeconsideredifcyclicloadingislikely3. Fireresistance thismaybeconsideredintermsof
resistancetocollapse,flamepenetrationandheattransfer4 S i l i t lik th k i t4. Specialcircumstanceslikeearthquakeresistance
The usual procedure is to decide which is the crucial limit state and base the design on it, then check to ensure that other limit states are satisfied.
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Characteristic Material StrengthCharacteristicMaterialStrength Thestrengthsofmaterialsuponwhichdesignisbasedarecalled
characteristicstrengths. Acharacteristicstrengthisonebelowwhichl l k l f llresultsareunlikelytofall.
Assumingnormaldistribution,thecharacteristicstrengthforconcreteistakenasthatvaluebelowwhichitisunlikelythatmorethan5 oftheyresultswillfall,andthatisgivenby:
sff 641 sff mk 64.1where :fk = the characteristic strength,fm = the mean strength and s = the standard deviation.
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CharacteristicLoadsWhatismeantbycharacteristicload?Characteristicloadisthatloadwhichhasa95%probabilityofnotbeingexceeded during the life of the structure.Theloadsarepredictedbasedonstatisticalapproach,whereitisassumedthatthevariationoftheloadsactingonstructuresfollowsthe
l di ib i Ch i i l d h ld b h h
exceededduringthelifeofthestructure.
normaldistributionCharacteristicloadshouldbemorethantheaverage/meanload
Characteristicload=Averageload+1.64(standarddeviationforload)The characteristic dead, imposed and wind loads have the notation Gk, Qk, Wkrespectively.
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Design Strengths of MaterialsDesignStrengthsofMaterials We obtain the design strengths of the materials by dividing the
characteristic strengths by the partial safety factor , i.e. mg y p y ,
takes account of possible differences between the material in the actualstructure and the strength derived from test specimens In concrete this
mkfstrengthdesign /m
mstructure and the strength derived from test specimens. In concrete, thiswould cover such items as insufficient compaction, differences in curing,etc. For reinforcement it would cover such items as the difference betweenassumed and actual cross-sectional areas caused by rolling tolerances,assumed and actual cross sectional areas caused by rolling tolerances,corrosion, etc.
The values of for each material will be different for the different limit states by virtue of the different probabilities that can be accepted
mstates by virtue of the different probabilities that can be accepted.
The recommended values for are as follows :-
Limit state m for m for Steel
m
Limit state m for Concrete
m for Steel
Ultimate: Flexure 1.5 1.05Shear 1 25 1 15Shear 1.25 1.15Bond 1.4
Serviceability 1.0 1.0
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DesignLoadsg We obtain the design load by:
Design loads characteristic load x partial safety factor f is introduced to take account of:
1. possible unusual increases in the load2. inaccurate assessment of effects of loading
ff
2. inaccurate assessment of effects of loading 3. variations in dimensional accuracy in construction 4. the importance of the limit state being considered.
i f diff t li it t t d l f diff t bi ti f varies for different limit states and also for different combinations of loading. Values of for ultimate limit state are given the following Table:
Load Combination
Ultimate Serviceability
ff
Combination Deadg
Imposedq
Waterw
Earth, Wind Water
ALL(g,q,w)
Dead Imposed (Earth Water)
1.4 (or 1.0) 1.6 (or 0.0) 1.4 - 1.0
Dead Imposed 1 4 (or 1 0) 1 4 1 4 1 0Dead Imposed Wind
1.4 (or 1.0) - 1.4 1.4 1.0
Dead Imposed Wind (Earth Water)
1.2 1.2 1.2 1.2 1.0
The arrangement of loads should be such as to cause the most severe effects, i.e. the most severe stresses.
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BASIC REQUIREMENT OF DESIGNED STRUCTURES
1. The structure must fulfil its intended function 1. The structure must fulfil its intended function throughout its intended(design) life and it shall do so without abnormal maintenance costs.
2. The structure must be safe. The consequences of collapse can be extremely serious and the possibility of
ll t b li ibl Th t t t bcollapse must be negligible. The structure must be designed so that if loads very much greater than the normal design loads are applied then adequtenormal design loads are applied then adequte warning of the danger of collapse shall be given(e.g. visible signs of cracks and large deflections) to permit g g )appropriate action to be taken.
3. The structure must be of least cost (economical).