Design of Compression members-Axially Loaded columns

byS.PraveenKumarAssistant ProfessorDepartment of Civil EngineeringPS College of !ec"nologyCoimbatorespk@civ.psgtech.ac.in,praveen9894861585@gmail.comIntroductionIntroductionAcolumnisanimportantcomponentsofR.C. Structures.Acolumn,ingeneral,maybedefnedasamember carryingdirectaxialloadwhichcausescompressive stresses of such magnitude that these stresses largely control its design.Acolumnorstrutisacompressionmember,the eective length of which exceeds three times the least lateral dimension.!hen a member carrying mainly axial load is vertical, itistermedascolumn,whileifitisinclinedor hori"ontal, it is termed as a strut.Columnsmaybeofvariousshapesuchascircular, rectangular, s#uare, hexagonal etc.2Classifcation of columnsClassifcation of columns3#asedon!ypeof $einforcementa$%iedColumns&where the main longitudinal bars areenclosedwithin closelyspacedlateral ties(allcrosssectional shapes$b$Spiralcolumns&where the main longitudinal bars areenclosedwithin closelyspacedand continuously wound spiral reinforcement'Circular, s#uare,octagonal sections$c$CompositeColumns&wherethereinforcement is in the form of structural steelsectionsorpipes, withorwithout longitudinal bars#ased on !ype of Loadinga$ Columns with axial loading 'applied concentrically$b$ Columns with uniaxial eccentric loadingc$ Columns with biaxial eccentric loading4%heoccurrenceof(pure)axialcompressioninacolumn 'due to concentric loads$ is relatively rare. *enerally, +exure accompanies axial compression , due to(rigidframe)action,lateralloadingand-oractual'or even, unintended-accidental$ eccentricities in loading. %hecombinationofaxialcompression'.$withbending moment'/$atanycolumnsectionisstatically e#uivalenttoasystemconsistingoftheload.applied withaneccentricitye0/-.withrespecttothe longitudinal centroidal axis of the column section. Inamoregeneralloadingsituation,bendingmoments '/x and/y$areappliedsimultaneouslyontheaxially loadedcolumnintwoperpendiculardirections,about thema1oraxis'22$andminoraxis'33$ofthecolumn section.%hisresultsinbiaxialeccentricitiesex0/x -. and ey 0 /y -., as shown in 45ig.'c$6. 5Columnsinreinforcedconcreteframedbuildings,in general,fallintothethirdcategory,vi".columnswith biaxial eccentricities. %hebiaxialeccentricitiesareparticularlysignifcantin the case of the columns located in the building corners.Inthecaseofcolumnslocatedintheinteriorof symmetrical,simplebuildings,theseeccentricities undergravityloadsaregenerallyofaloworder'in comparison with the lateral dimensions of the column$, andhencearesometimesneglectedindesign calculations.Insuchcases,thecolumnsareassumedtofallinthe frst category, vi". columns with axial loading. %heCode,however,ensuresthatthedesignofsuch columnsissu7cientlyconservativetoenablethemto be capable of resisting nominal eccentricities in loading 6#ased on Slenderness $atioColumns 'i.e., compression members$ may be classifed intothefollowingtwotypes,dependingonwhether slendernesseectsareconsideredinsignifcantor signifcant8 9. Short columns :. Slender 'or long$ columns. (Slenderness)isageometricalpropertyofa compressionmemberwhichisrelatedtotheratioof its(eectivelength)toitslateraldimension.%his ratio,calledslendernessratio,alsoprovidesa measureofthevulnerabilitytofailureofthecolumn byelasticinstability'buc;ling$,intheplanein which the slenderness ratio is computed..Columnswithlowslendernessratios,i.e.,relatively shortandstoc;ycolumns,invariablyfailunder ultimateloadswiththematerial'concrete,steel$ reaching its ultimate strength, and not by buc;ling. 7 and 9.= 10In such cases, the eective length ratio k varies between 1.0 and infnity Reinforcement in columnReinforcement in columnConcrete is strong in compression. Concrete is strong in compression.?owever, longitudinal steel rods are always provided?owever, longitudinal steel rods are always provided to assist in carrying the direct loads. to assist in carrying the direct loads.AminimumareaoflongitudinalsteelisprovidedinAminimumareaoflongitudinalsteelisprovidedin the column, whether it is re#uired from load point ofthe column, whether it is re#uired from load point of view or not. view or not.%hisisdonetoresisttensilestressescausedby%hisisdonetoresisttensilestressescausedby some eccentricity of the vertical loads. some eccentricity of the vertical loads.%hereisalsoanupperlimitofamountof%hereisalsoanupperlimitofamountof reinforcementinRCcolumns,becausehigherreinforcementinRCcolumns,becausehigher percentageofsteelmaycausedi7cultiesinplacingpercentageofsteelmaycausedi7cultiesinplacing and compacting the concrete. and compacting the concrete.@ongitudinalreinforcingbarsareAtiedBlaterallyby@ongitudinalreinforcingbarsareAtiedBlaterallyby AtiesBorAstirrupsBatsuitableintervalsothattheAtiesBorAstirrupsBatsuitableintervalsothatthe bars do not buc;le bars do not buc;le11Codal .rovisions'IS&C>D&Codal .rovisions'IS&C>D&:===$:===$12Page &o'() % (* +,S (-.-*///13Page &o'0(+,S (-.-*///14Page &o'(*+,S (-.-*///15Clause*..-1.)- Page &o'(2+,S (-.-*///165unctions of longitudinal 5unctions of longitudinal reinforcementreinforcement%o share the vertical compressive load, thereby reducing the overall si"e of the column.%o resist tensile stresses caused in the column due to 'i$ eccentric load 'ii$ /oment 'iii$ %ransverse load.%o prevent sudden brittle failure of the column.%o impart certain ductility to the column.%o reduce the eects of creep and shrin;age due to sustained loading..1718Clause *..-1.1.*Page &o'(0+,S (-.-*///19Clause *..-.1.* Page &o'(0+,S (-.-*///20Clause *..-.1.* Page &o'(0+,S (-.-*///5unctions of %ransverse 5unctions of %ransverse reinforcementreinforcement%o prevent longitudinal buc;ling of longitudinal reinforcement.%o resist diagonal tension caused due to transverse shear due to moment-transverse load.%o hold the longitudinal reinforcement in position at the time of concreting.%o confne the concrete, thereby preventing its longitudinal splitting.%o impart ductility to the column.%o prevent sudden brittle failure of the columns.2122Clause *..-.1.* Page &o'(0+,S (-.-*///Cover to reinforcementCover to reinforcement5or a longitudinal reinforcing bar in a column, thenominalcovershallnotbelessthan C=mm, nor less than the diameter of such bar. In the case of columns of minimum dimension of:==mmorunder,whosereinforcingbars does not exceed 9:mm, a cover of :>mm may be used. Clause *..(.*.) Page &o'(0+,S (-.-*///23SP 1(- )023 Page &o'22Assumptions in @imit State of Collapse Assumptions in @imit State of Collapse &Compression &Compression 24Clause 12.) Page &o'.0+,S (-.-*///25Clause 12.) Page &o'.0+,S (-.-*///26Clause 12.) Page &o'.0+,S (-.-*///27282930Eetailing of columns Eetailing of columns 31a$ %ermination of column bars inside slab b$ 5ixed end 1oint in a column32c$ %ypical detail of beam column 1unction at external column33