A REVIEW ON CONCRETE FILLED DOUBLE SKIN STEEL TUBULAR COLUMNS

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A REVIEW ON CONCRETE FILLED DOUBLE SKIN STEEL TUBULAR COLUMNS

Dalmiya Rajan1 , Juliet Raju M2

1Assistant professor, Dept.of Civil Engineering, St. Josephʼs College of Engineering &Technology, Kerala, India

2Student, Dept. Of civil Engineering, St. Josephʼs College of Engineering & Technology, Kerala, India

***

Abstract Concrete filled double skin steel tubular (CFDST) Columns considered as a new type of Concrete Filled Steel Tubular (CFST) Columns. It consists of an inner andouter steel tube with the annulus between the skins filled with concrete. The concrete is properly compacted and filled in between the steel surfaces. CFDST are high performance composite columns that are increasingly being used in bridges and high rise buildings as well as to reinforce CFST columns. CFDST has many advantages over CFST such as improved stability, section modulus, lighter weight and better damping characteristics. The inner tube increases their strength and ductility as compared to CFST. It is important to control the inner tube thickness to prevent the premature failure. The outer tube wall thickness increases the performance of the column therefore it reduces the susceptibility of the square tube to local buckling.

The hollow steel tubes can be chosen with any type of cross section, commonly circular and square tubes are preferred. This paper investigates the characteristics of square Concrete filled double skin steel tubular (CFDST) columns with circular steel tubes in the interior. Non linear FE analysis is performed in this paper on CFDST columns with SHS outer and CHS inner tubes under axial compression.

Key Words: Concrete Filled Double Skin Tubes, CFDST

Square Columns, Axial compression, Hollow Ratio.

1. INTRODUCTION

TheconceptofCFDSThasdevelopedfromtheuseofhollow steeltubesandtheywerefirstlyusedinoffshoreandinland construction. Concrete filled double skin steel tubular column is a composite construction in which two hollow steel tubes are concentrically positioned and the annulus between each tube is filled with concrete. Due to the presenceofinnersteeltube,it increasestrength to weight ratio,bendingstiffness,ductility,andseismicperformanceof the column[1].The inner steel tubes eliminate the use of externalformworkduringtheprocessofconcreting.Whenit iscomparedtoreinforcedconcreteandbaresteelthestrong mechanical characteristics of CFST reduces the cross sectional size of the column [2]. The choice of the cross sectionandgeometryisdependingupontherequirements such as the structural efficiency of the column, aesthetic criteria ,material availability, cost and method of

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construction.Commonlysquareandcirculararepreferred. Fig1showsdifferentconfigurationofCFDSTcolumn.

Fig 1:DifferentconfigurationofCFDSTcolumn

CFDSTcolumnswidelyusedintheconstructionofhigh rise buildings,factoriesandbridgesoverpasttimes.Therehave been several investigations are conducted on CFDST columnstounderstandthestrengthandbehaviorofCFDST columns.Asaresultofthesestudiesfoundthatdoubleskin tubes have an improved ductility and strength due to the “compositeaction”betweenthesteeltubesandsandwiched concrete.

2. STRENGTH OF CFDST COLUMN

A Series of test were conducted on concrete filled double skinsteeltubes(CFDST)columnsunderlong termsustained loads. The test was conducted by 2 stages , ie. Long term servicetestandultimatestrengthtest.Inthestudythereare six specimens such as two circular CFDST specimens, two square CFDST specimens and two reference conventional CFSTspecimensunderconcentricallylong termsustained loads. Also for comparing prepares ten CFDST and CFST referencespecimensweretested.Thetestisconductedto measure the ultimate loads without long term sustained loadings. It reduces the ultimate strength of the CFDST column and improves its deformation [3]. The structural characteristicsofshortCFDSTcolumnsundercompression with circular inner and outer steel tubes with the use of finiteelementmodeling.Duetotheconfinementprovidedby thesteeltubes,circularconcrete filleddouble skintubular columns (CFDST) greatly increases the displacement ductility and peak strength of sandwiched concrete [4] Tests was conducted on concrete filled double skin steel tube(CFDST)underaxiallypartialcompression.Testwere conductedonfourteenspecimenswithouterandinnersteel tubesofcircularhollowsection(CHS)andfifteenspecimens

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with outer and inner steel tubes of square hollow section (SHS).Thestudyexperimentallyinvestigatethebehaviourof partially loaded CFDST columns. Also the study compares CFDSTandCFSTcolumnsunderaxiallypartialcompression and study the bearing capacity of partially loaded CFDST sections.ThetestresultshowsthatCFDSTcolumnsbehaved inaductilemannerunderpartialcompression.Thebelowfig 2showstheCFDSTunder axiallypartialcompression[5]

3. MATERIAL PROPERTIES

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Fig 2:CFDSTunderaxiallypartialcompression

A circular concrete filled double steel tubular column (CFDST)withasquarehollowsection(SHS)asaninnertube filled with concrete that are axially loaded and tests that wereconductedontheCFDSTcolumnie.Compressiontest. Test are carried out on eight CFDST stub columns. For comparative analysis with CFDST columns, two circular CFST columns and one double skin concrete filled steel tubular(DCFST)columnareadded.CircularCFDSTcolumns withtheinnerSHShaveincreasedstrengthandalsoincrease the ductility when compared with the conventional CFST column and DCFST column [6]. The specimen consist of circular hollow section (CHS) outer and square hollow section(SHS)inner,withthespacefilledwithconcrete.And the inner tube of column is not filled with concrete. High nominal compressive strengths are in the parametric analysis, and steel tubes are cold formed from various designyieldstrengths.Twentycolumnswereinvestigated andmodelledinAbaqustoinvestigatethebehaviorofCFDST columnsunderaxialcompression[7]

Lately, stainless steel is more commonly used when comparedtocarbonsteelduetoitsbettercorrosion,fireand impactresistance,andmaintenances.Materialpropertiesof carbon and stainless steel given below in table 1 were obtainedusingtensiletest[8].

Table 1:Materialpropertiesofcarbonandstainlesssteel

Carbon steel(2.01mm)

Carbon steel(2.52mm)

Stainless steel(1.88mm)

275 351 2.08x10⁵ 0.22

276 384 2.05x10⁵ 0.25

322 703 1.91x10⁵ 0.46

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Topreparethetestspecimen,thetensilecouponswerecut from the steel tubes then tested according to the Chinese standardGB/T228.1 2010.Thetensiletestcouponswere shownbelowinfig3[6] Cold formedmaterialwasusedfor themanufactureofspecimens[20]

Fig 3: Tensileteststrength

4. EFFECT OF HOLLOW RATIO

Hollow ratio (χ) is an important factor affecting the compressivebehaviorofCFDST.InvestigatedtheCurvesof averagestressversuslongitudinalstrain,stressdistributions of concrete and steel tubes and hollow ratio effect. The hollowratios0,0.25,0.5,0.75weredefined.Ashollowratio χincreases,thelocationofmaximumconcretestressmoves fromcentretotheperipheryofthecross section.Commonly, hollow ratio on the concrete stresses with CHS outer is largerthanSHSouter[9].Itisimportanttocontroltheinner tube thickness to prevent premature failure. The performanceofCFDSTcolumnsoftheinnertubediameter were investigated by using hollow ratio (χ). The hollow

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ratios of 0.1, 0.2, 0.3, 0.5, 0.7 and 0.8 were defined by changing the inner tube diameter and other properties remainedunchanged.Theeffectofhollowrationontheload axial displacementcurveswereshownbelowinfig4[1] It wasshownthattheultimateaxialstrengthofCFDSTshort columnsimproveswithincreasingtheconcretecompressive strengthorwithdecreasingthehollowratio[10]

5. EFFECT OF STIFFENER

InvestigatesthebehaviorofstiffenedCFDSTmembersunder axial compression and cyclic loading. In the tests all the specimens are behaved in a ductile manner. For stiffened CFDST, the outward buckling was found in the outer tube andtheinwardbucklingwasfoundintheinnertube[11]

5.1 Effect of longitudinal stiffener

Due to the presence of stiffeners on the outer tube that reducesthelocalbuckling Additionally,theconfinementon theconcreteisimprovedasthestiffness oftheoutertube wasincreased.InCFDSTcolumnsthestiffenersareusedto increasetheductilityandenergydissipationability.Thefig5 showsthestiffenedCFDST.[11]

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CFDTcolumnsgenerallyusedinhigh risebuildingsandthe amountofsteelusedinCFDTcolumnishigh.Toreducethe steel utilization, thin walled steel tube with longitudinal stiffenersismaintainedintheoutertube.Accordingtothe testresults,thestiffenedcolumnsshowhighstrengthand gooddeformationcapacity[12]

5.2 Behavioural Analysis

Behaviour of square CFDT columns with longitudinal stiffenersindetailanalysed[11].CFDTcolumnsareallowing to local buckling when they have thin walled outer tubes. Whenusingthin walledoutertubesthatarenotstiffened, localbucklingmightbeasignificantfactor[13].

6. PARAMETRIC STUDY

6.1 Effect of steel yield strength

Whentheyieldstressofoutersteeltubeincreases,thenthe ultimateloadandductilityofCFDSTcolumnincreases[14]. A fiber based model was used to study the effect of yield stressofsteeltubecontrolthebehaviorofCFDSTcolumns. When the steel yield stress increases then the ultimate strength of column improves ie. Increased by 4.7%, 7.5%, 13.4%, and 19.4%. And the yield stress is increased from 250MPato300MPa,350MPa,400MPa,and450MPa.TheFig 6showstheeffectofsteelyieldstrengthontheaxialload straincurvesofcircularCFDSTcolumns.[15].

Fig 6 :Effectsofsteelyieldstrengthontheaxialload strain curvesofcircularCFDSTshortcolumns.

6.2 Concrete compressive strength

Infibreanalysisdifferentcompressivestrengthofconcrete areusedsuchas55MPa,75MPa,95MPa,and115MPa.Dueto theuseofhigherstrengthofconcretethenitimprovesthe ultimate strength of CFDST columns. Similarly, due to the high strength of concrete used in CFDST columns are less ductile due to their stiffness [13] . Strength of concrete rangesfrom40MPato100MPa.Asthestrengthofconcrete changesfrom40MPato100MPathentheultimatebending

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resistanceraisedby102.3%.Duetotheuseofhighstrength concrete, then the curvature ductility decreases. The fig 7 shows the effect of concrete strength on the strength envelopsofsquareCFDSTcolumns.[16]

When the compressive strength of concrete increases significantlyitincreasesthe initial bendingstiffeness,and the compressive strength ranges from 40MPa to 100MPa. When the strength of concrete changes then the ultimate moment capacity increases and ductility index decreases [17]. Intermsofductility,thecharacteristicsofHSandUHS aredifferentfromNSconcreteandithashighstrength.[19].

6.3 Ductility index

Ductility is defined as the ability to deform the member eitherfromitsbeginningofyieldingtothemaximumbearing capacityorafteritreachesthemaximumbearingcapacity.It is the ratio of total deformation at maximum load to the elastic limit deformation. It has been commonly used to calculate the axial ductility of columns. And also it is commonly used to calculate the ductility of composite columns. Due to the higher ductility index significantly it increasestheductilityofCFDSTcolumn[6].

DI=εu,0.92/εy

Whereεu,0.92istheaxialstrainafterthespecimens undergoultimateaxialstrength.εyistheaxialyieldstrain

Fig8showsthatthevalueofDIdecreaseswhenthestrength ofconcreteincreases.From theinvestigationitwasfound thatwiththeincreaseinthestrengthofconcretethecolumn hasthetendencytobrittlenessfailure.Infig8(a)showsthat whenthehollowratioincreasesthentheDIdecreases.Fig8

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(b),(c) showsthatthevalueofDIincreaseswhenthewall thicknessandyieldstressoftheoutertubeincreases[14].

(a) (b)

Fig 8: Ductilityindex

7. LOAD AND BOUNDARY CONDITIONS

Forapplyingloadandboundaryconditions,tworeference point ie. Top reference point and bottom reference point were assigned to the ends of the CFDST column. Top reference point was fixed against all the lateral

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displacementsandrotations.Andthebottomreferencepoint was fixed against all degree of freedoms[1] . For vertical bearing memberswithθ=90ᵒ,thenatthetopsurfaceofBM the translation in X direction is UX and in Z direction the translationisUz.Atthebottomsurfacethetranslationoccur inYdirectionie,UY andthesurfacewererestrained.Thefig 9showstheverticalbearingmemberθ=90ᵒ

Fig 9: Verticalbearingmember,θ=90ᵒ

Forinclinedbearingmemberswithθ=45ᵒ,thetranslatiomin Xdirectionie,UX andinZdirectionieUZisrestrainedatthe topsurfaceofthecolumn.Andforinclinedbearingmembers, theactionwasappliedintheYdirection.Thefig10shows theinclinedbearingmember,θ=45ᵒ[18]

Fig 10: Inclinedbearingmember,θ=45ᵒ

CONCLUSIONS

CFDSTcolumnssignificantlyincreasetheaxialloadcarrying capacitywithlessaxial shortening. Duetotheuse ofsteel tubesandconcrete,thereforeincreasesthestrengthofthe CFDSTcolumn.Finiteelementmethodisusedtoanalysisof concrete filled double skin tube columns. The outcome of thispapercanbesummarizedinbelowpoints:

Minimizing the local buckling by increasing the outertubethickness.

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UltimatestrengthofCFDSTcolumnincreasewhile increasingthestrengthofconcrete.

Elastic energy capacity absorption capacity of CFDST column increases while increasing the strengthofconcrete.

Innertubethicknessmustbecontrolledtoprevent theprematurefailure.

Delay the local buckling effect and reduce the bucklingdegreewhileusinglongitudinalstiffeners.

InCFDSTcolumnwhenthecompressivestrengthof concrete or hollow ratio decreased, significantly increasetheultimateaxialloadofCFDSTcolumn.

Increasing the strength of concrete then the ductilityofCFDSTcolumndecreases.

InCFDSTcolumntheultimatestrengthdependson thediameterandthicknessofoutertube.Andalso theyieldstrengthhighlyinfluenceontheultimate strengthofCFDSTcolumn.

Increasingthestrengthofconcretethenthe columnhasthetendencytobrittlefailure.

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