Question Number: 358

1.0 General In existing modern world, concrete and steel materials are most frequently usedfor structures extending from pavements to high rise structures, these materials countereach other although they own different material properties and characteristics in variousways. Steel structures has lesser weight ratio but has excellent resistance to tensile loading.Steel structures instruct more deflections and ductility to the structure but areuneconomical. Steel is prone to buckling phenomenon which can be resisted by concretewhich is good in compression. Prevention of corrosion and thermal insulation can be doneby concrete and on the other hand steel may be used to impart ductility which is animportant characteristic for high rise structures. Substantially, concrete has the capacity toresists buckling of steel. To get extreme assistances from both of the materials, mostlypreferred construction is the composite construction. In composite construction, differenttypes of columns are preferred, some of which are the Reinforced Cement Concrete (R.C.C)column, Concrete-filled-steel-tube column (C.F.S.T) and Encased column. In Civil Engineering, since last two decades, Composite members consisting ofconcrete-filled steel sections and Encased steel sections have become progressivelypopular. It is advantageous over the conventional structural sections in terms of ductility,strength, easy construction procedure, stiffness, energy absorption capacity and overalleconomy. Composite columns are structural members made of concrete and structural steelsubjected to compression and bending. Concrete-filled-steel-tubular column is a well compacted high-grade concretecolumn within mild steel tubes. Due to its effective shape of steel section it providesstructural benefit which offers a large moment of inertia and complete composite action ofsteel and concrete. Encassed Steel Composite Column is a composite column adequatelycomprising of structural steel surrounded with high grade concrete. This concrete acts ascorrosion resistance to the structural steel. 2Concrete-filled-steel-tubular Composite Columns are effectively used as bridge piers. AtAlmondsbury in U.K. in 1966 the four-level motor way bridge was constructed using 42"(1070 mm) diameter tubes with steel thickness of 50 mm and core concrete of M30 gradeas shown in fig 1.1. Due to advancement of design approaches and codes in U.K and U.S.Acuriosity in this type of construction is being revived. High strength, high performanceconcrete (up to 100MPa), admixtures and self-compacting concrete has been significantlydeveloped for this type of construction.1.1 Composite Column Sections with Classifications1.1.1 Reinforced Concrete Composite ColumnReinforced Cement Concrete (R.C.C) column comprises of concrete and steel whereconcrete can bear just compression whereas steel supports in taking both compression andtension. Combination of concrete and steel together is known as the composite material. Ithas high resistance to fire for a longer period. It has little maintenance cost and greaterservice life and least amount of deflection. It has higher compressive strength and is goodin tension, Reinforced Concrete Composite Column is economically designed andimplemented.1.1.2 Concrete-filled-steel-tube ColumnConcrete-filled-steel-tubular (C.F.S.T) column is a concrete filled hollow section of hotrolled steel. It is formed by well compacted concrete filled within the steel tubes. Theconcrete core gives stiffness to the column and compressive strength which resist inwardlocal buckling. The longitudinal and adjacent reinforcement is provided by the steel tubeto the concrete core which reduces shear force, bending moment and twisting moment andit provides concrete confinement. Concrete-filled-steel-tubular Composite Columns hasnumerous advantages in building construction but it is less adopted due to lack ofrecognition of the design codes, complexity of connection illustrations and constructionexperience.31.1.3 Encassed Steel Composite ColumnEncassed Steel Composite Column is a composite column comprising of structural steelsection surrounded by concrete. Construction load is carried by steel. Concrete assists byproviding fire resistance, strength and stiffness. It can be utilised when concrete finish isrequired and also if transitions are desired.1.2 Benefits of C.F.S.T Presently various types of composite materials are extensively used in theconstruction of concrete column, one such composite material is Concrete-filled-steeltubular (C.F.S.T) columns. It is being progressively adopted due to their enormous staticand seismic resistant properties, such as significant energy absorption capacity, bendingstiffness, favourable construction, fire performance, corrosion resistance, favourableductility, high strength and high ductility. C.F.S.T columns does not require shuttering assteel tube is provided for concrete. The steel tube wall buckling is prolonged by concrete.Concrete spalling, impact and abrasion is removed by the steel tube. It is economical asconstruction time and cost is reduced. Large part of the flexural rigidity of the column isimparted by the tubular section. It has lengthy service life with long term loading and theload carrying capacity of the C.F.S.T column is not affected. The steel tube within whichconcrete is confined has higher crushing strength. C.F.S.T columns are more beneficialover Reinforced Concrete Composite Columns and thus are extensively adopted in civilengineering structures.1.3 Practical uses In buildings the size of the columns can be reduced by using C.F.S.T columns.Canton tower a super high-rise structure located in China have adopted C.F.S.T in itsconstruction. C.F.S.T column also used in various types of bridges such as suspensionbridges, cable bridges, truss bridges and arch bridges. Used in the upgrade of transmissiontowers and poles as well.4Fig 1.4(a) Figure shows C.F.S.T columns with different cross sections1.4 Element Performance of Concrete-filled-steel-tubular CompositeColumns Three typical column cross sections are represented in Fig 1.4(a), where asquare or a rectangular hollow section, a circular hollow section is filled with concrete. Thelocal buckling is more prone to occur in square or rectangular cross-sections whereas thestrongest confinement to the concrete core is provided for the circular cross-section.Ultimately, the rectangular and square hollow sections of C.F.S.T are still being extensivelyused in construction, for the reasons of high cross-sectional bending stiffness, for easierbeam-to-column connection design and for artistic reasons. Round-ended, polygon andelliptical shapes of C.F.S.T are other types of cross-sectional shapes being used foraesthetical purposes, as shown in Fig 1.4(a). The tensile strength of structural steel is usually high and buckling takes placelocally under compression. Concrete has greater compressive strength when compared toits tensile strength. The usual and most prominent properties of steel and concrete havetaken advantage in concrete-filled steel tubular members. The local buckling of the steeltube is enhanced with the provision of the concrete core. The steel tube providesconfinement to the concrete core. Fig 1.4(b), shows failure modes for the concrete-filledsteel tubular column and the corresponding concrete and steel tube. It is shown that shearfailure is found in plain concrete column and both internal and external buckling isexhibited by the steel tube.The failure for the C.F.S.T column has only external buckling in the outer steel tube andthe inner concrete fails in a ductile behaviour. The failure mode for the non-prismaticinclined and tapered members are as shown in Fig 1.4(c), it is relatively similar to that ofthe prismatic member under compression, which is crushing of the concrete and theexternal buckling of the steel tube.