1.1 Advantages of composite columns Use of composite columns presents several advantages which improve the quality of our structure. Following some of those: Increased strength for a given cross sectional dimension. Increased stiffness,leading to reduced slenderness and increased bulking resistance. Good fire resistance in the case of concrete encased columns. "Corrosion protection in encased columns. "Significant economic advantages over either pure structural steel or reinforced concrete alternatives Identical cross sections with different load and moment resistances can be produced by varying steel thickness,the concrete strength and reinforc ent.This allo s the outer d n to be eld constant ors in a building,thus mplifyi h tubula ng tion Erection of high rise bu ilding inan extremely efficient manne As far as it concerns the last point Iam going to report an example in order to better explain it.In composite construction,the bare steel sections support the initial construction loads, including the weight of structure during construction.Concrete is later cast around the steel section,or filled inside the tubular sections.That is why with the use of composite columns along with composite decking and composite beams it is possible to erect high rise structures in an extremely efficient manner.There is quite a vertical spread of construction activity carried out simultaneously at any one time,with numerous trades working simultaneously.For example ■ One group of workers will be erecting the steel beams and columns for one or two storeys at the top of frame. Two or three storeys below,another group of workers will be fixing the metal decking for the floors. A few storeys below,another group will be concreting the floors e go down the building,another group will be tying the column reinforcing bars in below them will be fixing the formwork,placing the concrete into the column moulds s et Page 2124
A COMPARISON OF DESIGN METHOD FOR STEEL ENCASED CONCRETE COLUMNS AND STEEL REINFORCED CONCRETE COLUMNS P a g e 2 | 24 1.1 Advantages of composite columns Use of composite columns presents several advantages which improve the quality of our structure. Following some of those: Increased strength for a given cross sectional dimension. Increased stiffness, leading to reduced slenderness and increased bulking resistance. Good fire resistance in the case of concrete encased columns. Corrosion protection in encased columns. Significant economic advantages over either pure structural steel or reinforced concrete alternatives. Identical cross sections with different load and moment resistances can be produced by varying steel thickness, the concrete strength and reinforcement. This allows the outer dimensions of a column to be held constant over a number of floors in a building, thus simplifying the construction and architectural detailing. Formwork is not required for concrete filled tubular sections. Erection of high rise building in an extremely efficient manner. As far as it concerns the last point I am going to report an example in order to better explain it. In composite construction, the bare steel sections support the initial construction loads, including the weight of structure during construction. Concrete is later cast around the steel section, or filled inside the tubular sections. That is why with the use of composite columns along with composite decking and composite beams it is possible to erect high rise structures in an extremely efficient manner. There is quite a vertical spread of construction activity carried out simultaneously at any one time, with numerous trades working simultaneously. For example One group of workers will be erecting the steel beams and columns for one or two storeys at the top of frame. Two or three storeys below, another group of workers will be fixing the metal decking for the floors. A few storeys below, another group will be concreting the floors. As we go down the building, another group will be tying the column reinforcing bars in cages. Yet another group below them will be fixing the formwork, placing the concrete into the column moulds etc
A COMPARISON OF OESIGN METHOO FOR STEELENCDNCRETE COLUMNS AND STEEL REINFORCED CONCRETE 2 Material The cing steel used ir omp umns steel reinforced column ously in the case of composite column we need to define also the structura steel characteristics 2 1 Structural steel commonly used in construction as per /S:961-1975 and IS: Nominal steel grade Nominal thickness/diameter (mm)Yield stress,f(MPa) Fe 570-HT F<6 350 6st≤28 350 28<t≤45 340 Fe 540W-HT t<6 350 6≤t≤16 350 16<t≤32 340 Fe410-0 1<6 250 (not subiected to 6≤t<20 250 dynamic loading other 20<1≤40 240 than wind) Table 1:Yield strength fy of steel sections Nominal steel Nominal thickness diameter(mm) Yield stress,,fMPa grade Fe 410W A <20 250 20.40 240 >40 230 Fe 410W B <20 250 20-40 240 >40 230 Fe 410W C <20 250 20.40 240 >40 230 Table 2:fy of steelsection per
A COMPARISON OF DESIGN METHOD FOR STEEL ENCASED CONCRETE COLUMNS AND STEEL REINFORCED CONCRETE COLUMNS P a g e 3 | 24 2 Material The concrete and the reinforcing steel used in composite columns and steel reinforced columns are the same. Obviously in the case of composite column we need to define also the structural steel characteristics. 2.1 Structural steel Some of the structural steel grade commonly used in construction as per IS: 961-1975 and IS: 1977-1975 are given in Table 1. Table 1: Yield strength fy of steel sections Table 2: Yield strength fy of steel sections as per IS 2062:1992
2.2 Concrete Concrete strengths are specified in terms of the characteristic cube strengths,(fck)cu, measured at 28 days.Table 3 gives the properties of different grades of concrete according to /S:456-2000 and the corresponding EC4 values. Grade Designation M30 M35 (fek)cu (N/mm) 25 35 40 (fek)e (N/mm) 20 25 28 32 femn(N/mm 2.2 6 2.8 3.3 Eem=5700v(fek)eu(N/mm)28500 31220 33720 36050 Table 3Properties of Where (fckIcu characteristic compressive (cube)strength of concrete (fck)cy characteristic compressive(cylinder)strength of concrete,given by 0.8 times 28 days cube strength of concrete according to EC4 fctm mean tensile strength of concrete 2.3 Reinforcing steel Steel grades commonly used in construction are given in Table 4. Type of steel Indian Standard Nominal size (mm) Yield Stress f康Nmm Mild steel Grade I(plain IS:432Pat1)-1982 0 250 bars) 20<dk50 240 Mild steel Grade II (plain IS.432Pat1-1982 ds20 225 bars) 20<d≤50 215 Medium tensile steel IS:432(Partl)-1982 ds16 540 (plain bars) 16<d≤32 540 510 Medium tensile stee IS:1786-1985 415 (Hot-rolled deformed 500 bars and Cold-twisted 550 deformed bars) An important consideration has to be made about the influence of reinforcing steel in the both RCC and composite columns.It should be noted that although the ductility of reinforcing bars has a significant effect on the behaviour of continuous composite beams,this property has little effect on the design of composite columns.Concrete filled tubular sections may be used without any reinforcement except for reasons of fire resistance,where appropriate. Page 4124
A COMPARISON OF DESIGN METHOD FOR STEEL ENCASED CONCRETE COLUMNS AND STEEL REINFORCED CONCRETE COLUMNS P a g e 4 | 24 2.2 Concrete Concrete strengths are specified in terms of the characteristic cube strengths, (fck)cu, measured at 28 days. Table 3 gives the properties of different grades of concrete according to IS: 456-2000 and the corresponding EC4 values. Table 3 Properties of concrete Where: (fck)cu characteristic compressive (cube) strength of concrete (fck)cy characteristic compressive (cylinder) strength of concrete, given by 0.8 times 28 days cube strength of concrete according to EC4 fctm mean tensile strength of concrete 2.3 Reinforcing steel Steel grades commonly used in construction are given in Table 4. Table 4 Characteristic strengths of reinforcing steel An important consideration has to be made about the influence of reinforcing steel in the both RCC and composite columns. It should be noted that although the ductility of reinforcing bars has a significant effect on the behaviour of continuous composite beams, this property has little effect on the design of composite columns. Concrete filled tubular sections may be used without any reinforcement except for reasons of fire resistance, where appropriate
3 Partial safety factors 3.1 Partial safety factor yf for loads The suggested partial safety factory for different load combinations is given below in Table 5. Loading DL IL WI Dead Load (unfavourable effects) 1.35 Dead load restraining uplift or overturning 10 Imposed Load+Dead Load 135 15 Dead Load +Wind Load L35 Dead Load +Imposed Load wind Load (Major Load) 1.35 1.05 Dead Load Imposed Load (Major Load)+wind Load 1.35 1.5 105 Table5 Partial safety factors (According to proposed revisions) 3.2 Partial safety factor for materials The partial safety factor ym for structural steel,concrete and reinforcing steel is given in Table 6. Material Steel Section 1.15 Concrete 15 Reinforcement 1.15 4 Design nall current design provisions for reinforced concrete,steel and composite columns,the design action effects(the axial force N*and the maximum moment M*)resulting from the application of the design loads can be determined from a first-order elastic analysis but second-order effects must be accounted for by the use of a moment magnifier.alternatively.the second-order effects can be dete nined directly from a se rdar elastic analysis of the structure.In design,the member of a structure is proportioned so that its design strength is not less than the corresponding design action effect. 4.1 Design action effect In modern codes for reinforced concrete,steel,and composite structures,the direct design for structural stability and strength is allowed provided that second-order effects are taken into account including geometrical imperfections,material non-linearity,creep and shrinkage,concrete cracking,tension stiffening,three dimensional effects and construction sequence.This is called "rigorous structural analysis"in AS 3600,"advanced structural analysis"in AS 4100 and"general Page 5124
A COMPARISON OF DESIGN METHOD FOR STEEL ENCASED CONCRETE COLUMNS AND STEEL REINFORCED CONCRETE COLUMNS P a g e 5 | 24 3 Partial safety factors 3.1 Partial safety factor γf for loads The suggested partial safety factor for different load combinations is given below in Table 5. Table 5 Partial safety factors ( According to proposed revisions to IS 800) 3.2 Partial safety factor for materials The partial safety factor γm for structural steel, concrete and reinforcing steel is given in Table 6. Table 6 Partial safety factor for materials 4 Design In all current design provisions for reinforced concrete, steel and composite columns, the design action effects (the axial force N* and the maximum moment M*) resulting from the application of the design loads can be determined from a first-order elastic analysis but second-order effects must be accounted for by the use of a moment magnifier. Alternatively, the second-order effects can be determined directly from a second-order elastic analysis of the structure. In design, the member of a structure is proportioned so that its design strength is not less than the corresponding design action effect. 4.1 Design action effect In modern codes for reinforced concrete, steel, and composite structures, the direct design for structural stability and strength is allowed provided that second-order effects are taken into account including geometrical imperfections, material non-linearity, creep and shrinkage, concrete cracking, tension stiffening, three dimensional effects and construction sequence. This is called “rigorous structural analysis” in AS 3600, “advanced structural analysis” in AS 4100 and “general
method of design"in Eurocode 4.These approaches can be quite complex and,in general,simple methods are used for a wide range of regular structures with uniform members in which the axial force N*and the maximum end moment M2*for end-loaded braced members,resulting from the application of the design loads,can be determined from a first-order elastic global analysis. Second-order effects within the member are accounted for by the use of a moment magnifier 6b whereby the maximum moment M*in the column is given by M'=6pMi (1) Where 1 (2) And Cm=0.6-0.46≥0.4(AS3600) (3) Cm=0.6-0.4B (AS4100) Cm =0.66-0.44B20.44 (Eurocode 4) And B=Mi/M (4) atio of the smaller to the larger end moments and is positive when the member is Also Ner=元'EI/2 (5) where"I is the effective length of the column.However,the three codes differ in their determination of the flexural rigidity Elas follows 4.1.1 Reinforced concrete In the AS3600,the flexural rigidity El is given by EI=200Mubd/(1+Ba) (6 where EI is the secant flexural stiffness corresponding to the "balanced"moment capacity Mub of the cross-section taken to occur when the neutral axis is at a value of 0.6d(where d is the depth to the outermost layer of tensile steel)and the extreme concrete fibre strain in compression is 0.003 (Bridge 196).Values of Mub are also given in charts in handbooks(CCAA HB71-2002). 4.1.2 Steel InAS 4100,the flexural rigidity El is given by El=Esls where /s is the second moment of area of the steel section and Es is the elastic modulus. 4.1.3 Composite In Eurocode 4,the flexural rigidity El is given by EI=0.ls+ (8)
A COMPARISON OF DESIGN METHOD FOR STEEL ENCASED CONCRETE COLUMNS AND STEEL REINFORCED CONCRETE COLUMNS P a g e 6 | 24 method of design” in Eurocode 4. These approaches can be quite complex and, in general, simpler methods are used for a wide range of regular structures with uniform members in which the axial force N* and the maximum end moment M2* for end-loaded braced members, resulting from the application of the design loads, can be determined from a first-order elastic global analysis. Second-order effects within the member are accounted for by the use of a moment magnifier δb whereby the maximum moment M* in the column is given by (1) Where (2) And (AS3600) (3) (AS4100) . (Eurocode 4) . And (4) where β is ratio of the smaller to the larger end moments and is positive when the member is bent in double curvature. Also Ncr = π2EI/l2 (5) where “l” is the effective length of the column. However, the three codes differ in their determination of the flexural rigidity EI as follows. 4.1.1 Reinforced concrete In the AS 3600, the flexural rigidity EI is given by EI = 200Mubd/(1 + βd) (6) where EI is the secant flexural stiffness corresponding to the “balanced” moment capacity Mub of the cross-section taken to occur when the neutral axis is at a value of 0.6d (where d is the depth to the outermost layer of tensile steel) and the extreme concrete fibre strain in compression is 0.003 (Bridge 1986). Values of Mub are also given in charts in handbooks (CCAA HB71-2002). 4.1.2 Steel In AS 4100, the flexural rigidity EI is given by EI = EsIs (7) where Is is the second moment of area of the steel section and Es is the elastic modulus. 4.1.3 Composite In Eurocode 4, the flexural rigidity EI is given by (8)