CHAPTER 5 COLUMN - SKYSCRAPERS

1 R r CHAPTER 5 COLUMN INTRODUCTION columns are structural elements used primarily to support compressive loads. All practical columns are members subjected not only to axial load but also to bending moments, about one or both axes of the cross section. The bending action may produce tensile forces over a part of the cross sections, even in such cases; columns are generally refereed to as compression members. TYPES OF COLUMN According to the Ratio of Height to Least Lateral Dimension: (ACI Code and ACI Code ) Short COLUMN (Figure ): When 1.

2 Rklu< 34-12(M1/M2), where (M1/M2) is not taken less than 2. rklu< 22, when member is braced against sideways. Strength is governed by the strength of the material and dimension of the cross section Slender COLUMN (Figure ): When rklu > 100 Where, r = radius of gyration Strength is influenced by slenderness, which produces additional bending because of transverse deformations. lu lu (a) Short COLUMN (b) Slender COLUMN Figure : Types of COLUMN COLUMN 139 According to the Geometry of Cross Section.

3 Figure : Types of COLUMN (continued) TiesSpiral (c) Tied COLUMN (d) Spiral COLUMN (e) Composite ColumnConcrete filled Steel pipeOr COLUMN 140 Slab Beam COLUMN SHORT COLUMN CONCENTRICALLY LOADED COLUMN Design of Tied COLUMN Step 1: Determination of Factored Load Load contribution to the COLUMN : (See Figure ) For columns of intermediate floor: load from upper floor through the COLUMN above it + slab and beam loads from corresponding floor tributary area.

4 For columns of top floor: slab and beam loads from adjacent tributary area Pu = D + L (ACI Code-00) Pu = D + L (ACI Code-02) Figure : Live Load contribution to a COLUMN (ACI ) Top floor Intermediate floor COLUMN 141 Step 2: Steel Ratio Assumption Assumption of steel ratio depends on designer s experience. According to ACI Code < g < Step 3: Determination of Concrete Gross Area According to ACI Code , the design strength for tied COLUMN Pn,max = [ c(Ag Ast)+fyAst] (5-1) Where, = (ACI Code-02), consider Pu = Pn Equation (4-1) can be rewritten as: Pu = Ag [ f c (1- g) + g fy] (5-2) Using equation (5-2) Ag is determined Step 4.

5 Selection of COLUMN Size ACI Code Commentary R eliminates the minimum sizes for compression members to allow wider utilization of reinforced concrete compression members in smaller sizes for lightly loaded structures. With the Ag as obtained in step 3 select a square (h2) or rectangular (b x h) section, such that the side dimensions are integer multiplier. Step 5: Check for Steel Ratio Putting the selected Ag in equation (5-2), g is calculated. If g = ; Design is ok Otherwise change the COLUMN dimension and repeat this step.

6 Step 6: Calculation of Reinforcement Total reinforcement, Ast = Ag * g Now choose required bar size satisfying the ACI Code requirements minimum number of bars > 4 Nos. COLUMN 142 Step 7 : Selection of Ties Reinforcement for ties should be selected in accordance with ACI Code : Use ties of # 3 bar for longitudinal bar up to # 10 Use ties of # 4 bar for longitudinal bar of # 11, 14, 18 and bundled bar Step 8 : Determination of Vertical Spacing of Ties According to ACI Code the vertical spacing shall be smallest of.

7 16 db of longitudinal reinforcement 48 db of tie bar Least dimension of COLUMN section Step 9 : Arrangement of Ties ACI Code specifies the arrangement of ties: The ties shall be so arranged that every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie having an included angle of not more than 135 .No bar shall be farther than 6 in. clear on either side from such a laterally supported bar. Where the bars are located around the periphery of a circle, complete circular ties may be used. Details in Figure COLUMN 143 6"< 6"< < 6"< 6"< 6"< 6"< 6" 6" m ax6" m ax4 bar6 bar8 bar12 bar16 bar>6in >6in >6in >6in Max 6in Figure.

8 Arrangement of COLUMN ties COLUMN 144 For bundled bar = db 2 in ( ACI Code )( ACI Code )Minimum 112"Concrete d >b 112 inClear distance (ACI Code ) Step 10 : Concrete Protection for Reinforcement Details are shown in Figure : Figure : Concrete cover in COLUMN Step 11 : Bar Splicing in columns The main vertical reinforcement in columns is usually spliced just above each floor, or at alternate floors.

9 According to ACI Code , lap splices, mechanical splices, but welded splices and bearing splices shall be used. Lap Splices: Code requirements for lap splices are given in Table Details of lap splices at typical interior COLUMN are shown in Figure In addition, for offset bar following should be observed: ACI Code , offset bars shall be bent before placement in the forms. ACI Code , where COLUMN face is offset 3 in. or greater, longitudinal bars shall not be offset bent. Separate dowels, lap spliced with the longitudinal bars adjacent to the offset COLUMN faces shall be provided.

10 Details in Figure COLUMN 145 Mechanical and Welded Splices: According to ACI Code , a full mechanical splice or welded splice shall develop at least 125% of the specified fy of the bar. Table : Requirements for lap splices Longitudinal bars in Compression Reduction by (ACI Code and ) fy 60,000 psi ls = fydb 12 in.