CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTSUNIT – IV LIMIT STATE DESIGN OF COLUMNS Types of columns Braced and unbraced columns Design of short Rectangular and circular columns for axial, uniaxial and biaxial bending Compiled by: Mr. S. MANIKANDAN, Assistant Professor, Department of Civil Engineering, Shanmuganathan Engineering College. 2 CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTS 2 Marks [Questions & Answers] 01) What is pedestal? Ans.: A very short column having the ratio of effective length (leff) to least lateral dimension (b or B) as less than there is called pedestal. It is also called stub column. It is generally of plain concrete. i.e. un-reinforced. 02) On what condition intermediate column is more suitable? Ans.: The intermediate column is more suitable, when the slenderness ratio more than three and less than or equal to 12 (3 < leff/d < 12). It is also known as Short column. 03) Differentiate long column and short column. Ans.: Long column Short Column A Column having the ratio of effective A Column having the ratio of effective length length (leff) to least lateral dimension (b or d) (leff) to least lateral dimension (b or d) as less as more than 12. than or equal to 12. The failure pattern of long Column is The failure pattern of short column is Buckling. Crushing. 04) What are braced column? Ans.: A braced column is one which the side sway is insignificant, i.e., there is no significant relative displacement between the two ends of the column in the direction under consideration; such columns do not resist any significant horizontal load like wind or earthquake loads. Examples: Columns of water towers with bracings at different heights and columns in tall buildings provided with shear walls. 05) Distinguish between braced and unbraced columns. Ans.: Braced column Un-braced Column In Braced Columns, the side sway is In Un-braced Columns, there is significant insignificant. lateral displacement between the ends. It does not resist any significant horizontal It resists lateral loads in addition to vertical loads like wind and earthquake loads. S.M.K./A.P./CIVIL/S.E.C. loads. UNIT IV – LIMIT STATE DESIGN OF COLUMNS 3 06) Enumerate axially loaded column. Ans.: The column which carries a purely axial load is termed as axially loaded column. It is also called concentrically loaded column. 07) What are the types of reinforcements used to resist shear force in columns? Ans.: There are two types of reinforcements used to resist shear force in columns. Such as 1. Lateral ties for Rectangular columns. 2. Helical reinforcements for Spiral columns. 08) What are the advantages of providing helical reinforcement instead of lateral ties in a circular column? Ans.: The following are the advantages for providing helical reinforcements instead of ties. (i) Spiral column failed gradually. (ii) It exhibiting considerable amount of ductility and deformation. (iii) There will be a lot of warning before final failure. 09) Write any two support conditions of columns. Ans.: The following are the list of support conditions of columns: (i) End rotation unrestrained (ii) End rotations restrained (iii) One end restrained other unrestrained (iv) Both ends partially restrained (v) End rotations restrained (vi) One end rotation restrained other end unrestrained (vii) One end rotation partially restrained, other end unrestrained (viii) Partially restrained at both ends 10) Write down the expression for minimum eccentricity? Ans.: As per clause 25.4 of IS 456: 2000, the minimum eccentricity equal to the unsupported length (l) / 500 plus lateral dimension (b and D) / 30. L e (min) = b + 500 L and 30 e (min) = D + 500 30 TWO MARK Q & A and BIG QUESTIONS 4 CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTS 11) According to IS code, all columns shall be designed for minimum eccentricity. Justify the reasons for this statement. Ans.: According to IS code, all columns shall be designed for minimum eccentricity. Because to avoid the imperfections in constructions, inaccuracy in loading etc. 12) What is the formula used in the Bresler's Load contour method of approach for short columns subjected to axial load and biaxial bending? Ans.: IS 456: 2000 (clause 39.6) has recommended the formula proposed by Bresler for the design of columns subjected to biaxial bending. The method is based on interaction surface for the load system Pu, Mux and Muy. Mux Muy + Mux1 < 1.0 Muy1 Where Mux1 and Muy1 are the uniaxial moment capacities combining the given axial load Pu with bending moments about x and y axes, respectively. 13) On what condition long column is more suitable? Ans.: A long column is susceptible to buckling. i.e. its load carrying capacity is influenced by the slenderness effect which produces additional moments resulting from transverse deformation. 14) Name any two methods used for design of long columns. Ans.: The following are the methods used for design of long columns: (i) The strength reduction coefficient method (ii) The additional method (iii) The moment magnification method 15) Define overturning on columns. Ans.: The unbalanced moments tends to topple the columns, this effects is called as overturning on columns. S.M.K./A.P./CIVIL/S.E.C. UNIT IV – LIMIT STATE DESIGN OF COLUMNS 5 16 Mark [Questions] CE2306 (R 2008) - April / May 2015 (A.U. Chennai) – Q.P. Code: 71258 (Or) CE2306 (R 2008) – November / December 2014 (A.U. Chennai) – Q.P. Code: 91243 (Or) CE2306 (R 2008) – May / June 2014 (A.U. Chennai) – Q.P. Code: 51242 (Or) TWO MARK Q & A and BIG QUESTIONS 6 CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTS CE2306 (R 2008) – November / December 2013 (A.U. Chennai) – Q.P. Code: 31210 (Or) CE2306 (R 2008) - May / June 2013 (A.U. Chennai) – Q.P. Code: 21210 (a) Design a column 4 m long restrained in position and direction at both ends to carry an axial load of 1600 kN. Use M 20 concrete and Fe 415 steel. (16) (Or) (b) Design a column size 450 mm X 300 mm. Use M 30 concrete and Fe 415 steel. Take Lex = 6 m, Ley = 5.5 m, Pu = 1600 kN, Mux = 45 kN-m at top and 40 kN-m at bottom. Muy = 40 kN-m at top and 25 kN-m at bottom. The column is bent in double curvature and assumes a cover of 50 mm. (16) CE2306 (R 2008) – November / December 2012 (A.U. Chennai) – Q.P. Code: 11194 (Or) CE2306 (R 2008) - May / June 2012 (A.U. Chennai) – Q.P. Code: 10229 S.M.K./A.P./CIVIL/S.E.C. UNIT IV – LIMIT STATE DESIGN OF COLUMNS 7 (Or) CE1354 (R 2004) - April / May 2011 (A.U. Chennai) – Q.P. Code: 55232 (Or) Compiled By: Mr. S. MANIKANDAN M.E. (Structural Engg.) Assistant Professor, Department of Civil Engineering, Shanmuganathan Engineering College, Arasampatti, Pudukkottai – 622507. Email: [email protected] TWO MARK Q & A and BIG QUESTIONS