Unit IV - Limit State Design of Columns

CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTS

UNIT – IV 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. by: Mr. S. MANIKANDAN, Assistant Professor, Department of Civil Engineering, Shanmuganathan Engineering College.

CE6505 – DESIGN DESIGN OF REINFORCED CONCRETE ELEMENTS

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2 Marks [Questions & Answers] 01) What is pedestal? Ans.: A very short column having the ratio of effective length ( l eff eff ) 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 < l eff eff /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 (l eff ) 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 eart hquake loads. Examples: 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.

loads.

S.M.K./A.P./CIVIL/S.E.C.

UNIT IV – LIMIT LIMIT STATE DESIGN OF COLUMNS

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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 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

and

+ 500

L

30

e (min)

=

D +

500

30

TWO MARK Q & A and BIG QUESTIONS


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11) According to IS code, all columns shall be designed for minimum eccentricity. Justify the reasons for this statement. 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 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 capaciti es combining the given axial load P u with bending moments about 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.



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16 Mark [Questions] CE2306 (R 2008) - April / May 2015 (A.U. Chennai) – Q.P. Q.P. Code: 71258

(Or)

CE2306 (R 2008) – November November / December 2014 (A.U. Chennai) – Q.P. Q.P. Code: 91243

(Or)

CE2306 (R 2008) – May May / June 2014 (A.U. Chennai) – Q.P. Q.P. Code: 51242

(Or)



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CE2306 (R 2008) – November November / December 2013 (A.U. Chennai) – Q.P. Q.P. Code: 31210

(Or)

CE2306 (R 2008) - May / June 2013 (A.U. Chennai) – Q.P. 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, M ux = 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 November / December 2012 (A.U. Chennai) – Q.P. Q.P. Code: 11194

(Or)

CE2306 (R 2008) - May / June 2012 (A.U. Chennai) – Q.P. Q.P. Code: 10229



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(Or)

CE1354 (R 2004) - April / May 2011 (A.U. Chennai) – Q.P. Q.P. Code: 55232

(Or)

Compiled By:

Mr. S. MANIKANDAN M.E. (Stru (Stru ctural En gg.) gg.) Assistant Professor, Department of Civil Engineering, Shanmuganathan Engineering College, Arasampatti, Pudukkottai – 622507. 622507. Email: [email protected] Email: [email protected]


Unit IV - Limit State Design of Columns

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