Introduction to design of rcc column

In the Introduction of Design of RCC column, you will find types of the column,basic design fundamentals as per IS 456 2000

1. INTRODUCTION TO DESIGN OF RCC COLUMN AS PER IS 456-2000

2. COLUMN A column is defined as a vertical compression member which is mainly subjected to axial loads . Column transfer the load of superstructure to the foundation safely. Mainly column, struts and pedestals are used as compression members in buildings, bridges. If  the effective length exceeds three times its least lateral dimension it is called column. If the Effective length is less than three times its least lateral dimension is called pedestal.

3. Classfication of Column 4.Slenderness Ratio 3.Types of Loading 2.Materials of construction 5.Types of Lateral Reinforcement 1.Shape of cross section

4. 1.Shape of cross-section: Square Rectangular Circular Pentagonal Hexagonal Octagonal T-shape or L-shape etc.

5. 2.Material of Construction a. Timber Columns: Timber columns are generally used for light loads. They are used in small trusses and wooden houses. These are called as posts. b. Masonry Columns: These are used for light loads. c. R.C.C. Columns: R.С.С. columns are used for mostly all types of buildings and other R.С.С. structures like tanks, bridges etc. d. Steel Columns: Steel columns are used for heavy loads. e. Composite Columns: Composite columns are used for very heavy loads. They consist of steel sections like joists (I or H sections) embedded in R.C.C.

6. Timber RCC column Masonry Column Composite Column Steel Column

7. 3. Type of Loading a. Axially loaded column: The columns which are subjected to loads acting along the longitudinal axis or centroid of the column section are called as axially loaded columns. Axially loaded column is subjected to direct compressive stress only and no bending stress develops anywhere in the column section. b. Eccentrically loaded columns: Eccentrically loaded columns are those columns in which the loads do not act on the longitudinal axis of the column. They are subjected to direct compressive stress and bending stress both. Eccentrically loaded columns may be subjected to uniaxial bending or biaxial bending depending upon the line of action of load, with respect to the two axis of the column section.

8. Eccentric column

9. 4. Slenderness Ratio The slenderness ratio of a compression member is defined as the ratio of effective length to the least lateral dimension. The columns are classified as following two types depending upon the slenderness ratio: a. Short columns. b. Long columns. a. Short Columns: The column is considered as short when the slenderness ratio of column i.e ratio of effective length to its least lateral dimension (l/b) is less than or equal to 12. b.Long Column: If the slenderness ratio of the column is greater than 12, it is called as long or slender column.

10. 5.Type of Lateral Reinforcement An R.С.С. column has longitudinal and lateral reinforcement. They can also be classified according to the manner in which the longitudinal steel is laterally supported or tied. Column with longitudinal steel and lateral ties. Column with longitudinal steel and spiral ties.

11. Effective Length The vertical distance between the points of inflection of the compression member in the buckled configuration in a plane is termed as effective length le of that compression member in that plane. The effective length is different from the unsupported length l of the member, though it depends on the unsupported length and the type of end restraints. The relation between the effective and unsupported lengths of any compression member is Le = K x L

12. 1. Longitudinal Reinforcement in columns (as per IS 456:2000 ) The cross-sectional area of longitudinal reinforcement shall be not less than 0.8 percent nor more than 6 percent of the gross cross-sectional area of the column. Note: the use of 6 percent reinforcement may involve practical difficulties in placing and compacting of concrete, hence lower percentage is recommended. Where bars from the columns below have to be lapped with those in the column under consideration, the percentage of reinforcement steel shall usually not exceed 4 percent. In any column that has a larger cross-sectional area than that required to support the load, the minimum percentage of steel shall be based upon the area of concrete required to resist the direct stress and not upon the actual area. c) The minimum number of longitudinal bars provided in a column shall be four in rectangular columns and six in circular columns.

13. d) The bars shall not be less than 12mm in diameter. e) A reinforced concrete column having helical reinforcement shall have at least six bars of longitudinal reinforcement within the helical reinforcement. f) In a helically reinforced columns, the longitudinal bars shall be in contact with the helical reinforcement and equidistant around its inner circumference. g) Spacing of longitudinal bars measured along the periphery of the column shall not exceed 300mm.  h) In case of pedestals in which the longitudinal reinforcement is not taken into account in strength calculations, nominal reinforcement not less than 0.15 percent of the cross-sectional area shall be provided.  Note:  Pedestal is a compression member, the effective length of the which does not exceed three times the least lateral dimension.

14. (2) Transverse Reinforcement in columns: (a) A reinforced compression member shall have transverse reinforcement or helical reinforcement so disposed that every longitudinal bar nearest to the compression face has effective lateral support against buckling subject to provision The effective lateral support is given by transverse reinforcement either in the form of circular rings capable of taking up circumferential tension or by polygonal links (lateral ties) with internal angles not exceeding 135 degree. The ends of the transverse reinforcement shall be properly anchored.

15. Pitch and diameter of lateral ties (as per IS 456:2000) Pitch – The pitch of transverse reinforcement shall be not more than the least of the following distances: a. The least lateral dimension of the compression member b. Sixteen time the smallest diameter of the longitudinal reinforcement bar to be tied c. Forty-eight times the diameter of the transverse reinforcement. Diameter – The diameter of the polygonal links or lateral ties shall be not less than one-fourth of the diameter of the largest –longitudinal bar, and in no case less than 5mm.