The Constructables

Design of RCC Columns

RCC Column :

A column is a very important component in a structure. It is like the legs on which a structure stands. It is designed to resist axial and lateral forces and transfer them safely to the footings in the ground.

Columns support floors in a structure. Slabs and beams transfer the stresses to the columns. So, it is important to design strong columns.

A column is defined as a compression member, the effective length of which exceeds three times the least lateral dimension. Compression members whose lengths do not exceed three times the least lateral dimension, may be made of plain concrete.

The axial load carrying capacity of a column is deduced from the formula given below :

I would recommend using advanced structural design software like ETabs or Staad Pro for design of structures. Column design does not depend only on axial loads, but also on many other factors. There are bending moments and tortional forces induced due to beam spans, wind loads, seismic loads, point loads and many other factors.

In this article, we are going to discuss in detail the basics of classification of columns and different types of reinforcement required for a certain type of column.

A column may be classified based on different criteria such as:

1. Based on shape :

Rectangle
Square
Circular
Polygon

2. Based on slenderness ratio :

The ratio of the effective length of a column to the least radius of gyration of its cross section is called the slenderness ratio.

Short RCC column, =< 10
Long RCC column, > 10
Short Steel column, =<50
Intermediate Steel column >50 & <200
Long Steel column >200

3. Based on type of loading :

Axially loaded column
A column subjected to axial load and unaxial bending
A column subjected to axial load and biaxial bending

4. Based on pattern of lateral reinforcement :

Tied RCC columns
Spiral RCC columns

Minimum eccentricity :

E_min > l/500 + D/30 >20

Where, l = unsupported length of column in ‘mm’

D = lateral dimensions of column

Types of Reinforcements for columns and their requirements :

Longitudinal Reinforcement :

Minimum area of cross-section of longitudinal bars must be atleast 0.8% of gross section area of the column.

Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross cross-section area of the column.

The bars should not be less than 12mm in diameter.

Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column.

Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm.

Transverse reinforcement :

It maybe in the form of lateral ties or spirals.

The diameter of the lateral ties should not be less than 1/4^th of the diameter of the largest longitudinal bar and in no case less than 6mm.

The pitch of lateral ties should not exceed :

Least lateral dimension

16 x diameter of longitudinal bars (small)

300mm

Helical Reinforcement :

The diameter of helical bars should not be less than 1/4^th the diameter of largest longitudinal and not less than 6mm.

The pitch should not exceed (if helical reinforcement is allowed);

75mm
1/6^th of the core diameter of the column

Pitch should not be less than,

25mm
3 x diameter of helical bar

Pitch should not exceed (if helical reinforcement is not allowed)

Least lateral dimension :

16 x diameter of longitudinal bar (smaller)
300mm

TABLE FOR

AXIAL LOAD CARRYING CAPACITY OF COLUMN BASED ON PERCENTAGE OF REINFORCEMENT AND COLUMN SIZE FOR VARIOUS MIXES AND STEEL

Steel Grade Fy 415

CONCRETE GRADE	Axial Load carrying Capacity in KN (P)
M15	P= (2.7205 p + 6) b D/1500
M20	P= (2.7005 p + 8) b D/1500
M25	P= (2.6805 p + 10) b D/1500
M30	P= (2.6605 p + 12) b D/1500
M35	P= (2.6405 p + 14) b D/1500
M40	P= (2.6205 p + 16) b D/1500

Steel Grade Fy 500

CONCRETE GRADE	Axial Load carrying Capacity in KN (P)
M15	P= (3.29 p + 6) b D/1500
M20	P= (3.27 p + 8) b D/1500
M25	P= (3.25 p + 10) b D/1500
M30	P= (3.23 p + 12) b D/1500
M35	P= (3.21 p + 14) b D/1500
M40	P= (3.19 p + 16) b D/1500

Steel Grade Fy 550

CONCRETE GRADE	Axial Load carrying Capacity in KN (P)
M15	P= (3.625 p + 6) b D/1500
M20	P= (3.605 p + 8) b D/1500
M25	P= (3.585 p + 10) b D/1500
M30	P= (3.565 p + 12) b D/1500
M35	P= (3.545 p + 14) b D/1500
M40	P= (3.525 p + 16) b D/1500

Note :-

The axial Load carrying capacity column is arrived based on the formula

P_u=0.4 f_ck A_c + 0.67 f_y A_sc as per IS 456-2000.

Here in the Table P is Axial Load Carrying capacity of column in KN.

p = steel in percentage say percentage as 1.

b = breadth of column in mm.

D = depth of column in mm.