Columns and Classification of Columns

Introduction

A compression member column is an important element of every reinforced concrete structure. These are used to transfer the load of superstructure to the foundation safely. Mainly column, struts and pedestals are used as compression members in buildings, bridges, supporting system of tanks, factories and many more such structures.

A column is defined as a vertical compression member which is mainly subjected to axial loads and the effective length of which exceeds three times its least lateral dimension.

Classification of Columns

Columns are classified based on different criteria such as:

1. Shape of cross section.
2. Material of construction.
3. Type of loading.
4. Slenderness ratio.
5. Type of lateral reinforcement.

Shapes of Cross-section

On the basis of shape of the cross-section of the column, the column may be classified as following

1. Square
2. Rectangular
3. Circular
4. Pentagonal
5. Hexagonal
6. Octangonal
7. T-shape or L-shape etc.

Material of Construction

Columns may be classified as following, as per the material used for construction.

Timber Columns:

Timber columns are generally used for light loads. They are used in small trusses and wooden houses. These are called as posts.

Masonary Columns:

These are used for light loads.

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.

Steel Columns:

Steel columns are used for heavy loads.

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.

Type of Loading

A column may be classified as follows, based on type of loading:

1. Axially loaded columns.
2. Eccentrically loaded columns.

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.

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.

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:

1. Short columns
2. Long columns.

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.

Long Column:

If the slenderness ratio of the column is greater than 12, it is called as long or slender column.

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.

1. Column with longitudinal steel and lateral ties.
2. Column with longitudinal steel and spiral ties.
3. In this type of arrangement the longitudinal bars are tied laterally at suitable internals with the help of ties.
4. Circular column with spirals i.e. the longitudinal bars are tied continuously with the help of a spiral reinforcement. The columns with helical or spiral reinforcement are better in providing lateral support to bars as compared to links thus they increase the buckling resistance and ductility of the column.