Columns

COLUMNS

Column is a vertical structural member. It transmits the load from ceiling/roof slab and beam, including its self-weight to the foundation. Columns may be subjected to a pure compressive load or a combination of compressive load and bending moment. Columns may be constructed of Stone or Brick Masonry, Timber, Reinforced Cement Concrete (R.C.C.) or Structural Steel Sections. R.C.C. columns are the most widely used now-a-days.

 

1. DEFINITIONS

Effective Length: Effective length of a column is not necessarily its Actual or True Length. It depends upon the conditions of end supports of the column.

Column Failure: It depends on the Effective Length of the column compared to its cross-section.

Types of Columns: Based on the Effective Length, columns are classified as follows:

(i) Long Column or Slender:. If the ratio of the effective length to the least lateral dimension of the column (or diameter of the column of circular cross-section) is > 12, the column is known as Long Column or Slender. Slender means narrow in width.

In a long column, the ultimate load is governed by the compressing and bending strengths of the material of the column and the dimensions of the cross section. In addition, the ultimate load is also influenced by slenderness. Thus, for long columns, failure load is characterized by buckling or instability.

(ii) Short Column: If the ratio of the effective length to the least lateral dimension is < 12, the column is known as Short Column. In a short column, the ultimate load is governed only by the compressive strength of the material of the column and the dimensions of the cross section. For short columns, the failure load is characterized by yielding.

(iii) Intermediate Column: Column with length that is between the short and long columns is called Intermediate Column. Intermediate columns fail by both yielding and buckling.

 

2. R.C.C. COLUMNS

If the effective length of a compression member is equal to or less than thrice the least lateral dimension, the member is made of Plain Cement Concrete. If the effective length is more than thrice the least lateral dimension, the member should be of R.C.C.

1. Columns of Square, Rectangular and Circular Sections

See Fig. 1 (Square), Fig. 2 (Rectangular) and Fig. 3 (Circular).

R.C.C. columns are reinforced with Vertical Longitudinal Steel Bars (Vertical Main Reinforcement Steel Bars) tied by Lateral Steel Bars, which act as Lateral Ties. Lateral ties are also known as Stirrups.

Stirrups: Stirrups are tying the main bars so that the main bars should not buckle outward under load. Thus, lateral ties provide lateral rigidity to the column. Also, these ties keep the main bar in position without getting dislocated during concreting and compaction.

In the R.C.C. columns, longitudinal bars take the main load. Stirrups prevent the buckling of longitudinal bars and keep them in position with respect to each other.

Load Carrying Capacity of R.C.C. Column: If the percentage of steel used is large in a column, the concrete fails first.

If the percentage of steel used is less, the steel will reach its yield strength prior to concrete. The column will not fail, because it can take more loads at this stage. Thus, the strength of the column is fully utilized.

2. Spirally Reinforced Column (Fig. 4)

Sometimes, for circular column, the lateral ties are made in the form of a continuous Spiral. It is called Spirally Reinforced Column.

Pitch: Pitch should not be more than 1/6 diameter of the column.

3. Precautions to be observed in Column Construction

1. Process of Construction

Construction of concrete columns (i.e., concreting, compacting and curing) is similar to the construction of any R.C.C. works such as beams, etc.

2. Vertical Height of Concrete

Vertical height of concrete placed in one layer should not exceed one meter in height.

3. Columns for more than one Storey

Concreting of columns for more than one storey is to be carried out very accurately. Central lines of column must be accurately kept one above the other for columns placed in different floors. Otherwise, eccentricity in loading will result.

4. Load on the Column of the Top Storey

The loads on the top storey are relatively lesser. Hence, the cross section of the colum designed for the top storey should be relatively less.

Uses of R.C.C. Columns

R.C.C. columns are used in multi-storey buildings and heavily loaded structures.

 

3. STEEL COLUMNS or STANCHIONS

Steel Columns are also known as Stanchions. These are used in industrial structures to support the Beams and Trusses.

Steel Column is a vertical compression member. It supports the Floors or Girders in a building. Girder is a main load carrying member into which floor beams or joists are connected. Joists are beam members, which are used to carry floors and roofs of buildings of light loads supported on long columns.

Standard Structural Steel Sections

Structural steel is factory made in several forms called Rolled Steel Sections. See Figs. 5 and 6. Standard steel sections, namely, I-Section, Channel Section, Equal Angle Section, Tee Section, Z Section, Round Section and Tubular Section are used for light loads.

The shape of the cross section of the steel section and the sectional area should be designed carefully to avoid buckling.

The important property required for a compression member is a high value of Moment of Inertia. [Moment of Inertia of a body about any axis refers to the internal resistance of the body to rotation about that axis.]

1.  Built-up Column Sections (Fig. 7)

When the loads on the column are heavy Built-up Column Sections are used.

When two or three steel sections of rolled sheets and cover plates are connected to form a column, it is known as a Built-up Column or Box Column. Cover Plates (or Additional Plates) are properly welded with the main sections.

Fig. 7 shows different types of built-up sections for steel columns. The type of built-up ection to be used depends on the type of structure and the end connections of the column.

Uses: Built-up sections are used in steel skeleton constructions, roof trusses, etc.