Columns are the most important structural element in buildings. In this article, we will discuss the definition of columns. Also, a short description of all column types is presented here.
What is Column?
Columns are defined as vertical load-bearing members supporting axial compressive loads chiefly. This structural member is used to transmit the load of the structure to the foundation. In reinforced concrete buildings beams, floors, and columns are cast monolithically. The bending action in the column may produce tensile forces over a part of cross-section. Still, columns are called compression members because compressive forces dominate their behavior.
Concrete columns can be roughly divided into three categories- Pedestals, Short reinforced columns, and Long reinforced columns. Besides in modern days columns can be classified in different categories on a different basis.
Types of Columns
Columns can be of many types based on loading, length, column ties, frame bracing, etc. The types of columns used in construction are as stated below:
- Based on Loading
- Based on Column Ties
- Based on Slenderness Ratio
- Based on Shape of Cross Section
- Based on Construction Materials
- Based on Frame Bracing
- Other TYpes
All these types of columns are discussed below.
Classification of Column Based On Loading
Axially Loaded Column
If the compressive vertical loads act along the centroidal axis of the column, it is termed as an axially loaded column. This type of column without bending is not found practically so much.
Eccentrically Loaded Column: Uniaxial
When the loads are acting at a distance ‘e’ from the centroid of the column cross-section, the column is termed as an eccentrically loaded column. In an uniaxial eccentrically loaded column this distance ‘e’ could be along x-axis or y-axis. These eccentric loads cause moments along the x-axis or y-axis.
Eccentrically Loaded Column: Biaxial
In this type of column, loads are applied at any point of cross-section but not in axes. Loads cause moments about both the x- and y-axes simultaneously.
Classification of Column Based On Column Ties
In the tied column, the longitudinal bars are tied together with smaller bars. These smaller bars are spaced at uniform intervals up the column. Steel ties in column confine the main longitudinal bars. Over 95 percent of all columns in buildings in non-seismic regions are tied columns.
Spiral columns contain spirals to hold the main longitudinal reinforcement. Spiral is spring type reinforcement. The main bars are placed in a circle and ties are replaced by spirals. Spiral columns are used when high strength and/or high ductility are required. Because the spiral acts to resist the lateral expansion of the column bars under high axial loads. The main bars are placed in a circle and ties are replaced by spirals. Spiral columns are used more extensively in seismic regions.
Classification of Column Based On Slenderness
Short Compression Block or Pedestals
A pedestal is a compression member having a height less than three times its least lateral dimension. Pedestals need not be reinforced and may be designed with plain concrete.
Short Reinforced Column
The slenderness ratio (ratio of effective length to the least lateral dimension) is less than 12 in the short reinforced column. Short columns fail due to crushing or yielding of the steel bars. The loads that a short column may support depend on the dimension of cross-section and the strength of materials. Short columns show a little flexibility.
Long Reinforced Column
The slenderness ratio exceeds 12 in long columns. This type of column is also known as the slender column. As the slenderness increases, bending deformation increases. Long column fails due to buckling effect which reduces load-bearing capacity.
Classification of Column Based On Shape Of Cross Section
Column sections can be rectangular, round, square, octagonal, hexagonal as per requirements. Generally tied columns may be square and rectangular while spiral columns are circular. Circular columns are used when higher elevation is needed like in piles, bridges pillars. Circular columns provide a smooth and aesthetic finish. On the other hand, rectangular columns are found in residential and official buildings. They are easy and less costly to cast.
This type of column is unpopular. The L-shaped column can be used as a corner column in a framed structure. This design of the column can be a good replacement to resist both axial compression and biaxial bending of corners.
In the trapezoidal structure, this type of column can be used. V-shaped columns need more materials comparatively.
T-shaped columns may be used in bridge pillars depending on design requirements.
Classification of Column Based on Construction Materials
Reinforced Concrete Column
Reinforced concrete columns are the most widely used columns for framed structure. This type of column is composed of concrete as a matrix. The steel frame is embedded in concrete. Concrete carries the compressive load and reinforcement resists tensile load. The reinforcing materials can be made of steel, polymers, or alternate composite materials. For a strong, ductile, and durable construction the reinforcement needs to have some properties such as thermal compatibility, high resistance to tensile stress, good bond to concrete, anti-corrosive, etc.
Composite columns are constructed using various combinations of structural steel and concrete. The interactive and integral behavior of concrete and the structural steel elements makes the composite column a very stiff, more ductile, cost-effective, and consequently a structurally efficient member in building and bridge construction. This type of column has great fire and corrosion resistance also.
Steel, Timber, Brick Column
Steel columns are made of steel entirely. These columns are used in aircraft manufacturing warehouses, indoor shipyards, etc.
Timber columns are made of wood timber. They provide an aesthetic appearance creating a feeling of space and openness. Timber columns are designed for housebuilders, reception areas, and refurbishment properties.
Brick columns are found in masonry structures. They can be reinforced with concrete to increase strength or can be unreinforced. Brick columns can be a round-shaped, rectangle, or square, or elliptical in cross-section.
Classification of Column Based on Frame Bracing
Columns may be part of a frame that is braced or unbraced against sideways. Lateral stability to a structure as a whole is provided by bracing. Bracing can be obtained by using shear walls or bracings in the building frame. In braced frames relative transverse displacement of upper and lower ends of a column is prevented. Braced columns prevent gravity loads and shear walls prevent lateral loads and wind loads.
Unbraced columns resist both gravity load and lateral load. As a result, the load capacity of the column reduces.
Some Other Types Of Column
Prestressed Concrete Column
Prestressed columns can be used as an extension of the reinforced concrete columns when bending moments due to wind and earthwork forces, eccentric loads, or frame action are applied to columns. Prestressing transforms a cracked section into a non-cracked one and resists significant bending. This type can be found useful when the column is a high slender column and precast column.
Greek And Roman Column
Classical Greek and Roman architecture made use of four major styles of columns for their buildings and temples. These four types of columns were Doric, Ionic, Corinthian, and Tuscan. These columns look straight and uniform from a distance. But up close, they might actually tilt a bit, or lean left or right.