In the construction industry, a column is a vertical structural member that primarily supports loads from above. Columns play an essential role in providing strength, support, and sustainability for structure. In this article, we will discuss the column classification, reinforcement process, load calculation, developing length, and everything about columns.
What is a column?
A “column” is a vertical structure member that carries loads of roof slab, lintel, and ceiling in compression. Basically, “column” is also a “compression member”.
How to classify the column?
Many types of columns are classified into four types that are used in construction.
- Based on shape
- Based on construction Material
- Based on types of reinforcement
- Based on the type of loading
- Based on the slenderness ratio
Based on shape column types:
Different shapes are used according to the project requirement.
- Square or rectangular column: These types of columns are provided only if the shape of the room is square or rectangular. Much easier to construct and work with the shuttering.
- Circular columns: This type of column occupies/consumes less area that why it’s used as a pier and mostly where there is traffic expected. Use this column to avoid edges.
- L-Type column: They are the corner column and are generally used in the corners of the boundary wall and have the same features as a rectangular or square column.
- T-Type column: This kind of column is used depending on design requirements and in the construction of bridges etc. This has the same features as rectangular or square columns.
- V-Type column: As the name itself, it showcases the column is in a V shape and is generally used if the shape of the room is trapezoidal. As it requires more concrete when compared to the other columns.
- Hexagon Column: These columns are generally modified columns. It has six sides and it gives a good pictorial view. Generally used in elevation and provided in open verandahs, auditoriums, cinema halls, etc.
- Arch-type columns: These types of columns are used when the room has the shape of an arch. It is adopted where there is no chance of building square, rectangular, or circular types of columns. Also, it is difficult to cast.
- Y-Type column: these types of columns are carved from their edges or sides. Also, these columns are used in the construction of bridges, flyovers, etc, when there are heavy loads on the top.
- Y-Type column with arch: As the same as the Y-Type column but it has curved edges or sides that are provided below the bridges and flyovers where there is a congestion of buildings.
Based on the type of reinforcement:
These types of columns are divided into the following.
- Tied column: When the main longitudinal bars are confined within closely spaced lateral tie reinforcement, it is called a tied column. Approximately 95% of the columns used in buildings are tied. Mostly used in building/house construction.
- Spiral column: When the main longitudinal bars of the column are enclosed within closely spaced continuously wound spiral reinforcement, it is called a spiral column.
- Composite column: When the longitudinal reinforcement is in the form of a structure steel section or pipe with or without longitudinal bars, it is called a composite column.
Based on the type of loading:
Depending on the type of load, columns are classified into three types.
- Axially loaded column: When the vertical axial load acts/is applied on the center of gravity of the cross-section of the column, it is called an axially loaded column.
- Uniaxially eccentric loading: When the vertical axial load does not act/apply on the center of gravity of the cross-section of the column, it is called as uniaxially eccentric loading.
- Biaxial Eccentric Loading: When the vertical axial load does not act/ apply eccentrically on both the x-axis or y-axis, it’s known as biaxial eccentric loading.
Based on the slenderness ratio:
Depending upon the slenderness ratio the columns are classified into three types.
- Short Columns: If the ratio effective length of the column to the least lateral dimension is less than 12, the column is called as the short column.
- Long column: If the ratio effective length of the column to the least lateral dimension exceeds 12, the column is called as the short column
- Intermediate column: A column that fails by the combination of both crushing and bulking is called an intermediate column.
What is the reinforcement in a column?
Reinforced concrete columns are the structural members that are designed to carry the loads. Let’s discuss the step-by-step procedure of column reinforcement.
PCC formwork: Place the formwork to support the initial structure.
Cover block: Place the clear cover to maintain the distance between the reinforcement bars.
Main bars: To ensure the structure’s strength, place the main bars at the button.
Distributive bars: At the top of the main bars place the distributive bars to support loads by providing lateral stability.
Top bars: Top bars are located at the upper portion of structural elements to provide more strength and resistance to tensile forces.
Column bars: Now, column bars are longitudinal steel rods positioned inside vertical structural components providing strength and stability to withstand compressive loads.
Column Ties: Transverse steel supports that are placed around the column bars to secure and stabilize the vertical reinforcement in column.
Footing Formwork: Position the temporary footing formwork to shape and contain the concrete.
Footing concrete: Pour the concrete into the formwork.
Column starter formwork: This is placed in the initial temporary structure employed to shape and support the lower portion of the vertical concrete.
Column starter concrete: Pour the concrete into the formwork to ensure the proper bonding as an initial layer.
Remove the starter column formwork: Remove the formwork.
Column formwork column height: Now, place the formwork around the column reinforcement to shape and contain the concrete.
Column concrete: Pour the concrete into the column formwork.
How to Calculate Concrete Columns?
Here are several steps as a key point to calculate the column size as per ACI. You can use the online calculator or use the formula to Calculate Concrete Columns.
Concrete column load capacity:
Engineers commonly calculate the load capacity to determine the load capacity of the column to know strength, reinforcement area, and durability.
Load Capacity Calculation (ACI):
Load capacity = (0.85 * f’c * Ag) + (0.75 * fy * As)
- f’c= concrete compressive strength
- Ag=gross area of the column
- Fy= yield strength of reinforcement
- As=Area of the longitudinal reinforcement
Concrete Column Strength:
To determine the maximum weight a reinforced concrete column can support.
Strength Calculation (ACI):
Pn = 0.85 * φ * Ag * f’c.
- Pn= nominal axial strength
- Φ= resistance factor
- Ag= gross area of the column
- f’c= compressive strength of the concrete
Concrete Column Size:
Measure column size in terms of length, width, and cross-sectional area. Follow these steps to calculate.
- Determine the design axial load.
- Determine the moment on the column.
- Combine loads per design code.
Size Calculation (ACI):
Employing the formula, calculate the required cross-sectional area.
A = (P + Mc) / fc
- P=axial load
- Mc=moment load
- Fc=compressive strength of concrete
Note: Ensure the slenderness ratio to ensure stability. Also, use the column size calculator to determine the accurate result.
How to determine Column Load calculation?
To determine the load on the column follow several steps. You can calculate it online or by using the formula. Now, to know the process of column load calculation.
|Column Load factors
Types of Loads on Columns
- Self-weight of the column multiplied by the number of floors
- Self-weight of beams per running meter
- Load of walls per running meter
- Total Load of slab (Dead load + Live load + Self weight)
Find Self-weight column load calculation:
As we know,
- Concrete self-weight=2400 kg/m3
- Steel self-weight=7850 kg/m3
- Steel percentage= 1% of the concrete column
- Column self-weight=1000kg per flour
- The volume of the column=L×W×H
= 0.459 m³
- Weight of concrete= Volumeconcrete ×Densityconcrete
=0.459 m³ x 2400 kg/ m³
= 1101.60 kg
- Weight of steel=Vconcrete × 1% × weight concrete
=0.459 m³ x 1% x 7850 kg/ m³
= 36.03 kg
- Total self-weight=Weightconcrete + Weightsteel
Total weight in kilonewtons(1KN=101.9716):
- Total weight= WTotal/101.9716
Hence, the column total weight is 11.12kN
What is the development length of the column?
Length of steel bars that hold two concrete members together including column and beam. Minimum length of steel bar to be inserted inside the concrete column to ensure sufficient adhesion between concrete and steel. It is also known as anchorage length which is denoted as Ld.
Development length in the column:
In the column, the development length is an essential part that is placed between the longitudinal reinforcement and the surrounding concrete. So, the proper bond remains between them, and no chance of separating the beam and the column.
Development length formula for column:
This below given formula is used to calculate the required development length in mm for any given dia of bars.
Ld = (Φ × σs)/(4 × τbd),
- Ld= development length
- Φ=diameter of the bar
- σs=stress in bars at design load
- Τbd= design bond stress
Development Length in Compression:
It is the measurement of how much a column bar goes into the concrete footing vertically.
Development Length in Tension:
It refers to how much a column bar sticks into the concrete footing while bending.
The body strength between the beam and column would be insufficient to hold the beam in its position.
In that case, the beam will come out of the column which is why development length is necessary to be provided.
For bundle bars:
|3- Bar bundle
How to calculate the column ring size?
Column rings are used to keep reinforcement in place and enhance resistance against lateral forces. Also, the stirrups improve the load-bearing capacity. However, there are different shapes of columns with n number of bars. Now, we discuss the column ring cutting length formula.
Cutting Length of Rectangular Stirrups Formula:
- Rectangular column ring= 2(a+b)+2×number of hooks−3×number of 90° bends−2×number of 135° bends
Cutting Length of Square Stirrups Formula:
- Square ring size = 4a + 2× numbers of hooks – 3× numbers of 90° bends – 2 numbers of 135° bends
Cutting Length of Circular Stirrups Formula:
- Length of Circular ring = 2πr + 2 ×numbers of hooks – 2× numbers of 135° bends
Cutting Length of Helical Stirrups Formula:
- Length of one spiral ring: 2πR+2×number of hooks−2×number of 135° bends.
Cutting Length of Triangle Stirrups Formula:
- Cutting Length of ring=2H+a+2×number of hooks−4×number of 135° bends.
Cutting Length of Diamond Stirrups Formula:
- Cutting length of ring: 4H+2×number of hooks−3×number of 90° bends−2×number of 135° bends.
What is clear cover for column?
Clear column cover size is the minimum possible space between the outside face of the concrete fiber and the outer face of the main reinforcement except plaster thickness and concrete finishing.
Clear Cover for Columns:
|Column cover size
|40mm to 50mm
|Optimal for dry weather
|Optimal for wet/moist weather
What is lap length for column?
Lap length in column is provided to reach the required length where single bars fall short and to maintain the continuity of bars to safely transfer the load from one bar to another bar. So, the overlapping between the two bars is called lap length.
Formula for lapping length in column:
Lap length= Development Length Coefficient × Diameter of Reinforcing Bar
Lap length in column As per IS code 456-2000:
- The overlapping length should not be less than 75mm.
- In the case of columns generally, we take 24d-40d where d is the diameter of the bar.
Lap Splices and Welding:
- It is not recommended to use lap splices on bars larger than 36 mm in diameter.
- For larger bars, welding or adding extra spirals (6 mm) is suggested.
Column Lapping Length for Concrete Mix:
Tension lap length for mild steel (MS) bar in 1:2:4 nominal mix concrete is 58d. So, eliminating the anchorage value then
Lap length=58- 2*9d = 40d
For M20 concrete:
- Columns: 45d.
- Beams and slabs: 60d.
Note: To determine the lap length of reinforcement bars in concrete columns for structural engineering design that depend on the number of factors including structure king, concrete strength, and reinforcement bars diameter.
Frequently Asked Questions:
Q. How to decide the size of a column for a residential building?
Find out the dimensions of a column carrying an ultimate axial load of 950 KN and the grades of concrete and steel used for casting this column are M20 and Fe-415 respectively.
Q. Why do we provide columns?
Columns are mainly designed for compression as they support axial loads. Bending in the column may result from additional weights produced through the wind, snow, or other horizontal stresses. After that, columns need to be developed to withstand bending and axial load.
Q. What is column Beam?
A building component that experiences both transverse bending and axial compression simultaneously is referred to as a beam-column. A possible cause of the combined compression and bending is an eccentrically applied axial stress.
Q. What will happen if we do not provide development length?
If we provide less development length reinforcement bars will split from concrete.
Columns are usually made of materials like concrete, steel, or masonry, and are usable in different shapes and sizes depending on the specific structural requirements. Hopefully, this article will help you to understand column reinforcement, loud calculation, column cover, lapping length, and developing length.