Introduction
In structural engineering, the selection of column shapes plays a pivotal role in determining a building's overall structural integrity, aesthetics, and functionality.
Many shapes and types of columns are available, each offering unique advantages and disadvantages in terms of aesthetics, connection options, structural capacity under various loads, and suitability for different building types.
This comprehensive guide aims to provide structural engineers with a detailed analysis of the various column shapes available, along with their applications, material types, and considerations for optimal column design.
Table of contents
- Common column shapes: Aesthetic and connection considerations
- Structural capacity and material types
- Cost considerations
- Ideal building types and applications
Common column shapes: Aesthetic and connection considerations
Typically, the most critical decision regarding column selection relates to the desired aesthetic within the building and the types of loads that the column has to carry, such as live load, dead load, and lateral loads like seismic and wind loads.
Given that many columns of different sizes are available, the architect will often specify a particular column shape to achieve the desired appearance within the building.
The structural engineer can request to adjust the column shapes if there is a structural capacity or constructability issue, but typically, the engineer adopts the column shape specified by the architect.
In many instances, columns can be clad in a timber facade and painted to make them aesthetically more pleasing.
Depending on the nature and amount of load passed to the column, a suitable connection type will need to be designed. Typically, columns with a flat flange have many options for connection types, whereas circular columns are quite limited.
Let�s explore the aesthetic and connection considerations for each column shape.
Circular columns
Aesthetic
Circular columns often convey a sense of elegance and modernity, making them suitable for contemporary architectural designs. They offer a sleek appearance and smooth transitions between elements.
Figure 1: Circular columns in a modern design
Connection options
Circular columns typically utilise welded connections in steel structures and embedded reinforcement in reinforced concrete structures (refer to the Detailing of Reinforced Concrete to AS 3600 article for more information on reinforced concrete detailing).
These connections ensure seamless integration with beams and other structural elements. These connection types are more challenging to construct, more labour-intensive, and hence more costly.
Figure 2: Welded & bolted beam-column connection to circular hollow section
Square columns
Aesthetic
Square columns offer a more traditional or contemporary aesthetic compared to circular columns. They provide clean lines and symmetry, making them suitable for various architectural styles.
Figure 3: Traditional style square columns
Connection options
Due to their flat surfaces, square columns facilitate easy connections with beams and other elements. Through a welded end plate or equal angle flanges on a beam, bolted connections are commonly used in steel structures, while embedded reinforcement is utilised in reinforced concrete structures.
Figure 4: Simple bolted equal angle connection to square hollow section
Rectangular columns
Aesthetic
Rectangular columns offer versatility in architectural design, balancing the sleekness of circular columns and the simplicity of square columns.
They are suitable for both modern and traditional architectural styles.
Figure 5: Rectangular columns in an apartment building
Connection options
Rectangular columns offer similar connection options to square columns, with bolted connections through a welded end plate or equal-angle flanges on a beam.
They are common in steel structures and embedded reinforcement in reinforced concrete structures.
Refer to the Lap Length to AS 3600 article for further information on embedding reinforcement in concrete connections.
Figure 6: Reinforcement starter bars for rectangular column connection into a suspended slab
H-shaped columns (I-section)
Aesthetic
H-shaped columns, also known as I-section columns, offer an industrial aesthetic and are commonly used in warehouses, industrial, and commercial buildings.
Figure 7: Universal columns are commonly used in portal frames in warehouses
Connection options
H-shaped columns provide versatility in connection options, including welded connections in steel structures and bolted connections in certain applications.
For superior structural capacity, beams should have a welded end plate that is bolted through the flange of the beam.
In some situations, the web of the I-section can be packed with timber, and timber beams can be connected to this, but it provides less structural capacity, given that bending in the column will occur via the minor axis rather than the major axis.
Figure 8: Simple universal beam - column Connection via a bolted end plate
Structural capacity and material types
Once the architect has specified the desired aesthetic and hopefully considered the connection type (engineers may need to assist!), then the structural engineer needs to consider whether the structural capacity of the column shape is suitable for the application and consider the appropriate material type to achieve the required structural capacity.
Reinforced concrete exhibits the highest load capacity under bending shear and axial load in residential and light commercial buildings but takes up the most space and is the most costly.
Steel comes in next in terms of structural capacity, is relatively compact, and is mid-range in price.
Timber is the cheapest column material type but exhibits the weakest resistance to bending, shear, and axial forces.
In Australian Standards, slenderness is a key parameter used to assess the stability of columns under various loading conditions.
Slenderness is defined as the ratio of a column's effective length to its least radius of gyration. It quantifies the degree of column instability due to buckling.
The slenderness ratio (?) is calculated using the following formula:
? = L e f f r \lambda=\frac{L_{eff}}{r} ?=rLeff??
L e f f L_{eff} Leff? = Effective length of the column r r r = Least radius of gyration of the column cross-section
The effective length of the column is influenced by various factors, including end conditions, lateral support, and buckling behaviour. It is determined based on the column's boundary conditions and the presence of bracing or restraint.
The least radius of gyration (r) represents the measure of a column section's resistance to buckling about its principal axes. It is calculated based on the properties of the column cross-section, such as its moment of inertia and cross-sectional area.
Australian Standards, such as AS 4100 for steel design and structures or AS 3600 for concrete structures, provide specific criteria for assessing the slenderness of columns and determining whether they are susceptible to buckling.
These standards typically specify maximum slenderness ratios for different column configurations and loading conditions to ensure structural stability and safety.
For example, AS 4100 provides guidance on assessing the slenderness of steel columns based on factors such as end conditions, effective length, and material properties.
Similarly, AS 3600 outlines procedures for evaluating the slenderness of concrete columns considering parameters like boundary conditions and lateral support.
Slenderness is a key consideration when choosing a column shape, and it will be considered in the structural capacity comparison below.
Circular columns
Material types
Circular columns are constructed using reinforced concrete or steel. Reinforced concrete circular columns offer supreme structural stiffness, while steel circular columns provide compact versatility.
Structural capacity
Circular columns exhibit excellent load-carrying capacity for bending, shear, and axial forces, as their section shape offers uniform stress distribution. This makes them suitable for various structural applications.
Slenderness
Circular columns typically have lower slenderness ratios than other column shapes, reducing susceptibility to buckling.
Square columns
Material types
Square columns are commonly constructed using reinforced concrete or steel, with variations such as Parallel Flange Channels (PFCs) or universal columns. Reinforced concrete square columns offer simplicity and durability, while steel square columns provide enhanced load-carrying capacity and ease of connection.
Structural capacity
Square columns are commonly constructed using reinforced concrete or steel, with variations such as Parallel Flange Channels (PFCs) or universal columns. Reinforced concrete square columns offer simplicity and durability, while steel square columns provide enhanced load-carrying capacity and ease of connection.
Slenderness
Square columns may have higher slenderness ratios than circular columns, and proper reinforcement detailing in reinforced concrete columns or stiffeners in steel columns can mitigate potential buckling concerns.
Rectangular columns and H-shaped columns
Material types
Rectangular columns and H-shaped columns are commonly constructed using reinforced concrete or steel. The choice of material depends on structural requirements, architectural considerations, and construction methods.
Structural capacity
Rectangular columns and H-shaped columns offer similar structural capacity to square columns, with variations in load-bearing capacity depending on section dimensions and material properties.
Typically, square columns are preferred, but when bending moment is applied in across a predominate axis of a column (for example, a one-way slab), then, a rectangular column can be used with additional section area provided perpendicular to the axis that the load is applied for additional structural capacity.
As a result, the dimensions of rectangular columns can be adjusted to provide a more efficient structural design than square columns or circular columns, as the material is concentrated where it is required, based on the load path of the building.
Slenderness
Similar to square columns, rectangular columns, and H-shaped columns may exhibit higher slenderness ratios, but ensuring bending occurs via the major axis can mitigate buckling issues.
Cost considerations
It�s important to consider cost implications on the construction projects during the structural design phase.
Both the cost of the material itself and the amount of labour and plant required to construct the column should be considered, and the latter can vary depending on the column specified.
Material procurement
Circular columns may incur higher material costs due to the specialised formwork required for concrete construction or the fabrication costs for steel columns.
Square columns, rectangular columns, and H-shaped columns may offer cost advantages in material procurement, especially if standardised sections are readily available.
Construction costs
Circular columns may involve higher construction costs due to the complexity of formwork, the labour-intensive process of pouring and finishing concrete on construction sites, or the fabrication and welding of steel columns.
Square columns, rectangular columns, and H-shaped columns may offer cost savings during construction, as they generally require less complex formwork and simpler connection details.
Connection costs
Circular columns may entail higher connection costs, particularly in steel structures, due to the welding requirements and the need for specialized labour and equipment.
Square columns, rectangular columns, and H-shaped columns may offer cost savings in connection details, as bolted connections are generally simpler and more economical to implement.
Ideal building types and applications
Let�s consider some examples of where each column shape would be suitably applied.
Circular columns
Circular columns are ideal for high-rise residential towers, modern architectural designs, and structures with dynamic loading conditions.
They offer superior load-carrying capacity and aesthetic appeal, making them suitable for various structural applications.
Square columns
Square columns, especially those using PFCs or universal columns, are well-suited for light commercial buildings, industrial structures, and projects where high bending loads are anticipated.
They offer enhanced load-carrying capacity and ease of connection, contributing to cost-effective construction.
Rectangular columns and H-shaped columns
Rectangular columns and H-shaped columns are versatile options suitable for a wide range of building types, including residential, commercial, and industrial projects.
They offer flexibility in architectural design and structural performance, making them adaptable to various structural requirements.
Conclusion
In structural engineering, column shape selection involves careful consideration of aesthetic preferences, connection options, structural capacity, and building requirements.
Circular, square, rectangular, and H-shaped columns each offer unique advantages and considerations, catering to diverse architectural styles and structural demands.
Understanding the characteristics and applications of different column shapes can help structural engineers make informed decisions to achieve optimal design solutions that meet or exceed safety standards and client expectations.
Check out the complete guide on the Design of Reinforced Concrete Columns to AS 3600.
Want to start designing columns? Check out the ClearCalcs calculators for timber columns, steel columns, and concrete columns to Australian standards.
Designing to US standards or Eurocode? Check out the column calculators below:
- Wood Column (ASD) Calculator to NDS 2018 (ASD)
- Steel Column (ASD) Calculator to AISC 360-16 (ASD)
- Rectangular Concrete Column Calculator to ACI 318-19
- Timber Column Calculator to EN 1995-1-1:2004+A1:2008 (Eurocode 5) & EN 1990:2002
- Steel Column Calculator to EN 1993-1-1:2005 (Eurocode 3)
- Rectangular Concrete Column Calculator to Eurocode 2
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