Steel columns and beams are the building blocks of most major commercial construction these days. Therefore, knowing how to design a steel column is one of the first things any engineer worth their salt needs to master.
The main reason steel is so popular is due to the fact it’s relatively light and easy to work with, speeding up the construction process. There are some disadvantages too, namely that steel is ductile and malleable, and can therefore be easily deformed under high loads if the overall structural design is incorrect. It is also more expensive than other materials and requires more maintenance over the long term. Overall, however, the advantages of building with steel outweigh the disadvantages.
Columns have been a feature of buildings throughout history, from the ornate pillars of ancient temples and arenas to present day steel-frame structures. Although they are far humbler in appearance in the modern age, there is still a lot of thought and analysis that goes into their design. It is also imperative the correct safety precautions are taken when making the calculations.
How to Design a Steel Column
The design of steel columns is a complicated process and this is just a brief overview of the main steps in the process. For more information refer to Eurocode 4: Design of Steel and Concrete Composite Structures, or BS EN 1990: Basis of structural design.
Conditions of an Ideal Column:
When considering how to design a steel column, here are the conditions that contribute to the ideal column:
- Initially straight
- Axially loaded
- Uniform stress (no residual stress)
- Uniform material (no holes)
- No transverse loads
Steel column design process
The design process around how to design a steel column can be broken down into the following steps:
- Calculating the influence area
- Calculating all the loads acting on columns from the influence area
- Calculating the cross-sectional area
- Checking long/short columns and eccentricity of columns
- Determining lateral ties
Let’s take a look at these steps in some more detail.
STEP 1: Calculation of the influence area for steel column design
Influence area refers to the area upon which most of the load is acting. In other words, the area of the column that has the greatest load being applied to the column. This is a simple area calculation once the section has been highlighted.
STEP 2: Calculation of the loads acting on columns from the influence area
The second step involves calculating all the loads acting within the specified influence area. These loads can either be dead loads or live loads. Live loads are calculated from moveable and variable objects and forces namely, humans, chair, table etc., whereas dead loads refer to immovable objects like brick work, slabs and beams. The total load is calculated by:
Total Load on each floor = Dead load on the floor + Live load on the floor
Using this, Pactual can be calculated:
Pactual = Total Load on each floor X no. of floors
STEP 3: Calculating the cross-sectional area required for steel columns
The third step in the process is to figure out the cross-sectional area required. A crucial point to remember here is: to avoid buckling of the steel column, a larger cross-sectional area should be considered. However, steel is expensive so increasing the cross-sectional area can vastly increase the cost of a project. Safety should not be compromised to keep costs low, so adequate calculations of stresses within the beam should be carried out and an appropriate cross-section chosen from national standard tables.
The most common type of column used in engineering is the Universal Column (UC) as they perform well under compression and have thick flanges to resist buckling. There are various cross sections available, including:
- Circular Hollow Section (CHS): Highly resistant to local buckling, but high connection cost and can be weak when there is a combination of bending and compression loads. These are often used for big offshore structures such as oil rigs.
- Rectangular Hollow Sections (RHS): Used widely in high-rise buildings as they have good resistance to compression loading.
- Angle sections: Used for lighter loads, e.g. in roof trusses. They are very easy and fast to connect, making them convenient for large-scale construction projects.
Other types of cross section are rolled I-sections, welded I-sections, and welded box sections.
Checking for long/short column buckling and eccentricity of steel columns
Buckling is caused by a sudden bending force applied to a straight column that is under axial compression loading. A short column has a lesser chance of failure by buckling. A long column is more prone to buckling, which means it may bend under the force of loading. Eccentricity is where the loading is not directly through the centre of the column.
To assess buckling under compression, a concept called ‘effective length’ is used. To calculate effective length (KL) we use the actual length (L) of the column and consider the rotational and buckling effects of loading. There are equations in BS EN 1990 that can be used to this end.
Determining the diameter and spacing of lateral ties for steel columns
This last step in how to design a steel column is to determine the spacing of transverse links and binders. The objective here is to reduce the spacing and minimise the value of diameter. This will make the structure more stable and sound.