What is a steel portal frame? This question often comes to mind when you look at large warehouses, factories, or farm sheds. Actually, a steel portal frame is one of the most widely used building systems today. What makes it so popular is its ability to create wide, clear areas without using internal columns.
In other words, steel portal frames give businesses and builders the freedom to use the space as they want. But if you are new to these steel frames, you must know what they actually are and how they work. Hence, in this article, I will explain everything about steel portal frames, including their types and components. So let’s get started!
What Is a Steel Portal Frame?
A steel portal frame is a structural system consisting of steel columns and steel rafters. A steel portal frame gets its name from the shape it creates. When the two vertical columns and the two sloping rafters are joined, the structure resembles a large doorway or “portal.”
What stands out about the steel portal frame is its ability to create large, clear spans. It means you get a big open area inside the building with no pillars blocking the space. For this reason, portal frames are commonly used in warehouses, factories, and farm sheds. In addition to being strong, steel portal frames are also fast to build and cost-effective. How?
Most parts are made in factories and transported to the site. After that, you just need to assemble those parts on-site with nuts and bolts. This speeds up construction, reduces labor costs, and helps you start your business earlier. That’s why steel portal frames have become a popular choice for many building systems worldwide.
Main Components of a Steel Portal Frame
A steel portal frame is a highly stable, durable, and strong building structure. To ensure all this, it comprises several key components. Let’s discuss the key parts of a steel portal frame that form a steel structure building.
- Columns: These are the vertical steel elements on each side of the building. They support the roof rafters and transfer loads to the foundation.
- Rafters: Rafters are the sloping beams that form the roof. They meet at the top and form the roof’s angle, or pitch.
- Haunches: Haunches are thickened or strengthened areas. They are present at the joint between the rafter and the column, handle bending forces, and strengthen the frame.
- Apex: The apex is the top center point of the frame where the two rafters meet. This is the spot where the left rafter and right rafter join together to form the top angle of the building.
- Purlins: Purlins are horizontal steel members installed across the rafters. This part supports the roofing sheets and transfers the load to the main frame.
- Girts: Girts are extended, horizontal steel members attached to columns. Their main job is to support the building’s wall panels.
How a Steel Portal Frame Is Built: Step-by-Step Process
Building a steel portal frame is not as simple as it sounds. It comes to life through a series of organized steps. To clarify, let’s walk through how a steel portal frame is built, step by step.
1- Prepare the Site & Build the Foundation
The first step in building a steel portal frame is preparing the construction site and building a strong foundation. A proper foundation is crucial because it supports the entire weight of the steel frame. So first, site preparation starts with clearing the land of trees, rocks, debris, and any other obstacles. The ground is then leveled and compacted to distribute the weight evenly.
Sometimes, extra soil is added or removed to achieve a flat and solid base. After the site is ready, the next task is to build the foundation. For steel portal frames, the foundation is typically reinforced concrete. Interestingly, concrete itself is robust in compression. When it is combined with steel reinforcement bars, it becomes stronger.
During foundation construction, anchor bolts are placed in the concrete. These bolts hold the steel columns firmly in place. However, when installing these bolts, I recommend carefully measuring their positions. Even a small error can make it difficult to erect the columns later and may compromise the building’s stability. Once the foundation is set and the concrete has cured, it provides a strong platform for the steel columns to be erected.
2- Install the Steel Columns
After the foundation is ready and fully cured, the next step is to install the steel columns. These vertical columns are the backbone of a portal frame building. Why? They support the entire weight of the roof and transfer loads safely down to the foundation. To install the steel columns, they are first carefully lifted by cranes.
They are precisely positioned on top of the anchor bolts you set into the foundation earlier. Here, accuracy is critical, as even minor misalignment can affect the assembly of the entire steel portal frame. Therefore, use measuring tapes and plumb lines to ensure each column is perfectly vertical. Once the columns are in position, they are temporarily supported using props or braces.
This is done to check the alignment before permanent welding. After alignment checks, the columns are bolted or welded to the foundation anchor bolts. At this stage, workers also ensure that the column spacing meets the design requirements.
3- Assemble & Lift the Rafters
Once the steel columns are firmly in place, the next step is to assemble and lift the rafters. Rafters are horizontal or slightly angled steel beams that connect the tops of the columns. These rafters actually create the portal roof structure of steel frames. Generally, the rafters are pre-fabricated in a factory.
It means they are cut, shaped, and drilled before arriving at the construction site. On-site, workers only need to lay the rafter sections on the ground and connect them using bolts or welding. This welding gives the complete rafter shape for each frame. After assembling the rafters on the ground, cranes lift them into position on top of the columns.
Here, the rafters must align perfectly with the tops of the columns and the apex. Once the rafters are correctly aligned, temporary supports and safety ropes are used to hold them steady. After that, they are bolted and screwed to the columns. However, this connection between the rafter and the column must be very rigid to resist bending and rotation.
4- Add Bracing, Purlins, & Girts
After the main portal frame (columns and rafters) is assembled, the next step is to add bracing, purlins, and girts. These elements essentially make the building strong, stable, and ready to hold the roof and wall panels. So, first, bracing is added. Bracing consists of diagonal or horizontal steel members that connect the columns and rafters.
Its main job is to prevent the frame from swaying or twisting due to wind or other forces. Two types of bracing are mainly used. The first is roof bracing, installed along the rafters. The second type of bracing is wall bracing that connects columns across the length of the building. After adding bracings, it’s time to install the purlins. Purlins are horizontal steel members that run across the rafters.
They act as the support for the roof sheets and distribute the load evenly to the main frame. After the purlins, the girts are installed. Girts are similar to purlins, but they are installed on columns to support the walls. Wall panels or cladding are attached to girts, which help the walls stay straight and strong. Girts also transfer wind loads from the wall panels back to the main steel frame. This ensures the building remains stable even in strong winds.
5- Install Roof & Wall Sheets
The final step in building a steel portal frame is to install the roof and wall sheets. This step is all about protecting the steel portal frame. Roof sheets include insulation, waterproof layers, and features for snow and temperature changes. It all depends on what you need. First, the roof sheets are fixed to the purlins you installed in the previous step.
When fixing the roof sheets, ensure that there are no gaps, overlaps are correct, and all fasteners are secure. Once the roof sheets are installed, the wall sheets are attached to the girts along the columns. These are the metal panels that cover the building’s sides. The wall sheets are attached to the girts (horizontal supports on the columns).
They must be lined up correctly and fastened tightly so they do not come loose. If the building needs doors, windows, or vents, they are added during the installation of the wall sheets. Once all roof and wall sheets are installed, the building becomes fully enclosed and functional. At this stage, any final adjustments, sealing, and finishing touches are done. After completing this step, the steel portal frame building is ready for use.
Types of Steel Portal Frames
Steel portal frames are not all the same; they are highly versatile. Depending on the design, roof shape, and size, there are several types of steel portal frames. Each of these is designed to meet specific needs. So let’s dive in and explain the main types of steel portal frames.
1- Single-Span Portal Frame
A single-span portal frame is the simplest type of steel portal frame. It consists of a single continuous frame spanning the entire width of a building. It means that single-span portal frames do not contain internal columns. This creates a large, open space inside the building, which is ideal for storage and movement.
The main structure of the single-span portal is made up of two vertical columns and two sloping rafters. These rafters and columns are connected at the top by a rigid joint, forming the characteristic portal shape. Single-span portal frames are often used for small to medium-sized buildings. They are very cost-effective because fewer materials are needed, and construction is faster.
2- Multi-Span Portal Frame
A multi-span portal frame is a type of steel portal frame that has more than one span. It means it includes internal columns to support the roof across longer distances. Unlike a single-span frame, a multi-span frames divide the building into sections. The main frame of a multi-span portal consists of several portal frames connected side by side.
To support the rafters, there are some internal columns. These internal columns strengthen the structure and enable it to carry heavier loads. That’s why they are used in large factories, industrial plants, warehouses, and aircraft hangars. No doubt, multi-span portal frames require more materials and construction time. However, due to their greater strength, they are essential for load-bearing buildings.
3- Mono-Pitch Portal Frame
A mono-pitch portal frame is another type of steel portal frame in which the roof has only a single slope. As you know, in the above steel portal frames, there are typically two sloping rafters. But in mono-pitch frames, there is only one sloping roof. It means the roof rises from one side of the building to the other, creating a lean-to style roof.
This single slope allows for easy drainage of rainwater and snow. How? Water flows naturally from the higher side to the lower side of the roof. That’s why mono-pitch portal frames are used for extensions, carports, or sheds. Moreover, despite having only one slope, the frame is still strong due to the rigid rafters’ joints.
4- Tapered Portal Frame
A tapered portal frame is one where the columns are sloped or tapered. It means they are not perfectly vertical. The columns lean slightly outward or inward, and the rafters are adjusted accordingly to form a strong, rigid frame. This design creates an aesthetic, modern look and can also improve structural efficiency.
Thus, tapered portal frames are often used in large industrial and exhibition halls where looks matter more. From an aesthetic standpoint, the sloping columns can also handle heavier loads thanks to the internal columns. These columns are fewer in number, ensuring more storage and movement space while remaining durable.
5- Curved Rafter Portal Frame
Some steel portal frames are designed with curved roofs rather than straight ones. This type is called a curved or bow-top portal frame. In this type, the rafters form a gentle arch, while the columns may remain straight. The curve gives the building a modern look and helps it handle wind and snow better.
Rainwater and snow naturally slide off the roof rather than staying on top. This reduces the weight and pressure on the roof, helping the frame last longer and making it safer. The arch also gives more headroom in the center, which is helpful for tall machinery, vehicles, or large storage racks.
Advantages and Disadvantages of Steel Portal Frames
As I said above, steel portal frames are widely used. However, like any building system, they have both benefits and drawbacks. Before you make a final decision on their selection, take a look at the pros and cons of a steel portal frame.
| Advantages | Disadvantages |
|---|---|
| Wide, clear spans | High initial cost |
| Strong & durable | Steel temperature issues |
| Quick to build | Prone to corrosion |
| Cost-effective long-term | Needs skilled labor/equipment |
| Flexible design | Limited architectural variety |
| Low maintenance (with coating) | Requires proper foundation |
Conclusion
Steel portal frames are one of the most popular and reliable building systems used today. They provide strong, stable structures that can span large open spaces without internal columns. The clear, open interior space offers flexibility for storage and various work activities. That’s why these steel frames are an ideal choice for warehouses, factories, workshops, hangars.
What further makes the steel portal frames special is their versatility. These steel frames come in various types, as discussed in this article. All of those types have specific designs to meet the needs of particular projects. Not only this, but steel portal frames are also cost-effective. Most of their components are prefabricated. This speeds up construction and reduces labor costs.