Large-span structures that are lightweight and have fast erection or construction processes are ideal for industrial and commercial spaces. These structures are becoming increasingly popular in modern construction due to the various other benefits that they offer. A portal frame is one such structure that is commonly used across the world to create functional industrial buildings. Keep reading to learn all about what portal frames are, their components, types, advantages and limitations in construction.
What is a Portal Frame?
A portal frame is a rigid structure formed by the composition of columns, beams and pitched or curved rafters. Portal frame structures are typically single storeyed and have expansive uninterrupted floor spaces. They were developed during the Second World War to fulfill the need for large equipment storage spaces that could be built quickly. Although portal frame design is possible with wood and aluminium, mild steel and structural steel are the most commonly used materials.
What are the Components of a Portal Frame Structure?
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Primary Structural Frame
- Footings: These are generally made of RCC, although the type of footings may vary based on the design and structural requirements.
- Columns: Structural steel (typically in I, C or box sections) with a minimum gauge of 6mm is used as columns, which are erected on footings with welded or bolted base plates.
- Tie Beams: These are straight members that connect the columns on two parallel sides (typically the longer sides).
- Rafters/ Roof Beams: Inclined I, C or box section members that form a rigid A-frame with the columns and majorly bear the roofing loads.
- Roof and Wall Braces: These are diagonal members placed between column and rafter bays that provide additional support when the distance between subsequent bays is too large.
Secondary Members
- Purlins: These support the roofing material and hold it in place. Typically, purlins are fixed on top of rafters at a maximum spacing of 1.4m from each other.
- Side Rails: These provide additional support to the columns when bay sizes are too large and also enable the mounting of wall cladding elements.
- Gutters: Fixed at the bottom edges of the roofing material, these guide water away from the building and into drainage systems.
- Wall Cladding: Materials like sandwich panels or metal sheets are used to cover the side walls to form enclosed portal frame structures. These offer the added benefit of being lightweight as well as cost effective.
- Roof Cladding: Different lightweight roofing options like profiled metal or PVC sheets are commonly used to cover the roof.
In India, the IS code SP 40 (1987) guides the design of steel portal frame structures.
Advantages of Portal Frame Construction
- Lightweight: Typically made with steel sections and lightweight roofing and wall cladding materials, portal frame structures are atleast 50% lighter than conventionally built structures (such as RCC framed structures or masonry structures).
- Large Spans: Structures with very large spans up to 100m can be constructed with portal frames. Since these do not consist of any intermediate columns, huge uninterrupted spaces can be formed, which is especially beneficial for industries and factories that require large machinery.
- Quick Erection: With mass-manufactured and/or pre fabricated components, portal frames require only welding, bolting and fabricating processes after the foundation is ready. Thus, the construction time is drastically reduced.
- Cost-Effective: These structures typically do not require deep foundations. This, combined with the mass manufacturing of most components, significantly reduces construction costs.
Types of Portal Frames
Portal frame types can be classified on two factors:
- Based on Structural Support and Span:
- Single Span Portal Frame: A-frames without any intermediate columns extend across the entire span of the structure and form its main load-bearing element. In this type, the maximum span of the shorter side (the sloping or curved side) is around 80m.
- Multi Span Portal Frame: The sloping or curved shorter side consists of one or more intermediate columns that provide additional support to the main A-frame and divide it into bays.
- Portal Frame With Internal Mezzanine: An internal mezzanine floor—formed by horizontally and vertically fixed steel sections furnish the A-frame with enhanced rigidity and offer extra usable floor space.
- Tied Portal Frame: The main curved or sloping A-frame has a horizontal tie and short vertical members that give it additional support and enable the structure to have large uninterrupted floor spaces.
- Based on Shape:
- Pitched Roof Portal Frame: This has a roof which slopes down along two opposing directions, creating the typical A-frame. It is the most commonly used type of portal frame.
- Mono-Pitched Portal Frame: Also referred to as a lean-to shed roof, this portal frame has a roof that slopes along one direction and is typically used for small structures with a maximum span of 15m.
- Mansard Roof Portal Frame: In this type, the roof can be divided into 4 sections, in which 2 sections slope towards the same side, but at different angles. This portal frame is used when larger clear heights are required at the central zones.
- Curved Portal Frame: A curved steel member (rather than diagonal members) forms the main portal frame structure along with the columns. Curved portal frames are mainly designed for aesthetic reasons and require split curved sections for transportation purposes if the span is more than 20m.
- Cellular Beam Portal Frame: This consists of tapering steel sections with circular holes in them. They are mainly designed to enhance the aesthetics of the structure.
Limitations of Portal Frames
- Low Fire Resistance: Although not combustible, steel can undergo bending and deformation when subjected to high temperatures. This can significantly weaken the structure.
- Effect of Weather: Steel portal frames are vulnerable to moisture, which can corrode and rust its surface, leading to the structure losing its strength. Very humid regions like coastal areas have an increased adverse effect on steel due to high salt content in the air.
- Labour Requirement: Skilled labour is required to construct portal frames. Extensive scaffolding is also needed in case of large heights and bigger spans, driving up the construction cost.
- Thermal Insulation: Steel has a very high thermal conductivity and its use can create very high indoor temperatures. Although ideal for storing certain goods in warehouses, such structures may be inconvenient for prolonged human occupation unless mechanical ventilation systems are incorporated (which again increases the cost).
Getting Portal Frame Structures Built
Despite their limitations, portal frames offer a wide range of benefits that make them popular in all types of modern construction. Brick & Bolt, a tech-enabled company in India, offers end-to-end construction services for all your building needs. With 7000+ completed projects, the company is a pioneer in residential and commercial construction. Premium quality is guaranteed through the 470+ quality tests (QASCON) conducted for every project. To get your dream project constructed with portal frame structures, reach out to Brick & Bolt today!
Hemali Patel 