Deciding when you might need a portal frame for your project can be hard. So we've compiled some crucial information to help you decide when and why you might need one. For architects, we've also made some portal frame bracing unit tables that could help you in your designs What is a Portal Frame?
A portal frame is a structural system commonly used in buildings, usually in a commercial or industrial setting. It consists of vertical columns and horizontal beams connected by rigid connections, creating a rectangular or square shape that provides structural stability and support for the building.
The rigid connections between the beams and columns allow the frame to resist lateral forces such as wind or seismic loads. While common in commercial or industrial buildings, portal frames can also be used in residential construction.
To skip down to the portal frame bracing unit table, click here: Portal Frame Bracing Unit table
In New Zealand, the use of portal frames is regulated by the Building Code. According to section B1 of the Building Code, buildings must be designed and constructed to withstand the effects of all loads and forces likely to be imposed on them during their intended life. This includes wind loads, earthquake loads, and other environmental factors such as snow or rain.
Why might I use one? So, why might you want a portal frame in your residential house? Here's a few reasons why:
Larger open interior space
A portal frame can allow for larger open spaces without the need for interior columns or walls. This can create a more spacious and open feel to your home, making it ideal for those who value open-plan living.
Large openings in the exterior wall
Portal frames can allow large doorways and windows in exterior walls.
Strength and stability
Portal frames are incredibly strong and stable. They are designed to withstand a range of environmental factors, including wind and earthquakes. This can provide peace of mind and ensure your home is safe and secure. Flexibility in material use
Portal frames can also be constructed using a variety of materials, including steel, concrete, or timber. This provides flexibility in design and construction, allowing you to choose the materials that best suit your needs and preferences.
What's are the disadvantages?
It is important to note that portal frames are not necessarily suitable for all residential homes.
Design cost Portal frames must adhere to the New Zealand Building Code, particularly Section B1. To ensure the portal frame adheres to the building code, a CPEng Engineer is required to sign off on its design. This adds extra cost to implement in the design phase.
Construction cost In smaller houses, the construction cost of portal frames is higher than other structural systems.
Can I avoid portal frames and still have open space?
If you are wanting large open spaces in your building, portal frames are perhaps the best structural system to use. However, if you are wanting to avoid the extra cost, there are a number of design choices you can make that you can discuss with your engineer to reduce the requirement of a portal frame :
For multi-storey houses, design the open plan living space on the top floor, where bracing requirements are not as high.
Avoid openings for windows and doors larger than 2m.
What about diaphragms?
Using diaphragms, most commonly the floor and ceiling, to achieve the required lateral load resistance may be possible without the need for portal frames. However, these have limitations and would also need to be designed by a Chartered Professional Engineer (CPEng).
Portal Frame Bracing Unit Table
To help architects or building owners in the design of their new building, we've calculated and created a bracing unit table for single and two-storey buildings. Using different Parallel Flange Channels (PFCs) sizes, load cases (wind and earthquake) and spans you may require. If you'd like access to this for your own designs, you can find a downloadable copy at the bottom of our Knowledge Hub page, here
If you need help with the bracing unit table or are after a portal frame design for your building, contact us today!