Overall lateral stability: Things to consider when designing large structural openings

In recent years, open plan living spaces and large bi-fold doors have become popular design choices for people undertaking residential extensions and new build projects. These modern designs prioritise spaciousness, natural light, and seamless indoor-outdoor living, creating a sought-after aesthetic homeowners. However, the removal of existing walls and the addition of large openings inherent in these designs introduce unique engineering challenges in terms of lateral stability.

In this blog, we’ll shed some light into why lateral stability matters, the engineering solutions to provide it for residential properties, and the pros and cons associated with these solutions.

When considering Residential extensions and new build houses in the UK, Lateral Stability refers to a building’s ability to resist horizontal forces such as wind pressures, or even accidental impact. While vertical loads are well understood and managed through beams and walls, lateral forces are equally important and are best considered right at the start of the design process.

One way to think about Lateral Stability is to compare a home to a sail boat. We want to avoid the boat capsizing in the wind. This example shows the side elevation of a house and its wind catchment area to allow for the large opening to the existing rear elevation i.e. its sail. Imagine the wind hitting the house and the highlighted area on the side elevation acting as a sail. Engineers need to provide a solution to provide sufficient strength in the house to resist the lateral wind pressures within the highlighted area.

Understanding and addressing lateral stability is crucial for several reasons. Firstly, it ensures the safety of occupants and protects property by preventing collapse or excessive deformation during extreme events. Compliance with Building Regulations mandates adequate lateral stability, emphasizing its significance in structural design. The Property should also be considered as a stable building in its own right without the help from any adjoining buildings.

When it comes to integrating large openings, collaboration between clients, architects and structural engineers is crucial. Together, we evaluate different engineering solutions that align with their vision, budget, and environmental considerations.

The structural design solutions for large openings will usually involve the placement of wind portal frame. Alternatively, retaining a portion of the masonry elements can remove the need for a wind portal. There are other engineering solutions available, however these are not typically used in residential projects but could be adapted to be used in some cases.

Wind Portal

Also known as a portal frame or goal post, offers an effective solution for restoring lateral stability in buildings, particularly those featuring expansive openings. A Wind portal in its simplest form is a steel frame consisting of 2 columns and a beam with stiff rigid bolted connections and concrete pad foundations. This creates a stiff frame that limits the horizontal deflection of the house, protecting the building from potential structural compromise.

1/3 masonry rule

This rule has come from the interpretation of the Building Regulations Approved Document A, which serves as a foundational framework for acceptable construction methods. The rule can be explained as the retained or new masonry surrounding the large opening being equal to 1/3 of the overall width of the house. The calculated length of masonry can be distributed in any combination but must be solid masonry or cavity wall.

To give a more in detailed example, the total width of the building is 10.9m (measuring from the centre of the cavity wall). 1/3 of 10.9m is 3.63m, which is the amount of masonry that needs to be retained to meet Building Regs requirements. By retaining 3 sections of masonry at 1.6m, 1.3m and 0.75m, a total of 3.65m of masonry is retained and removes the need for a wind portal.

When it comes to choosing between a portal frame structural solution and using 1/3 masonry for openings, there are several key factors to consider

Wind Portal

Pro’s

  • Allow the large openings that are desired which the Client & Architect have worked hard to develop.
  • They provide the opportunity to create expansive, open spaces that contribute to the aesthetic appeal and functionality of the building.
  • In some cases it allows for even larger opening than already proposed.

Cons

  • The more intricate construction process of wind portals can result in increased expenses, potentially adding up to £10,000 more compared to the 1/3 masonry option.
  • Wind portals have higher embodied carbon than retaining 1/3 masonry, highlighting the need for mindful decision-making if environmental impact is a consideration for the project. 

1/3 Masonry

Pros

  • The construction cost is significantly less as there are less materials and easier to construct.
  • Typically, no additional work is needed to the existing foundations when implementing 1/3 masonry solutions.
  • If a steel beam is used this has a lower Embodied Carbon than a Wind Portal. Alternatives to steel such as Glulam engineered timber further reduce the embodied carbon of the structure.

Cons

  • Compromises to the layout may need to be made to accommodate the additional masonry elements. This could potentially impact the original design vision.

In summary, while portal frames offer greater architectural freedom and spaciousness, 1/3 masonry solutions prioritise cost-effectiveness and reliance on existing materials, albeit with design constraints. A good engineer will take a collaborative approach, understand your priorities, explore the structural opportunities and visually demonstrate the proposals to help you find a solution that is right for you.

Lateral stability is paramount in all building projects, but a one size fits all approach shouldn’t be taken. Ultimately, collaboration between clients, architects, and engineers is key to finding the best structural solution that aligns with the vision, budget, and environmental considerations of the project.

Do you have a project that requires an elegant, efficient and cost effective structure?

0114 442 0232

info@jamstructures.co.uk