Fire safety in the frame

David Atkinson from Metsec discusses the increased focus on through-wall fire testing post-Grenfell, and how steel framing systems can contribute to improved performance as well as thermal efficiency

Light gauge galvanised steel framing systems (SFS) are primarily used for wall constructions such as infill walling, continuous walling, high bay walling and load bearing structures in low to medium rise situations.

Their light weight and versatility deliver benefits to the entire construction team, providing the designer, developer and end client with cost-efficient, sustainable design solutions for a wide variety of residential, hotel, student accommodation, social housing, healthcare and education projects.

Whilst efficiency, versatility and sustainability are desirable attributes for any building system, they count for little without certified proof of the system’s ability to meet the required technical performance standards in the areas of acoustics, thermal efficiency, load bearing, and fire protection.

Walls’ fire performance

Fire performance and thermal efficiency are achieved through the combination of materials that are used in conjunction with SFS to construct the wall. For external wall constructions, from inside to out, this would typically consist of dry lining, SFS (possibly including insulation), sheathing board, and insulation. The final wall construction can vary according to the external facade materials that are used. These can range from brickwork to insulated render, timber cladding, composite panels, ventilated rainscreen and many others that might be selected to achieve the desired exterior finish or budget.

Traditionally, performance of the wall construction could be assessed by compiling the technical data from manufacturers of each of the materials used in the construction; boards, sheathing, SFS and insulation. Since the Grenfell Tower fire, there has been increasing recognition amongst SFS manufacturers of the need to undertake fire testing of the wall construction itself.

By carrying out a ‘through-wall’ performance test, designers and contractors can have added assurance of the product’s performance capabilities.

The through-wall concept

While there is currently no strict definition of what ‘through-wall’ means, it is widely regarded as the area from the inside face of the internal dry lining to the outside face of either the sheathing board or insulation. This part of the wall provides the majority of its fire, thermal and acoustic properties.

The reason for the emergence of the ‘through-wall’ concept is that with so many exterior facade materials available, it would be impossible to test every possible wall construction. The ‘through-wall’ approach to performance allows manufacturers to test a meaningful number of material combinations (SFS, internal and external boards plus insulation). The tested solutions will either be from the dry lining through to the sheathing board or from the dry lining to external insulation.

This approach provides the design team with the freedom to choose the desired external facade, provided the necessary performance requirements are met.

Measuring performance

Building Regulations set out requirements for time performance, particularly with regard to integrity (passage of smoke and flame), insulation (temperature on the opposite side of the wall) and load bearing capacity where the wall is being used in a structural capacity.

In order to achieve ‘through-wall’ fire performance data, SFS manufacturers construct different complete wall build-ups measuring 3 metres x 3 metres, comprising light gauge steel frames fitted with various combinations of boards and insulation materials to UKAS-accredited laboratories for testing.

Each build-up is tested using a large-scale, high heat test (reaching up to 600°C in 5 minutes), which measures the performance of the whole wall by simulating fire trying to break out through a solid wall. Tests are conducted to British Standards BS EN 1364-1 and BS 476-22 (for non-loadbearing walls) to achieve the 120 minute, 60 minute and 30 minute fire ratings typically required by Building Regulations.

In most cases, fire performance is an inside-to-out requirement, aiming to prevent the spread of fire from its source to adjacent compartments. Where buildings are in close proximity to each other, fire resistance will also be required from outside in.

Whatever the circumstances, ‘through-wall’ fire testing and the performance data it returns for the combinations of materials used in wall constructions is a positive step forward from SFS manufacturers. As more manufacturers commit to this approach, the array of certified wall constructions available to designers will increase – providing greater choice and flexibility in the solutions they can use. This will not only meet fire performance requirements but will also achieve other performance criteria – such as thermal and acoustic insulation – as well as meeting aesthetic and budgetary targets.

David Atkinson is specification manager for Metsec’s framing division