‘Through the wall’ is a relatively new term used to describe a system approach to external wall systems using light gauge steel framing, but what does it mean and is it achievable? FIS Technical Director Joe Cilia explains.
SFS light gauge steel framing systems provide the structural framework for the inner skin of external walls. Associate Director at the Steel Construction Institute (SCI),
Andrew Way, said steel has become the market leading material for this application owing to its quality, speed of installation and design versatility.
So what does ‘through the wall’ actually mean?
‘Through the wall’ could be defined as the composite build-up of the external wall of a building using plasterboard, light gauge steel frame, insulation and sheathing board which, when constructed together, provide the primary fire resistance and weather tightness of the wall.
Structural, fire, thermal, acoustic requirements will define the required performance for the elements and products specified in the wall construction. The final choice of cladding must be considered during the design stage to ensure the load can be carried and the correct cavity barrier is specified.
Managing Director of the purlins and framing division at voestalpine Metsec plc, Stephen Ginger, said ‘through the wall’ is typically interpreted in different ways by different
product manufacturers but significant focus should be given to whole wall construction as the specification and pointed out that choice of products will affect the building performance. “The ‘through the wall’ performance from internal to external finishes is not just about the performance of individual components,” he stressed.
What are the benefits of SFS?
SFS offers a fast, efficient and cost-saving alternative to blockwork and a non-combustible alternative to timber framing, that can improve weather tightness, help meet target dates on a building, making for a safer, more comfortable and drier environment for the fitout phase of construction.
Where is it used?
SFS is used in several built environments, including Housing, Apartments, Student Accommodation, Healthcare, Hotels, Education and industrial warehousing.
What guidance is available?
The Steel Construction Industry (SCI) and FIS have a freely available guide to design and installation of light gauge steel external wall systems available on their websites, known as ‘Technical report ED017’. Andrew said: “This guidance encompasses all aspects of light steel external wall systems, from design to installation and from specification to sign-off.”
What are the likely performance requirements of SFS?
Structural (ADA), fire resistance and reaction to fire (Approved Document B which will be referred as AD B here forward), Acoustic (AD E), thermal insulation (AD L), air and water tightness will all be a consideration.
Senior Certification Engineer – Warrington Fire, Dr Mostafa Jafarian, said: “Through the wall fire performance will depend on the interpretation and direction for performance which could be from either side, so the requirement could potentially change.”
What tests are used for evaluating the performance of SFS?
Structural performance is substantiated using structural calculation methods to the relevant Eurocode or British Standard. Calculations should ideally be signed off by a chartered structural engineer to ensure the sections are used appropriately and are structurally adequate.
Mostafa advised that fire resistance is dependent on the requirements of AD B, which may require tests to confirm the fire resistance from the inside of the building and fire from outside the building. This can be demonstrated using fire resistance performance: to BS 476 Part21 or BS 476 Part 22 or European Standard equivalent BS EN 1364-1 or BS EN 1365-1 (for partition systems), BS EN 1364-2 (ceilings) or BS EN 1365-2 (Floors and roofs). Reaction to fire performance can be demonstrated using the test methods given in EN 13501-1 which should be conducted at an accredited third party body such as Warringtonfire.
It is also possible to meet the requirements of AD B using an assessment based on fire tests from a UKAS recognised body such as Warringtonfire and the BRE, which would need to be approved by the building control officer for the project.
For projects where the cladding / insulation is not A1 or A2, it may be necessary for the external cladding to be tested as a complete assembly to meet the requirements of external spread of flame/reaction to fire when tested to BS 8414, although above 18m in residential construction this option has been removed.
Mostafa said: “Based on our experience, the system made of non-combustible or material of limited combustibility could also fail a BS8414 test if they are not suitability designed. So, even for those systems, it would be recommended that they be tested to be about their safety.”
There may be an additional requirement to test the fire barriers using the BS 8414 test which then should be approved by building control on project bases.
Acoustic performance can be demonstrated via testing or computer modelling and thermal performance can be demonstrated by calculations to BS EN ISO 6946 or thermal modelling.
Stephen Ginger said: “The important thing to take into account is that SFS is a structural system and should be designed by SFS manufacturers who employ qualified structural engineers. Clients and contractors should demand that the chosen SFS supplier supports the project with a warranty which has full professional indemnity and evidence of this should be provided from the SFS supplier before a project order/ design is awarded.”
What issues should be considered when specifying /installing infill SFS?
Issues to be considered if related to “through wall” construction within the structural and fire design are as follows:
• SFS Sections are CE marked.
• Calculations provided and signed off by chartered structural engineer.
• Interfaces and fixings to primary structure considered and shown on drawings.
• CDM requirements are met.
• The chosen SFS supplier can offer professional indemnity to the value the client/contractor requires.
• The chosen SFS supplier who can offer “through wall” construction has appropriate fire resistance test or assessment as substantiation from a UKAS registered facility.
• Requirements for fixing the components to match the fire test/assessment is understood and followed on site.
• Products are not substituted for “similar” products without approval of the test / assessment sponsor and building control.
• Interfaces of the SFS and primary structure are considered and detailed.
• If the primary structure is steelwork, then consideration and connections to intumescent painted steels or boxing out need to be detailed.
Stephen emphasised that since the tragic events of the Grenfell tower, the industry is rightly reacting, reviewing and taking decisive actions over which products should be used in the wall construction on all buildings going forward.
Andrew added that SFS has been used for a long time and is therefore a well-established construction system. However like any system it must be designed, specified, installed and signed off by suitably qualified and competent people.
It’s clear that specifying and installing SFS is a specialist task and should not be undertaken without consultation with other disciplines such as structural engineers and designers who will work with the ‘Through the wall’ suppliers to ensure a compliant design and specification is developed in conjunction with the building control bodies and that the installation process is regularly inspected and signed off at each stage of erection and before closing out the framework.
The FIS SFS Working Group was formed to provide guidance and address issues in this growing part of our sector.
Last year FIS launched a joint guidance note with the Steel Construction Institute (SCI) which we will build on 2020 to:
• Create a new specifiers guide
• Provide guidance for service penetrations
• Review BS8000:8 Workmanship On Building Sites’ code of practice for plasterboard partitions and dry linings
• Develop a standardised structure for ‘Through the Wall’ fire performance data
• Develop a quality check list
• Develop a guide for the safe ingress of materials to site
• Develop a competency framework
Full meetings are held twice a year with small task and complete working group meetings between times.