CROSS Safety Report
Fire protection to light gauge steel frame walls
A disagreement between fire engineers and manufacturers on testing for the loadbearing performance of light gauge steel frame walls in case of fire has been reported.
Key Learning Outcomes
For Light Gauge Steel Frame manufacturers and suppliers:
- Provide relevant information to help ensure that designers and builders provide adequate protection to all elements of a structure, including walls that are not separating compartment walls
- Internal loadbearing walls could be exposed to fire on both sides simultaneously and should therefore provide the required loadbearing fire resistance for such exposure
- Panelised light gauge steel frame construction is considered a modern method of construction, according to an MHCLG Joint Industry Working Group.
- Approved documents may not provide appropriate guidance for some buildings that are not considered as “common buildings situations” and incorporate modern construction methods, according to the MHCLG’s Manual to the Building Regulations
- Any design should be tested against the functional requirements of the relevant building regulations, and not only against the provisions of technical guidance
- Internal walls that may not need to be fire-resisting for means of escape purposes (i.e. not separating walls) may need additional fire protection if they form part of the structure
- Light gauge steel frame elements may need additional measures to ensure they remain structurally stable in order to perform their intended function
For fire and rescue services:
- Light gauge steel frame structures that do not have all-round fire-resisting protection may be vulnerable in a fire situation, potentially leading to the progressive collapse of the whole structure
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A disagreement between fire engineers and manufacturers on testing for the loadbearing performance of light gauge steel frame walls (LGSF) in case of fire has been reported.
Light steel framing (LSF) is a type of structural system in which thin, cold-formed steel sections, called light gauge steel (LGS), comprise the structural elements which are arranged to build the ‘structural frame’ in buildings. The reporter was involved in a project where LSF was chosen as the solution and a discussion was had regarding the expected load-bearing performance of internal non-separating walls in case of fire. The term ‘non-separating’ refers to the function of the wall as a fire separating element, and not as an architecturally separating element.
Cause for concern
The concern was raised when the solution proposed by a leading manufacturer was considered, by the reporter, to be outside the limits of fire-resistance tests, and in their opinion 'without reasonable skill and care'. This led to further communication efforts with other leading manufacturers and suppliers, who seconded the original position of the LGS manufacturer, making this a common position among the LGS industry leaders. However, the reporter and the other consulted fire engineers disagree.
The core of the disagreement is that load-bearing fire-resistance test evidence for LSF assemblies and systems is based on fire exposure to one side only. The reporter considers that the real-world application of LSF includes non-separating loadbearing walls inside apartments which can reasonably be expected to be exposed to fire on more than one side simultaneously. Since these internal walls are part of the structure’s loadbearing frame, a certain level of functionality needs to be guaranteed in terms of structural performance.
An example of a typical floor plan in a residential building is shown in Figure 1, in which highlighted in red are the compartment walls that are expected to be exposed only on one side in a fire situation (fire separating elements), whereas highlighted in blue are the internal non-separating walls that can be expected to be exposed to fire on both sides.
Approved Document B employs the performance classification for fire resistance as it is set out in BS EN 13501. Part 2 of this series of documents, covering the 'Classification using data from fire resistance tests, excluding ventilation services', addresses in paragraph 7.2.2 the issue of classifying loadbearing walls without a separating function; it is stated that 'Loadbearing walls without a separating function shall be tested as columns by the method given in EN 1365-4'. Columns, when tested in a furnace, are fully exposed to the heating conditions on every side.
The Steel Construction Institute (SCI) has published guidance P424 on the fire resistance of light steel framing. Figure 2 below, extracted from that guidance, shows the temperatures of the loadbearing C sections recorded during a furnace test in which the system was tested as a separating wall being exposed on one side. The criterion was the time it would take the section to reach the critical temperature of 550 ºC; in this case approximately 75 minutes in testing conditions.
The reporter, using that figure, iterated that if a test were undertaken on the non-separating loadbearing wall for exposure to more than one side, the heating of multiple exposed sides simultaneously would, logically, reach the critical temperature sooner than indicated in SCI guidance and potentially in less than the intended 60 minutes of fire resistance in a furnace. This raised the concern that, in case of a fully developed compartment fire in an apartment, there is potential for the failure of a loadbearing non-separating LSF wall system earlier than what is currently assumed in available guidance. This also raises the associated possibility of structural failure through progressive collapse.
In the summary of the SCI guidance document, it is stated that LSF 'can be used in buildings up to 15 storeys high'. For such cases, assuming a 3.0m floor height for a typical residential occupancy, the reporter states that the requirement would be 120 minutes of load-bearing fire resistance for the structural elements.
During a design meeting between the fire engineers, the contractors (who chose LSF as a solution), and the LSF manufacturer, this issue was raised. The requirement to test according to BS EN 13501-2:2016 based on clauses 22.214.171.124 and 126.96.36.199 was brought forward, but the LSF manufacturer 'did not have an answer' and referred only to the guidance by the SCI. Subsequently, further support to the argument was found in BS 476-21, which in clause 8 (determination of the fire resistance of walls) states that 'This clause describes a method for determining the fire resistance of vertical walls with or without unventilated cavities, which have both loadbearing and separating functions, and which are required to withstand exposure to fire on one face'. The reporter’s interpretation of this is that single-sided testing is for separating walls only.
Further on technical guidance, Clause A.6.1 of Appendix A in BS 476-21 states that:
'Some walls, used in practice, act as wide columns which are not designed to provide fire separation, but are required for their loadbearing capacity. In such cases the methods specified in clause 8 may be used but normally the criteria for integrity and thermal insulation are not required. Owing to modern building design, situations can develop in a building, due to open plan design or the provision of doors that are not inherently fire resisting, where a wall that acts as a wide column can be exposed either partially or fully to fire on both faces simultaneously. Very few facilities are capable of exposing a realistic length of walling to fire exposure on both faces simultaneously. However, where the facility does exist, the basic methodology used in evaluating the single face exposure is appropriate for such situations'.
The reporter has some concern that the clause in Appendix A of BS 476-21 creates a weak route on system performance verification as it is based on a constraint of a test house rather than a worst-credible thermal exposure. As an example, they consider it more responsible to test LGS as a 2.5m wide wall (i.e. can fit in a test rig) and then extrapolate the expected performance under each specific field of application. Testing under reasonably expected thermal exposure is for them a defensible approach when compared to just testing to one side. Professionally, they would find it difficult to provide technical defence of a system that relied on validation by virtue of a weaker thermal exposure.
The reporter also considers that the LGS manufacturers 'appear to have a status quo bias', where an emotional bias to maintain the current state of affairs affects human decision-making, due to statements such as 'we’ve never been asked this before'.
LSF manufacturers’ positions
Insight was also provided on the position and justification of the position by some LSF manufacturers to support one-sided testing. The main argument is that Item 2 on loadbearing walls, in Table B3 of Approved Document B, indicates testing on 'each side separately' as the type of exposure.
The reporter disagrees because in the SCI P424 guidance is clarified that 'systems formed of closely spaced members, or continuous panels or slabs might be considered as both a "structural frame" and a "load bearing wall"', with the document additionally clarifying that 'Loadbearing panelised systems, such as loadbearing light steel walls, perform a structural function, often forming a building’s primary structural frame' (Page 15). Following that rationale, the reporter is of the opinion that Item 1 on structural frame, in Table B3 of Approved Document B should be followed, which indicates testing at the exposed faces of the element, interpreted as testing on both sides simultaneously. Despite the SCI clarification, the reporter could not find any reference on testing with exposure to more than one side in their guidance.
Further justification for one-sided testing was provided on the basis that ‘test houses’ can only test walls from one side only. The reporter is of the mind that 'a constraint of testing is not a reasonable defence if the consequence is a significant overestimation of the performance of the structure in fire' and welcomed any comment on the feasibility of undertaking testing with LSF arranged as a column, therefore exposed to fire on more than one side.
One of the LSF manufacturers acknowledged the need to consider exposure to more than one side for loadbearing walls inside an apartment, and indicated that 'they would usually try not to have LSF structural frame in a non-separating wall'. However, when that does happen, they use a consultant to undertake finite element analysis (FEA) in relation to the load-bearing fire resistance performance.
Finite element analysis as an alternative
The reporter was exposed to some reservations by other colleagues on FEA, and its suitability to accurately model LSF behaviour in a fire event. Any comment by CROSS is welcome on this topic.
Another manufacturer could not provide fire test evidence for 90-minute fire resistance when asked to, and this led them to engage another party to provide finite element analysis to satisfy the request. Again, the reservations on FEA, in this case, were raised by the reporter.
Comparison with other modes of construction
To iterate the point, the reporter made a comparison between hot rolled steel and concrete. Hot rolled steel columns used in residential buildings are protected by plasterboard, the resistance of which is dictated by a fire test that exposes the steel column on all sides, as it is indicated in Table B3 of Approved Document B. Similarly when concrete ‘blade’ columns (more than 4:1 length to width ratio) are used in residential occupancies, the concrete cover that provides the necessary fire-resistance is derived through fire testing which exposes the column to fire on all sides. Industry guidance from the Concrete Centre on blade-column design re-iterates this, evident through the phrase 'When designed as a wall it is recommended that the blade column is considered as a wall exposed to fire on both sides even when it forms part of a compartment wall' (page 9).
Issues about responsibility
Of further concern to the reporter is the fact that the manufacturer on this specific project was also not taking responsibility for the fire resistance performance of the structure; for example, providing details for boarded protection. They also could not identify who would be responsible, just that it would not be them.
The reporter is of the mind that when it comes to well-established and now common construction methods, such as hot rolled steel column design, then an architect could detail the boarded protection without the input of a structural engineer. However, when it comes to modern methods of construction and novel systems, the abdication of fire safety responsibility from the parties who are pushing and promoting these solutions appears irresponsible to them. The minimum the reporter expects for good practice is for the provider of the LSF system to connect the contractor and design team with those who have the relevant skills, knowledge, and experience to deliver the necessary fire safety performance. This would also ensure ‘handing over the baton’ securely and enabling a golden thread of information.
Concerns about the impact of uncertainties and occupant behaviour on light steel framing
Concerns were raised about other potential ‘weaknesses’ in this type of construction. One of them is the unknown effect on tested fire performance that could occur when building services (combustible pipes, wires, and more) are routed inside these walls. Another is the unknown risk associated with sockets and switches being recessed into these walls, which have the potential to create weaknesses in the load-bearing fire resistance. Finally, uncontrollable resident behaviours, such as fixing furniture into or onto the fire protection of LSF walls may unwittingly weaken the structure’s ability to withstand fire. The reporter considers that any of these anticipated end-use conditions issues should not result in a building’s structural fire resistance being weakened, or progressive collapse occurring as a result of that weakening.
Current SCI guidance provides advice on detailing service penetrations through LSF walls and floors, but tested examples are not quoted, and the focus is on ensuring compartmentation, rather than loadbearing fire resistance. Similarly, the critical temperature of the steel is the key factor in this case, and this is rarely addressed by products on the market to make good any of these service penetrations. The reporter is only aware of products on the market whose sole focus is on maintaining fire integrity and fire insulation performance to the unexposed side of a separating wall, rather than maintaining the temperature of steel in the middle of a separating or non-separating wall below the critical one.
It is the reporter’s impression that loadbearing non-separating LSF walls have been, and are being, used frequently in the design of multi-storey residential buildings. However, that is being done with no available fire test evidence using exposure of these structural systems to fire on more than one side, and this needs addressing by both the manufacturers and authors of industry guidance who don’t identify this issue with clarity.
A consequence of the LGS being used in loadbearing non-separating walls without testing to more than one side is that structural failure due to fire could occur much earlier than assumed by the manufacturer and designers. On this basis, the reporter considers that residential buildings which have been, or are being, constructed using LSF as the structural frame with non-separating loadbearing walls would not satisfy the Building Regulations, or may not satisfy the Regulatory Reform (Fire Safety) Order 2005.
The reporter considers that any Light Steel Frame system should have fire-test results that show loadbearing fire-resistance performance that reflect the intended end-use application as follows:
- Fire testing results should be made available for loadbearing non-separating LSF walls exposed to fire on more than one side. That requires testing the LSF system as a column as per BS EN 13501-2:2016 (clause 188.8.131.52 and 184.108.40.206) using EN 1365-4:1999.
- Fire testing should include anticipated end-use features like sockets, switches, TV mountings, and other expected penetrations in the board protection to determine with accuracy the steel temperatures.
- An independent review of the SCI publication ‘Fire-resistance of Light Steel Framing’ publication by a suitably qualified and competent structural fire engineer is recommended. The construction industry appears to rely entirely on fire-resistance advice from those manufacturing or supplying the LSF systems - who in turn form the committee that write the SCI LSF guidance. The SCI LSF fire-resistance guidance appears to have been originally issued in 2015 and updated again in 2021 with the same author and committee organisations for both editions. Since Grenfell, all construction organisations, institutes, and professionals have been called to action to search for weaknesses in the training of people, the processes, and systems used, along with how products are being marketed and installed, with a focus on correct testing, awareness of limitations, and review of associated technical publications. How can the LSF sector and SCI demonstrate their publication ‘fire-resistance of light steel framing’ is robust, serves residents and owners of flats and is without bias?
Expert Panel Comments
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This report presents well a very good point. The proliferation of lightweight steel construction is an area of interest, largely because it is not yet as well understood as other forms of construction. Issues like the one brought forward by the reporter need to be investigated further and certainly bought to the attention of the wider sector.
Light gauge cold-formed steel, when compared to hot rolled steel, is usually thinner, which allows a slightly easier rise in its temperature when exposed to heat. Similarly, it is potentially more vulnerable to local buckling, due to the thinner section and potential for localised heating of the section. On both these counts, it can be said that the section is likely to be more vulnerable to loss of strength in a fire than traditional hot rolled sections, and this needs to be taken into account while acknowledging that light gauge steel is different from hot rolled steel.
Fire resistance testing
There is still some confusion around fire testing in the construction industry. Fire resistance is a metric for three different things; integrity, insulation, and loadbearing performance. There are cases where some or all three criteria need to be satisfied, depending on the function of the rated element and the nature of the building. Further on that, there may be cases where different fire resistance ratings are required between the criteria – for example, the insulation criterion for a separating wall might require 30 minutes of fire resistance, but the resistance criterion might require 60 minutes of fire resistance to ensure the loadbearing capacity of the element as part of the structure. However, in this type of construction, almost every single internal and external wall (irrespective of whether they are classified as fire compartment walls) will be containing structural light gauge steel elements and it follows that every wall needs to achieve the loadbearing performance even if they don't need to achieve integrity and insulation.
The panel agrees that these internal loadbearing walls could be exposed on both sides simultaneously and should be designed as such because exposure to both sides by fire is possible. Avoiding such confusion is even more crucial when the structural (resistance) requirements are higher than the separating requirements (insulation and integrity).
Technical guidance and codes
The cited Approved Document B requirements for 'each side separately' is based on traditional forms of construction, and it can clearly be argued that this is a very different situation. Technical guidance is in place to assist designers, not to be followed blindly at the expense of common sense. Concerns have been raised in the past when safety justifications were based purely on literal interpretations of codes. Panelised light gauge steel frame construction is considered a modern method of construction, according to an MHCLG Joint Industry Working Group. Approved documents may not provide appropriate guidance for some buildings that are not considered as “common buildings situations” and incorporate modern construction methods, according to the MHCLG’s Manual to the Building Regulations.
The issue reported also shows some of the limitations when applying common guidance to new methods of construction, and there appear to be commonalities with other reports published by CROSS in the literal application of an Approved Document, when the method of construction may well sit outside the scope of the Approved Documents.
There are ways to demonstrate fitness for purpose, and there are organisations that will give independent validation. When innovating it is good practice to seek an independent opinion from experts and to challenge ideas and solutions from many perspectives - this instance is no exception.
The reporter’s comment referral of the Regulatory Reform (Fire Safety) Order 2005 (FSO) is relevant when considering the 'clarification' of the scope of the FSO through the Fire Safety Act 2021 (FSA), with the FSA clarifying that:
Where a building contains two or more sets of domestic premises, the things to which this order applies include—
(a)the building’s structure and external walls and any common parts;
Finite Element Analysis
When it comes to the reporter’s request for comment on the suitability of Finite Element Analysis (FEA), a point that is reiterated in CROSS reports is that FEA is only a tool and the results it gives depend on the input parameters. To improve confidence in the approach, an industry agreed methodology that at the very least covers inputs and the confidence with which they are known is a prerequisite. Additionally, a number of FE analyses are needed to establish a sensitivity assessment when employing the method.
Changes to the construction by occupants
A typical fire compartmentation drawing normally only shows the walls that need to achieve integrity and insulation, so one might think that any walls that are not highlighted do not need any fire resistance. This can be relevant in the case of those who take over a completed structure and may alter it without recognising the potential impact of that change, hence inadvertently altering the structure with unintended consequences.
The thin steel sections rely inherently on encapsulation for fire protection. The boarded fire protection also acts as bracing, in some cases, to the light gauge steel sections. Any damage or alteration to the plasterboard inhibits this bracing role. The encapsulation can be damaged by structural alterations, rewiring, or differential movement between the concrete core (where present; dominated by bending deflection). Poor constructional tolerances can also affect it.
The reporter is to be thanked for raising the issue and presenting it with such a very good understanding of the situation. It speaks highly of the fire engineers who seem to be considering this properly and pushing in the direction of safer structures by applying common sense to this situation.
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