Technical
Supporting a boundary elevation
For decades, the detailing of boundary walls of single-storey buildings has followed guidance in P313 and before that, in P087, and going back further, advice from CONSTRADO. Following that advice has been shown to be successful in practice – boundary elevations have stopped the spread of fire to neighbouring property. Increasingly however, questions have been asked about the resistance of the cladding and secondary support steelwork when assessed using the EN 1993-1-2 standard fire curve. To meet this requirement an industry group has prepared a new technical specification for the design and detailing of boundary elevations, available from BCSA (link at end of article). David Brown of the Steel Construction Institute explains the structural engineering in the new guidance.
Single-storey buildings and boundaries
In England and Wales, (and similarly in Scotland and Northern Ireland) Approved Document B (ADB) implements the requirements of the Building Regulations and has a special section addressing single-storey buildings. Because the Building Regulations are mostly concerned about loss of life, there is generally no need for fire resistance of the structural frame in a single-storey building. The Building Regulations do want to stop the spread of fire, so if the structure is defined as being near a boundary (the definition depends on several factors), the elevation becomes a “boundary elevation” and must be constructed to prevent fire spreading to a neighbouring structure. The requirement is generally to stop fire spreading out from the inside of the structure, but if the boundary is very close, the elevation must also have fire resistance from the outside.
ADB refers to SCI publication P313 where the concept to provide a boundary elevation is provided by the following features as illustrated in Figure 1:
- Cladding with appropriate fire resistance, tested to BS EN 1364-1 or BS 476-22 (note that reaction to fire in accordance with BS 476 will be removed from the England Building regulations from March 2025 and the standard withdrawn entirely in 2029)
- Primary columns which remain upright by:
a. Protecting the column from base to eaves level and
b. Providing a moment resisting base.
In some cases, preference is to avoid a moment resisting foundation, which can be expensive. If a moment resisting base and foundation are not provided, the primary frame must be protected to prevent the boundary elevation column from collapsing. Guidance on the extent of the necessary protection was presented in New Steel Construction, July 2023.
In a fire, the unprotected rafters lose strength and drop into catenary, applying a force at the top of the column pulling inwards, which leads to the calculated overturning moment at the base.
Successful past practice
Despite heightened awareness of fire design and demands for analytically robust solutions, experience in the UK demonstrates that the provisions in P313 to prevent fire spreading to neighbouring properties have been successful. It should also be recognised that this performance is based on a number of engineering assumptions, including:
- The calculation of the overturning moment, which has some engineering basis, but is unlikely to be accurate;
- The entirely empirical assessment that a base moment of 10% the plastic moment of resistance of the column is appropriate for gable columns;
- The extrapolation of cladding performance from a typically 3m × 3m non-loadbearing test in accordance with BS EN 1364-1 or BS 476-22 to the panel sizes used in reality.
Challenged assumptions
In recent years, interest in all forms of fire performance has been heightened. For boundary elevations of single-storey buildings, questions have been asked of the secondary support steelwork – the side rails and their performance in the fire condition. The cladding will have been tested to BS 476-22 or BS EN 1364-1 and the primary steel column will be protected, but what of the light-gauge side rails? If the inside of the structure is assumed to be a compartment, then at the commonly required resistance period of 60 minutes, the temperature of the standard fire (specified in BS EN 1991-1-2) reaches 945°C. At this temperature, the cold-rolled steelwork has only 4% of its original strength (according to Table E.1 of BS EN 1993-1-2), which seems more of a mathematical curiosity rather than something to place undue reliance on.
Of course, there are many potential reasons why a theoretical approach is inappropriate:
- In real fires, the cladding generally remains attached;
- In real fires, purlins often remain in place, despite huge deformation;
- The temperature of 945°C may not be reached, perhaps due to venting through the roof;
- The lower side rails will inevitably be cooler and retain some strength – the temperature is unlikely to be uniform.
Equally, it could be argued that in some circumstances there may be a high fire load in the structure, the roof cladding may remain intact (no venting) and the temperatures reach those predicted by BS EN 1991-1-2. A solution must be put in place that will provide a reliable fire-resistant boundary.
Industry Group
A group of interested parties, who each have a contribution to the fire boundary, was established to prepare recommendations. The group included:
- Steelwork contractors, responsible for the main frames
- Secondary steelwork manufacturers, responsible for the light-gauge steelwork
- Cladding manufacturers, responsible for the integrity and insulation of the cladding (composite panels or built-up systems)
- BCSA and SCI
Whilst steelwork contractors, secondary steel manufacturers and cladding manufacturers will have their own areas of responsibility, a reliable solution requires input from all three parties – collaboration is required. The output from the industry group was to define the essential features of a robust solution with some flexibility over which party provides (and charges for) certain parts of the system. The coordination of the various contributions is a responsibility for the Principal Designer.
System features
The concept for the boundary elevation is simple. It is assumed that in the common fire condition preventing fire spread from the inside of the structure, the fire-resistant cladding is attached to and hangs like a curtain from a so-called “capable member”, as illustrated in Figure 2. No reliance is placed on the unprotected side rails. Each part of the system is discussed below.
If the property boundary is very close to the structure, it may be necessary to consider fire spread from the outside of the structure. In these situations, the secondary steelwork is protected by the cladding and may be assumed to remain competent. It is unlikely that a boundary is only required to resist fire from the outside, so in most circumstances the prevention of fire spreading from the inside will dominate the boundary system requirements.
“Capable member”
The “capable member” is something at high level to which the cladding is attached. It must be designed to carry the vertical load of the cladding in the fire condition, as an accidental combination of actions. No other variable actions need including as part of the member verification. The capable member could be a hot-rolled member, or a cold-formed member.
Hot-rolled (or hot-finished members) will need to be designed in accordance with BS EN 1993-1-2, for a fire resistance period equal to that of the internal compartment (normally 60 minutes for single-storey buildings). The necessary protection depends on the member utilisation and A/V value for the member. This data must be communicated to the party responsible for specifying the fire protection – it is unacceptable to simply state “the member must be protected”. Judicious member selection is important, as the protection of some member types can be practically impossible or prohibitively expensive.
It may be possible to demonstrate, either by physical testing or by analysis, that a cold-rolled member with appropriate protection has sufficient resistance in the fire condition to perform as a “capable member”. The limiting temperature for the member must be communicated to the party responsible for providing the protection – it is unacceptable to simply state “the member must be protected”.
The capable member will be positioned such that the outside face is on the sheeting line and therefore is almost certain to be supported by some steelwork from the main steel frames. The supports to the “capable member” must also be adequately protected as they are essential to maintain the resistance of the boundary.
In tall elevations, it may be necessary to introduce one or more intermediate “capable members” if the full height of the cladding cannot support itself from a single member.
In some cases, the cladding may be supported from the structure within a parapet, which may be used as the “capable member”.
Cladding
The cladding will typically have been tested in a 3m × 3m test furnace, which is clearly not representative of its use in practice. No change is proposed to the assumption that the tested cladding remains equally capable in the large areas of cladding used in practice. No load-bearing tests are proposed to demonstrate that the cladding will support itself when hung from a “capable member” – instead, it is anticipated that cladding companies will demonstrate that in the fire condition, an adequate load path is maintained from the “capable member” into the cladding, and that the cladding is capable of hanging from that support for a specified height. The demonstration of cladding performance may involve some component testing, or analysis, or structural design and is expected to utilise the outer sheet (when the assumed fire is on the inside of the structure) as the main load-carrying component.
Vertically laid cladding
The cladding is to be attached to the “capable member”. It should be shown that in the fire limit state the fixings to the capable member (usually screws) either maintain their resistance (since the interface is usually protected between the cladding and the capable member) or the fixings designed on a reduced resistance. In each case, the fixings must be appropriate for the weight of cladding. An adequate load path from the “capable member” to the vertical load-carrying elements of the cladding is required.
If the cladding is not continuous over the full height of the elevation, the joints must be shown to be capable of carrying the design vertical actions in the fire condition. The internal liner and fixings may be critical as they are exposed to the compartment fire. It may be possible to show that the load at joints can be carried by transfer to the outer sheet and its lap connections, or by bracketry within a built-up cladding system. If a joint cannot be detailed to be adequate, an additional capable member should be introduced to carry the weight of the lower cladding.
Horizontally laid cladding
Horizontally laid cladding is generally attached to vertical members, running between horizontal rails. The vertical members are not continuous, so if they are to be used to carry force to the “capable member” they must be verified at elevated temperature. The joints between vertical members where they are interrupted by the horizontal rails, and the more heavily loaded connection to the “capable member” must also be verified at elevated temperature.
At elevated temperature, the cladding must be shown to span between the vertical supports. It may be that the outer sheet provides adequate resistance. If cladding systems rely on internal bracketry in the fire limit state, an adequate load path to the capable member must be demonstrated.
Slotted side rail connections
For many years, some authorities have insisted that the conditions in a fire test – which generally have slots at the supports to allow expansion and contraction – are reproduced in practice and therefore insist that slots be provided at the connections of the side rails to the primary steelwork. Both SCI and BCSA consider that the opportunity for side rails to buckle over their length (and thus accommodate expansion) means that slots are not required. In the normal design condition, side rails provide restraint to the column, so providing slots is detrimental to their performance.
The slot length generally provided is much less than the theoretical expansion. Where two side rails meet, the gap to allow expansion is generally in the order of 60mm, implying that the cladding, which is fixed to the side rails at intervals, can accommodate 60mm of crushing at the cleat locations as illustrated in Figure 3.
The recommendation from SCI and BCSA is that slots need not be provided at the connections between the side rails and the primary steelwork.
Allocation of responsibility and information exchange
Whilst the foregoing recommendations define the features of the system, the implementation of a competent boundary requires coordination by the Principal Designer. Responsibilities of the various parties are identified below, reflecting typical practical arrangements (which may differ between contracts).
Steelwork contractor
- Design of the primary steelwork (and sometimes the secondary steelwork);
- Design of the “capable member” if hot-rolled / hot-finished sections and communicating the member details to other parties;
- Performance specification for the fire protection of the primary steelwork and “capable member”.
The steelwork contractor will need design loading from the cladding manufacturer and secondary steelwork manufacturer, with any requirements for intermediate “capable members”.
Secondary steelwork manufacturer
- Design of the secondary steelwork, including the performance of any load-carrying members assumed to act in the fire limit state;
- Design of the “capable member” if cold-formed and communicating the member details to other parties;
- Performance specification for the fire protection of the “capable member”
The secondary steelwork manufacturer will need design loading from the cladding manufacturer, with any requirements for intermediate “capable members”.
Cladding manufacturer
- Justification of the cladding hanging as a curtain from a capable member;
- Justification of the fixings to the capable member and at laps (if any).
Main contractor
- The design of any moment-resisting foundations;
- The design and application of fire protection systems.
Protection of the primary steelwork
Although the new guidance primarily concerns the secondary steelwork and cladding, it is self-evident that the primary steelwork must also be adequately protected. The specification of adequate protection requires the calculation of a critical temperature, which will depend on the utilisation of the member and is determined by the designer of the structure. It should be noted that in the fire condition, the main columns may be highly utilised, since the calculated base moment already includes reduced partial factors to reflect the accidental limit state.
Conclusions
Although the recommendations of P313 appear to be adequate in practice, it is clear that the secondary steelwork – which supports the elevation cladding – cannot be verified for the usual fire resistance period of one hour, if the temperature within the structure follows the standard fire curve specified in BS EN 1991-1-2.
The new guidance proposes the engineering justification of a load path to ensure the cladding remains supported by the structure. The proposed solution requires collaboration between the main parties involved in construction, with the essential coordination the responsibility of the Principal Designer.
The technical specification for the design and detailing of boundary elevations is available from BCSA at https://bcsa.org.uk/resources/fabrication-technical-design/industry-specifications/
