Designing Steel Framed Buildings for Fire


I’m writing this article on the 4th anniversary of the Grenfell Tower tragedy, an event that shocked the entire nation and shone a much needed spotlight on the issue of fire safety in buildings. Of course there are several significant differences between a highrise residential block and a single storey agricultural building and many of the issues identified at Grenfell Tower do not apply to grain stores or animal housing. Nevertheless, fires do break out on farms, sometimes with tragic consequences, so it seemed timely to dedicate this article to the subject.

Regulations and fundamentals

As most readers will be aware, agricultural buildings that are not used as dwellings are exempt from the Building Regulations provided that they meet certain conditions. RIDBA members, however, also supply a wide range of non-agricultural buildings (e.g. buildings for industrial, retail or educational applications), so knowledge of the basic requirements of Approved Document B, the part of the Building Regulations in England and Wales dealing with fire, is important. Furthermore, farm buildings are sometimes built close to dwellings (e.g. a barn next to the farm house) or are converted for other uses (e.g. a riding school), so the Building Regulations exemptions don’t always apply. Fortunately, even when a single storey building falls within the scope of Approved Document B, the rules are far less onerous than they are for multi-storey buildings.

There are two fundamental issues that need to be considered when designing a building against the risk of fire:
• Saving the lives of occupants within the building
• Preventing the spread of fire to neighbouring buildings.

The former is usually critical for multi-storey buildings, where there is an emphasis on preventing the spread of fire within the building (compartmentalisation), providing escape routes, the installation of sprinklers and preventing collapse of the structure by protecting critical structural elements. By contrast for single storey buildings, the emphasis is very much on preventing the spread of fire to neighbouring buildings, in particular through the collapse of a burning building onto its neighbour.

Saving lives

The overriding priority of Approved Document B is saving the lives of building occupants in the event of a fire. This is achieved by a combination of minimising the time needed to egress from the building and maximising the time taken for the fire to spread. The former is enabled through the provision of escape routes leading to fire exits, while the latter is often achieved by the use of fire doors, fire-proof barriers, compartmentalisation and sprinklers. Since the majority of agricultural buildings are single storey, the simplest way of saving lives is to ensure that everyone within the building has easy access to an exit. For this reason, the exemption of agricultural buildings to the Building Regulations states that nowhere within the building may be further than 30m from an exit.

Preventing the spread of fire

Of greater concern for single storey buildings is the spread of fire to neighbouring buildings, especially where a neighbouring building is a dwelling. Preferably, agricultural buildings (i.e. those that are generally exempt from the Building Regulations) should be at least one and a half times their height from any building with sleeping accommodation. Where this is not possible, or for industrial or commercial buildings where there is no exemption from the regulations, it is necessary to design the building such that in the event of a fire. For braced frames, this could be achieved by protecting the external walls only (i.e. the columns and bracing), but this approach is inadequate for portal frames, because the columns and rafters act together as if they were a single structural element. For standard frames with nominally pinned bases, if the roof structure were to collapse in a fire, the walls would also collapse allowing the fire to spread. Of course, in theory, this problem could be overcome by applying fire protection to the entire portal frame.

However, applying fire protection to rafters is difficult and expensive so an alternative solution is needed. SCI publication Single Storey Steel Framed Buildings in Fire Boundary Conditions (SCI-P-313) presents an alternative method in which engineering principles are applied to the design of the columns and bases to demonstrate that the columns alone could withstand the overturning moment applied to them even if the entire roof structure were to collapse in a fire. In this method, the overturning moment at the point of rafter collapse is calculated for a special fire limit state case, in which the loading is less severe than that normally used to design the structure.

The column base strength and stiffness are estimated based on the actual base dimensions and thickness and the size and strength of the holding down bolts. Sprinklers are recognised to have a considerable beneficial impact on the intensity and spread of fire and a significant relaxation in the rules is permitted when they are used.

Fire protection

Where columns along external walls require fire protection, this should extend up to the underside of the haunch, or to the rafter where there is no haunch. The level of fire resistance of the protected columns should be the same as that of the wall. Values of fire resistance (i.e. duration in minutes) are given in Approved Document B. Building designers and frame manufacturers have several options when it comes to the means of fire protection. The most common are summarised (right).

Boards – This is probably the simplest solution and is especially suitable for commercial and retail premises, where the boards provide the additional benefit of hiding the steelwork within a neat box. The boards are fitted as a dry trade after the completion of the structure, so do not interfere too much with the construction programme.
Sprays – This solution is less common in the UK, but is sometimes used where complex shapes would be difficult to protect using boards. The end result, while effective as fire protection, is not aesthetically pleasing, so sprays are not used where appearance is important. Spays are messy to apply and no other construction work is possible during this operation.
Blankets – This solution combines the advantages of boards and sprays. In common with boards, blankets are applied as a dry trade to the completed structure, but like sprays they are suitable for complex shapes. They are especially useful for protecting truss structures, since the blankets can be wrapped around the individual elements of the truss.
Intumescent coatings – Unlike the first three options, which all offer passive fire protection, intumescent coating react to temperature, foaming up to provide fire protection in the event of a fire, but otherwise resembling a painted finish to the steel.

Intumescent coatings can either be thin or thick film and may be applied in the frame manufacturer’s workshop or on site. Off-site applied thin film intumescent coatings are probably the most appropriate for portal frame structures and have a significant market share in the UK.