Single storey buildings in fire

In the UK, single storey buildings do not normally require fire protection (Approved Document B , Section 8.4, excludes from the definition of elements of structure, structure which only supports roof).  Exceptions may occurs where the structural elements form part of:

• A separating wall
• A compartment wall or the enclosing structure of a protected zone
• An external wall which must retain stability to prevent fire spread to adjacent buildings (ie a boundary condition)
• A support to a gallery or a roof which also forms the function of a floor (eg a car park or a means of escape).

By far the most common structural form for single storey non-domestic buildings are portal frames and the most common scenario in which fire protection is required is a boundary conditions.  Boundary conditions occur as a result of the requirement for adequate space separation between buildings as outlined in Part B of Schedule 1 of the Building Regulations 1991:

“The external walls of the building shall offer adequate resistance to the spread of fire over the walls and from one building to another, having regard to heights, use and position of the building”.

Where fire resistance is required in a boundary condition, it has been widely accepted that it is necessary only for the affected wall and its supporting stanchions to be fire protected.  The rafters and other walls may be left unprotected but the stanchion base must be designed to resist the overturning moments and forces caused by the collapse of the unprotected parts of the building in fire.  The method of calculation used to derive the horizontal forces and moments created by rafter collapse is given in the Steel Construction Institute publication, Single Storey Steel Framed Buildings in Fire Boundary Conditions, Figure 46 shown below.

This document contains guidance not just on simple portal frames but also on portal frames with lean-to structures, two storey sections etc as well as the design of single storey buildings utilising truss and lattice rafters.

Most authorities expect engineers to design single storey buildings for boundary conditions in this way. In England, Wales & Northern Ireland it is not necessary to apply for a relaxation if it is shown that The Steel Construction Institute document has been used as the basis for design. On the same basis, a class relaxation is available in Scotland.

The SCI document advises on the use of sprinklers in single storey boundary conditions:

It advises that Approved Document B (see page 4) recognises that there is a reduced risk of fire spread in buildings where sprinklers are installed. The boundary distance for a building with sprinklers may be halved or the unprotected area in the wall may be doubled. Also, where the recommendations of the SCI document are followed, the requirements to design the foundation to resists the overturning moment from the collapse of the roof need not be followed.

In Scotland, although the England & Wales approach is considered reasonable, it is up to local authorities to grant relaxations to the regulations on an individual basis.

In Northern Ireland, the regulations follow the England & Wales approach although there is no specific statement as to the issue of design for overturning moment.