Fire resistance of steel framed buildings – single storey buildings in fire

In the UK, single storey buildings do not normally require fire protection (Approved Document B clause 7.4, page 4). Exceptions may occur where the structural elements form part of

  • a separating wall.
  • compartment wall or the enclosing structure of a protected zone.
  • an external wall which must retain stability to prevent fire spread to adjacent buildings (i.e. a boundary condition).
  • supports a gallery or supports a roof which also performs the function of a floor (e.g. 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 in a boundary condition.

In a boundary condition when fire resistance is necessary, it has been widely accepted (Approved Document B clause 13.4) that it is sufficient for only the stanchions supporting the walls designated as forming the boundary to be fire protected, The rafters may be left unprotected but the stanchion base must be designed to resist the overturning moments and forces caused by the rafter collapse 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 document, The Behaviour of Steel Portal Frames in Boundary Conditions.

Most authorities expect engineers to design portal frame buildings in this way. In England, Wales and 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 a basis for design. A class relaxation is available in Scotland provided that the Steel Construction Institute design method has been used.

The SCI guide also advises on the use of sprinklers in portal frames;

“A sprinkler system is often installed in portal frame buildings which are to be used for retail or warehouse purposes. In the event of a fire their effective operation would control the size of the fire and may even extinguish some fires. The resulting risk of fire spreading beyond the building of origin is greatly reduced. It follows that in a sprinklered building rafter collapse would be unlikely and the dependence on base fixity would be greatly reduced.”

In England and Wales the Department of the Environment has indicated that they consider this to be a reasonable approach.

Source: Corus

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