FPASA BULLETIN FD 12

FIRE RESISTANCE RATINGS OF LIGHTWEIGHT
WALL CONSTRUCTIONS

Increased use is being made of lightweight or dry-wall methods of construction for the internal walls in multi-storey buildings.

There are both advantages and disadvantages inherent in the use of lightweight wall constructions when compared to the use of conventional plastered brick walls. The greater mass and nature of brickwork and most concrete blockwork ensures that, for internal walls, a far greater resistance to fire, impact loads and noise transmission is achieved.

The quality of site workmanship seldom affects these properties in the case of plastered brickwork or blockwork. However, workmanship is of the utmost importance insofar as the fire resistance of lightweight walls is concerned and the fire-resistance time given in the table will be achieved only if the workmanship is of a high standard. The details at cross-wall junctions need to be designed and then executed on site in a manner that will ensure that the fire resistance at these positions is the same as that of the wall.

The values in the table are intended merely to indicate to designers what fire resistance ratings can be expected from different types of lightweight wall constructions. General terms have been used to describe the types of wall constructions that utilise patented components such as, for instance, studs, board fixing devices, etc, which have achieved similar fire resistance ratings in wall furnace tests.

Notes to table

  1. GP board means gypsum plasterboard.
  2. UA-C sheet means unpressed asbestos-cement sheeting, wet-cured or autoclaved. The same fire behaviour will not be achieved if pressed sheets are used.
  3. X-rated GP board means a special type of gypsum plasterboard which has a small percentage of glass fibre incorporated in the gypsum core of the board.
  4. When two layers of cladding material are used on one side of a wall, a joint between the cladding sheets of the second layer must not be on the same stud as a joint in the first layer.
  5. When fixing cladding materials to the studs or nogging pieces, it is important that the distances between fixing screws or nails should not be more than 300 mm both on gypsum plasterboard and asbestos-cement sheet materials. Also, when two layers of cladding are used on a side, the first layer must be fixed in the same manner as if only a single layer were to be used. The screw fixing of the second layer should be staggered vertically relative to the fixing of the first layer.
  6. By “steel studs” is meant cold-rolled galvanised steel.
  7. Class I impact is achieved by swinging a 30 kg sandbag twice against the face of the wall from a height of 0,45 m above the point of impact.
  8. When timber studs are used in walls, the walls are considered to be semi-combustible.

Type and thickness of cladding as well as studs used in the construction of walls / Stud spacing (mm) / Fire resistance rating (min.) / Impact resistance Class /

Remarks on construction

1 x 12,7 mm thick GP board on either side of 65 x 35 mm steel studs / 450
600 / 30
20 / < 1
< 1 / Board fixed vertically or horizontally with or without glass wool or mineral wool in the cavity
1 x 12,7 mm thick GP board on either side of 75 x 38 mm timber studs. Boards fixed vertically or horizontally / 450
600 / 30
20 / < 1
< 1 / This type of construction is considered to be semi-combustible
2 x 9,5 mm thick GP boards on either side of 65 x 35 mm steel studs or of 75 x 38 mm timber studs. / 405
600 / 60
30 / 1
< 1 / First layer of cladding can be vertical with second layer horizontal or vice-versa.
2 x 12,7 mm thick GP boards on either side of 65 x 35 mm steel studs or of 75 x 38 mm timber studs. / 450
600 / 60
30 / 1
1 / As above. If both layers are fixed vertically, steel studs placed back-to-back and fixed to each other at 300 mm centres must be used.
1 x 12,7 mm thick GP board on one side and 2 x 9,5 mm thick GP boards on other side of 65 x 35 mm steel studs or of 75 x 38 mm timber studs. / 450 / 30 / 1
(or < 1) / The double layer of boards must be used on the face most likely to be impacted during normal use eg the side bordering on a passage. The other side will have an impact of <1.
1 x 12,7 mm thick X-rated GP board on either side of 65 x 35 mm steel studs. Boards fixed vertically or horizontally. / 450 / 60 / 1 / If boards are fixed horizontally, then horizontal joints between boards must be backed by and fixed to nogging joints.

Composite cladding on walls

1 x 9,5 mm thick GP board and 1 x 9,5 mm UA-C sheet on either side of 65 x 35 mm steel studs. / 450 / 60 / 1 / Cladding materials can be either horizontal or vertical. Joints between boards must be staggered.
Unpressed asbestos-cement board cladding to walls
1 x 9,5 mm thick U-AC sheet on either side of 65 x 35 mm steel studs. / 450
600 / 20
20 / <1 / Cladding sheets fixed to the steel studs with screws, so that the joints between the boards on each side of the wall are staggered.
As above but with mineral wool in the cavity. / 450
600 / 30
30 / <1 / As above
1 x 9,5 mm thick UA-C sheet on either side of 75 x 38 mm timber studs with cavities filled with mineral wool. / 450 / 30 / <1 / As above
2 x 9,5 mm thick U-AC sheets on either side of 65 x 35 mm steel studs. / 405 / 60 / 1 / Boards can be fixed either vertically or horizontally or vertically and horizontally. The joints between the boards must be staggered.
1 x 12,7 mm thick U-AC sheet on either side of 65 x 35 mm steel studs or 75 x 38 mm timber studs. / 450
600 / 30
20 / <1 / Cladding sheets fixed to studs with steel screws at 300 mm centres
As done above but with a nominal 50 mm thick mineral wool insulation blanket in the cavity. / 450
600 / 30
30 / > 1 / As above.

References:

Anon, Fire Protection of buildings, FPASA Journal “Fire Protection” Vol. 4, No. 2 June 1977

E G Williams, Fire resistance ratings of some common lightweight wall constructions, FPASA journal “Fire Protection” Vol. 7 No. 1 March 1980.

Published by

Fire Protection Association of Southern Africa

(Incorporated Association not for Gain)

(Reg. No. 73/00022/08)

P O Box 15467

Impala Park

1472


4/2000