Part G3 Atrium construction
Application of Part
This Part does not apply to an atrium which—
To clarify which atriums must comply with Part G3.
The BCA defines the term “atrium”. It is important to understand this BCA definition before considering Part G3. See definition in Schedule 2.
Part G3 does not apply to an atrium if it connects two storeys in a non-sprinkler protected building, or up to three storeys if each of the connected storeys is sprinkler protected and one of those storeys is at a level giving direct egress to a road or open space.
This provision is consistent with D2D17, which deals with the number of floors that can be connected by a non-required non-fire-isolated stairway, ramp or escalator.
Dimensions of atrium well
To minimise the risk that radiant heat from a fire will affect people or materials on the other side of an atrium well.
The BCA defines the terms “atrium” and “atrium well”. It is important to understand these BCA definitions, before considering Part G3. See definitions in Schedule 2.
An atrium well is not limited to having a maximum size, nor is there a limitation on its shape, except that it must contain a space throughout which could contain a cylinder with a diameter of not less than 6 metres. This 6 metres is equal to the minimum distance between unprotected openings in external walls of adjoining buildings required by C4D3. This diameter void is intended to reduce the effects of radiant heat from a fire:
- affecting occupants evacuating on the other side of the atrium well; and
- igniting materials on the other side of the atrium well.
Separation of atrium by bounding walls
An atrium must be separated from the remainder of the building at each storey by bounding walls set back not more than 3.5 m from the perimeter of the atrium well except in the case of the walls at not more than 3 consecutive storeys if—
To minimise the risk of spread of fire and smoke from or to the atrium or other parts of the building.
G3D3 requires the use of bounding walls, which limit the risk of fire and smoke spreading from the atrium to other parts of the building and other parts of the building to the atrium.
The detailed requirements for bounding walls are explained in the comments on S31C5.
The bounding wall and maximum 3.5 metre setback requirements do not apply where there are three or less consecutive storeys and:
- one of these storeys is located at the level from which direct egress is provided to a road or open space; and
- the total floor area of these storeys is limited to that specified in Table C3D3 (the floor area referred to in G3D3(b) is that of the three storeys, and not the total floor area of the atrium).
Figure G3D3 illustrates the location of an atrium’s bounding walls.
Section showing floors of an atrium included in the area permitted by Table C3D3.
Construction of bounding walls
Bounding walls must—
- have an FRL of not less than 60/60/60, and—
- be constructed of fixed toughened safety glass, or wired safety glass in non-combustible frames, with—
To limit the spread of fire between different parts of a building by way of an atrium.
The walls bounding an atrium are not fire walls as defined in the BCA. Therefore, these walls do not create separate fire compartments within a storey of the building. The bounding walls define the extent of the part of the building regarded by the BCA to be an “atrium”. See Figure G3D3.
G3D4 prescribes two options for construction of bounding walls:
Examples of bounding walls complying with G3D4.
Construction at balconies
To maximise the safety of people who have access around an atrium well.
To maximise public safety, a barrier is required at the edge of a balcony to an atrium well. The barrier must:
- have no openings and be non-combustible, to act as a shield from heat during a fire; and
- have a height of not less than 1 metre—which is consistent with D3D18.
Separation at roof
To minimise the risk that the roof of an atrium will not remain in place during a fire.
Clauses S31C7 to S31C14 of Specification 31 require smoke extraction through the roof, or near the top of an atrium - this smoke extraction is required to be by mechanical exhaust or by smoke and heat vents.
It is therefore important that the roof remains in place during a fire and not allow the re-circulation of smoke. To achieve this, G3D6 requires:
- the roof to have the FRL required by Table S5C11g of Specification 5; or
- the roof structure and membrane to be protected by a sprinkler system (other than a FPAA101D or FPAA101H system).
The materials used in the roof must comply with:
- the fire hazard properties specified in Specification 7; and
- when the roof is required to have an FRL, G3D6(a) does not allow the use of the concessions contained in Clauses S5C14, S5C15 or S5C16 of Specification 5 for the roof of the atrium.
Means of egress
To require that safe egress is available from an atrium.
Although the building may have an effective height of less than 25 metres, two exits are required from any area within an atrium. This is due to the additional fire and smoke hazards associated with atriums.
There may be other parts of the BCA which permit buildings to have only a single exit. G3D7 overrides any such provisions insofar as they relate to atriums.
All other aspects of the means of access and egress from an atrium must comply with Section D.
Fire and smoke control systems
To specify the additional fire and smoke control systems required for an atrium.
Because of the additional fire hazard associated with an atrium, special fire and smoke control measures are required to reduce the likelihood of conditions being reached which could endanger the safety of occupants before they have time to evacuate. These measures are set out in Specification 31.