Part G3 Atrium construction

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.

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.

Figure G3D2 illustrates the method of measuring the atrium well to comply with G3D2 and the location of an atrium’s bounding walls.

Figure G3.2

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 floors included in the area permitted by Table C3D3.

Figure G3D3 illustrates the location of an atrium’s bounding walls.

Figure G3D3

Section showing floors of an atrium included in the area permitted by Table C3D3.

G3D4 establishes the required FRL and acceptable construction requirements for the bounding walls required by G3D3. See Figure G3D4.

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:

• G3D4(a) applies where the bounding walls are constructed of materials with a FRL of 60/60/60; or
• G3D4(b) applies where the bounding walls are constructed of fixed toughened or wired safety glass in non-combustible frames.

Figure G3D4

Examples of bounding walls complying with G3D4.

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.

G3D5 takes precedence over the provisions of D3D17 to D3D21 for barriers around an atrium well.

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.

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.

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.