Specification G3.8 Fire and smoke control systems in buildings containing atriums

Specification G3.8 includes the requirements for fire and smoke control systems in buildings containing an atrium.

Where a sprinkler system is required, it must be installed in the whole building, not just that part in which the atrium is located.

Generally, a sprinkler system must comply with Specification E1.5. The remainder of Clause 2 of Specification G3.8 sets out additional requirements for sprinklers in a building containing an atrium.

If any conflict exists between Specification E1.5 and Clause 2, then Clause 2 takes precedence.

Clause 3 requires smoke extraction through the roof, or near the top of the atrium. It is therefore important that the roof remains in place during a fire and does not allow the re-circulation of smoke. To achieve this, G3.6 requires the roof:

• to have the FRL required by Table 3 of Specification C1.1; or
• structure and membrane to be protected by a sprinkler system (other than a FPAA101D or FPAA101H system).

Figure Spec G3.8 illustrates the alternative protection of an atrium roof.

When the roof is required to have an FRL, G3.6(a) does not allow the use of the concessions contained in Clauses 3.4, 3.5 or 3.6 of Specification C1.1 for the roof of the atrium.

The sprinkler protection requirement only applies to a roof located in an area where a fire could affect its integrity. The distances stated in Clauses 2.2(a) and (b) differ because of the different fire loads assumed in the different Classes of building. The heights of the atrium roof have been selected as those beyond which a smoke plume produced by a typical fire would no longer be hot enough to damage an unprotected roof.

The sprinkler heads need only be arranged to give a wetting effect to both the underside of the roof membrane and any part of the supporting structure.

The higher temperature rating of the sprinkler heads is to make sure they do not falsely discharge due to the potentially higher normal temperatures under an atrium roof, especially if a glass or translucent roof is used (as is common practice).

Clause 3 requires smoke extraction through the roof, or near the top of an atrium. The smoke extraction is required to be by mechanical exhaust or by smoke and heat vents.

Because of the height of the roof in an atrium, the sprinklers protecting it are unlikely to be effective at the lowest level of the atrium. Additional sprinklers are therefore normally required to protect that level, as this area is normally a large open space, and is potentially the primary source of a major fire. Adequate protection of the floor often requires the use of sidewall sprinkler heads, as required by Clause 2.3(a). Clause 2.3(a) requires a “performance-type” decision as to the types of sprinklers (sidewall and overhead) and their combination.

Because of the additional fire hazard associated with atriums, Clause 2.3(b) requires the use of fast response sprinkler heads.

Although a “fast response” sprinkler head can activate at the same temperature as a “normal” sprinkler head, its reaction time is much shorter and it has different discharge characteristics. Therefore, Clause 5 of Specification E1.5 requires that the sprinkler system be designed specifically for the use of these heads.

Clause 2.4.1 sets out the requirements for sprinkler protection of glazed bounding walls. These requirements are illustrated in Figure G3.4.

Under Clause 2.4.2, the location of the sprinkler heads must allow full wetting of the glazing without wetting an adjacent head. Consequently, water should run down the window, cooling the glass and therefore creating a resistance to the spread of fire or smoke through the glazed wall by reducing its potential to failure during a fire. It is also important that the adjacent sprinkler head not be wetted, because the cooling effect of the water may delay its activation.

Because of the additional fire hazard associated with atriums, Clause 2.4.3 requires the use of “fast response” sprinkler heads.

Although a “fast response” sprinkler head can activate at the same temperature as a “normal” sprinkler head, its reaction time is much shorter and it has different discharge characteristics.

Clause 2.4.4 sets out the minimum requirements for water discharge rates. Clause 5 of Specification E1.5 therefore requires that the sprinkler system be designed specifically for the use of these heads.

Clause 2.4.4 sets out the required water discharge rates on any glazing in a bounding wall. Because the atrium can have a high fire load and large volume, the higher flow rate of 0.25 L/s.m² is required on the atrium side of the glazing when the wall is not set back from the atrium well.

The importance of the wall sprinklers to glazed walls in a fire is such that Clause 2.4.5 sets out the minimum potential coverage to be provided by such sprinklers, which in turn determines the characteristics of the water supply in terms of pressure and quantity.

The water supply for the wall wetting system referred to in Clause 2.4.5 is additional to the water supply for the other sprinklers in the building.

It is assumed that the size of a fire will be controlled by the sprinkler system installed in the building. The greater height required to be covered by a wall wetting sprinkler system in a Class 6, Class 7 or Class 8 part of a building (see Clause 2.4.5(a)(ii)) reflects the greater fire load contained in such buildings.

When referring to sprinkler and wall wetting stop valves, Clause 2.5 requires that:

• they be monitored to detect and warn of any unauthorised closure; and
• the sprinkler and wall wetting systems have independent valves.

As generally required by the BCA, mechanical air-handling systems must comply with AS 1668.1. However, if any conflict exists between AS 1668.1 and Specification G3.8, the Specification takes precedence.

The basic operation of the mechanical air-handling system during a fire is to limit the spread of smoke. To achieve this it must:

• maintain a tenable atmosphere along balconies to allow the occupants to evacuate; and
• avoid smoke being drawn into the atrium when a fire occurs in another part of the building, which requires that—
  • the atrium smoke exhaust fans activate only when smoke enters the atrium;
  • the atrium cannot be used as a return air path; and
  • the normal relief or exhaust fans in the atrium must stop normal operation. If necessary, these fans may be designed for exhausting smoke from the atrium.

The intent of the details contained in Clause 3.2(d) and (f) is to create a negative air pressure on the fire floor or in the fire affected compartment, so that air and any entrained smoke is drawn to it. This creates positive pressure on non-fire floors, so as to prevent the migration of smoke to non-fire floors while exhausting smoke from a fire affected floor.

The smoke control system may be activated by any of the methods listed in Clause 3.3(a).

The location of the controls for the system may be any of those listed in Clause 3.3(b). Note that the locations are in areas normally accessed and used by the fire brigade during a fire.

Clause 3.4 specifies the characteristics on which a smoke exhaust system design must be based, which are:

• the size to which the sprinkler system will limit a fire, in terms of its heat output and perimeter;
• the extent of any smoke plume; and
• the system’s discharge rate, as calculated under Figure 3.4 of the BCA.

Figure 3.4 of the BCA must be used to determine the minimum smoke extraction rate from the atrium. The required rate depends on the fire load specified in Clause 3.4(a) and the height of the smoke plume above the floor of the atrium well specified in Clause 3.4(b).

The exhaust rates are based on the need to maintain the smoke plume safely above any egress path.

If the smoke is allowed to travel up the atrium due to its own buoyancy, in higher atriums it will reach a stage where it will not travel any higher. The reasons for this include:

• the smoke will not have enough buoyancy to reach the top of the atrium; and
• the smoke will entrain air, and cool as it rises.

Accordingly, to make sure the smoke continues to travel up the atrium well, minimum (see Clause 3.5 (a)) and maximum (see Clause 3.5(b)) air velocities are specified. The maximum velocity specified in Clause 3.5(b) only applies to an atrium well with a constant plan cross section.

To make sure that exhaust fans operate effectively during a fire, they must be designed to operate for at least 1 hour at a temperature of 200°C (see Clause 3.6(a)).

Under Clause 3.6(b) and (c), to make sure that at least one fan continues to operate during a fire:

• at least three fans are required in atriums adjoined by Class 2, Class 3 or Class 9 parts, because of the heightened risk levels in such areas. This is due to such factors as the possibility that people will be sleeping and, in Class 9a buildings, the presence of non-ambulatory occupants; and
• at least two exhaust fans are required in all other atriums.

Smoke and heat vents are permitted in low-rise atriums (i.e. atriums less than 12 metres high), instead of a mechanical smoke extraction system.

The reason for the Clause 3.7(a) limit is that 12 metres is considered to be a height to which smoke will travel due to its own buoyancy and still be effectively vented.

Clause 3.7 does not apply where a Class 6 part of a building adjoins the atrium.

Clause 3.7(b) requires that vents be fitted with a manual override switch, for use as necessary by emergency services personnel.

Clauses 3.1 to 3.7 require that smoke-laden air be exhausted from an atrium. Clause 3.8 sets out how this exhausted air is to be made-up (i.e. replaced) in the atrium.

Clause 3.8(a) is a performance criterion that requires make-up air to be provided from outside the atrium at a level at or near the lowest storey of the atrium and non-fire storeys.

To make sure that the area where the bounding walls are set back from the atrium well is kept smoke free, an air velocity of 0.1 m/s is required by Clause 3.8(b). This velocity is consistent with that required through an open door by a fire-isolated stairway pressurisation system which accords with AS 1668.1.

Clause 3.8(c) provides a means of achieving Clause 3.8(a). It is important that the make-up air assists (and does not disturb) the exhausting of the smoke layer. To achieve this, the make-up air should enter the atrium at as low a level as possible, preferably at the base of the atrium. The make-up air must be provided from:

• openings designed to open to outside air on detection of a fire in the atrium;
• a system of ducts to supply outside air to the atrium; or
• a combination of the above.

If a system of ducts is used to supply the make-up air, to make sure the system operates when needed during a fire, the ducts must have an FRL of 60/60/60 if they pass through a different fire compartment to the atrium.

A building’s fire detection and alarm system must, in general, comply with AS 1670.1. However, if any conflict exists between AS 1670.1 and Specification G3.8, the Specification takes precedence.

Clause 4.2 sets out, in detail, the requirements for smoke detectors within an atrium. The aim of these requirements is to make sure that the smoke detection system operates effectively and false alarms are minimised.

Clause 4.3 sets out the requirements for smoke detectors at return and relief air openings.

Alarm systems required in a building containing an atrium, must include a break glass alarm at each door to a fire-isolated exit. The aim of this provision is that a person is able to break the glass setting off the alarm as they evacuate the building.

Where a sampling type smoke detection system is provided, a staged alarm must be given. The stages are set out in Clause 4.4(b)(i) to (iii). The reason for the staged alarms is to minimise the occurrence of false alarms.

Under Clause 4.4(c), beam and point type smoke detectors (as with a sampling type smoke detection system) must also operate as set out in Clause 4.4(b)(i) to (iii), but at the levels set in AS/NZS 1670.1. This provision only applies to beam and point type smoke detectors required by the BCA.

Clause 5 requires the installation of an emergency warning and intercom system in any building containing an atrium. The system must comply with:

• AS 2220.1 and AS 2220.2; and
• the additional requirements set out in Clause 5(b).

The system is a combination of an emergency warning system and an emergency intercom. The main function of an integrated system is:

• to generate an alert and evacuation signal;
• to facilitate communication with evacuation zones; and
• to provide communication between the building management or emergency services personnel and strategic points within the building.

The operation of the system may be by a number of measures referred to in Specification G3.8, including the break glass alarm required by Clause 4.4(a).

Clause 6 only applies where a required path of travel to an exit passes through an atrium (see Clause 6(a)).

Under Clause 6(a), because of the need to continue the operation of emergency services, a standby power supply is required. This is particularly important because of the increased fire hazard associated with atriums, and the dangers for occupants if they have to evacuate through atriums.

To make sure it operates when needed during a fire, the standby power supply must:

• automatically come into operation when the normal power supply fails;
• if located within the building, be protected by fire-resisting construction;
• be connected to the safety systems by means of cabling suitable to resist fire; and
• come from two supply sources.

Clause 6 is consistent with the protection required for electricity supply systems in C2.13.

Clause 6(c) sets out means of achieving Clause 6(a). To make sure that the necessary power to operate the emergency equipment is available during a fire, it requires the standby power to be from one of the listed alternatives.

Reference to E2.2, and in particular Table E2.2(a), means that all fire-isolated exits serving an atrium must be provided with a pressurisation system in accordance with AS 1668.1. The reason for this is the additional fire hazard associated with atriums and for occupants evacuating them.