The design brief for the Castlereagh Street building was based around Stockland’s desire to overcome the problem of ‘silos’ in the business. It also wanted to exemplify how existing buildings can achieve a good environmental performance. Bligh Voller Nield, architect for the project, addressed this issue by designing an open atrium throughout eight-storeys of the 32-storey building. This atrium was then interconnected with an open stair. But the atrium proved to be a challenge for Arup, who was responsible for fire engineering.
“Developing an open atrium high up in an existing building was a challenge. With eight floors open to each other, in the event of a fire, it would mean everyone would need to be evacuated at the same time. The zone smoke control system couldn’t achieve required pressure differentials for this number of floors, and fire fighting would be difficult, particularly in the unlikely event of sprinklers failing to control a fire,” says Marianne Foley, senior associate at Arup and project engineer for the project.
The problem was in the event of a fire, smoke could easily spread to other floors in the building. Foley says one standard solution is to use large quantities of smoke exhaust from the top of the atrium, as well as high supply rates onto non-fire floors. But this solution was not possible at the Stockland headquarters as the atrium was not located at the top of the building and would have meant huge smoke exhaust ducts punched up through existing tenancies.
The design solution Arup applied was through the use of fire curtains to recreate compartmentation in the event of a fire. It also utilised a standard zone pressurisation system to minimise smoke spread between floors, as is done in standard high-rise buildings without atria.
Horizontal fire curtains were used on two floors, levels 25 and 27, to recreate the fire compartmentation that would have been given by floors. Vertical curtains were also used to close around the stairs at these levels to complete the compartmentation. This gave three fire compartments in the tenancy in the event of a fire — two of three storeys and one of two storeys.
“Fire curtains have not been widely used in Australia and even the use of vertical curtains would have been good to enable the creation of the open void. The real breakthrough came with the finding of the horizontal fire curtains,” Foley says. “The horizontal fire curtains are contained on a roller at the edge of the void and are carried on fine wires to seal the void on the fire trip. Because there is a stair running through the void, to achieve the sealing, vertical curtains were used in conjunction with the designing of the staggered void to complete the separation.”
The fire curtains were supplied and installed by Smoke Control and manufactured in Germany by Stoebich. The Fibershield fire curtains are made from a reinforced and coated glass fibre fabric, run in simple terms on a roller contained within a header box. The curtains run on side guides to achieve the sealing required to achieve the fire rating of -/120/-.
The curtains are electrically operated using electric motors and an electromagnetic brake to control the rate of closure. For the horizontal curtains, stainless steel cables spaced at 1,500 mm nominal centres are required to be installed across a horizontal opening to support the travel of the bottom bar. Unlike vertical fire curtains, horizontal curtains use a gas propulsion system rather than a gravity drop.
As the horizontal curtains hadn’t been used in Australia before, Arup undertook extensive research into the product. It carried out a review across its 150 global engineers on the performance of fire and smoke curtains in completed buildings to identify problem suppliers or manufacturers.
“Three specific issues came up that we hadn't expected. The first was a query about the weight of water that could be supported by the curtains, related to fire fighting. Although we didn't expect large amounts of water to end up on the curtains, brigades were concerned about the possibility of sudden failure of a curtain falling onto firefighters below,” Foley says.
To mitigate this, the manufacturer tested the curtain and amended its design slightly to ensure there would be leakage through the curtain to prevent unexpected failure. Bligh Voller Nield also designed the balustrade to minimise water flow onto the curtains.
The second issue was again a brigade concern about people jumping onto the curtains when they were in the closed position. “Although it's hard to imagine when and why this would happen, and although the curtains can take significant weight, as a potential safety issue it was taken seriously and signage included on the curtain to say 'do not walk',” Foley says.
Finally, curtains need relatively dust free guide rails and hence cannot be commissioned until the project is no longer a building site, or at least not until all dusty works are finished. Also, the pressures generated in buildings with zone pressurisation systems may be significantly greater than the required 20Pa, and may cause some interference for the drop of the vertical curtains. This is overcome by dropping the curtains before activating the zone pressurisation.
The fire curtains not only cater to the immediate threat of fire, but also cater for the future. There are no rules in terms of use of the curtains and there are no restrictions. The only maintenance requirement is stairs need to be kept clear to allow the curtains to drop.
