Branngardiner - en drøm for en arkitekt og et mareritt for en branningeniør?
Branngardiner er ikke et nytt fenomen, men har likevel fått et nytt fokus. En automatisk branngardin kan ved utbrudd av brann begrense og kontrollere spredning. Men hvor nyttige er de når det både stilles krav til å ivareta arkitekturen, uten at det skal gå på bekostning av brannsikkerheten?
What are fire and smoke curtains?
Fire curtains are an electrically operated system used for achieving compartmentation requirements, mainly in open plan buildings and designs that tend to be a bit more complex. They can provide separation between two adjacent spaces, they can offer protection to elevator doors, and can protect evacuation routes in open plan layouts. Fire curtains are expected to be exposed to fire and are thus rated to achieve protection in ranges of 1000 °C. However, smoke curtains are expected to only be exposed to smoke and thus should achieve protection up to 600 °C. Different manufacturing companies can produce different ratings.
Active fire protection
They are an active fire safety system, meaning they need a trigger to be activated, usually that trigger is in the form of an alarm and detection system. They can achieve compartmentation the same way that fire rated doors or compartment walls do. When triggered, they drop vertically to the floor from the ceiling, hence sealing the required area. Errors are also minimized by relying on gravity to pull the curtain down. They can also be used horizontally for level separation in an atrium.
Aesthetically pleasing function
Due to the fact that they are triggered by an alarm, detection system, or a manual switch, smoke and fire curtains thus remain in their retracted position until they are triggered. This offers complete invisibility when not functional, thus maintaining the required open plan design, an architect’s dream. When employing fire and smoke curtains, open plans spaces are conserved, atriums are deemed acceptable, and large architectural features such as huge spiral staircases can be used for escape. People can evacuate from buildings while passing beside those curtains, fire damage is reduced by avoiding smoke accumulation, and fire fighters can safely fight fires.
Besides the levels of heat that these curtains can withstand, they are usually tested to achieve fire resistance for a period of 60 minutes. Most manufacturing companies mention that testing to be undertaken to BS 476 part 22 in accordance with BS 476 part 20. This is usually attained through using a glass fibre fabric which is then treated with a flame-retardant aluminium filled polymer. This allows for maintaining minimum thickness and weight with maximum fire protection. The fabric can supposedly achieve class 0 classification according to BS 476 6:1989 and BS 476 7:1997.
As an electrical and mechanical system, continuous maintenance is essential to make sure that it remains in a good working condition as the consequences of a failure can be devastating to both human lives and property. Abiding by regulations and insurance requirements also requires timely check-ups.
The other side of the truth
All the details mentioned above seem like smoke and fire curtains can be the perfect solution to any problem, especially when the architect and the fire engineer cannot agree on the architect’s vision. Fire regulations are there for a reason, and that’s mainly to protect lives, while also maintaining an acceptable level of property protection. Then what’s the problem? Why can’t fire and smoke curtains be used in all building and save the fire engineer the hassle and explanations they need to offer architects?
Guidance requirements – Approved Document B
As impressive as fire curtains can seem, they cannot be used to solve all the problems we as fire engineers might face. First of all, according to Approved Document B (the main guidance used in the United Kingdom), a compartment wall separating occupancies should achieve a fire resistance of a minimum of 60 minutes in terms of loadbearing capacity (when required), integrity, and insulation.
Also, the compartment wall forming a separate part of a building should run the full height of the building in a continuous vertical plane. The above two requirements suggest that fire curtains cannot be used as an alternative to full wall. This is because they cannot achieve the required insulation (based on the material they are made of and their thickness) and they cannot run the full height of the building. However, they can be used as an alternative to certain doors or shutters pending that they do not form more than 25 % of the wall area.
The insulation requirement becomes an issue again when discussing escape routes that pass beside a fire or smoke curtain. As these elements cannot achieve insulation, the person evacuating through that area would be subject to very high temperatures thus rendering the escape route untenable if it had a small width.
Another issue is when fire or smoke curtains are to be used in lieu of a door. Approved Document B also states that fire door-sets should be classified in accordance with BS EN 13602 part 2. This requires that fire doors are to be tested for smoke leakage when the temperatures reach 200 °C. This is often referred to as Sm or medium temperature leakage as per BS EN 1634 part 3.
Clause 220.127.116.11.1 of BS EN 13501-2:2016 specifies that “Smoke leakage is the ability of the element to reduce or eliminate the passage of smoke from one side of the door to the other. The following performance levels are defined:
a) Smoke leakage S200 - when the maximum leakage rate measured at both ambient temperature and 200 °C and up to a pressure of 50 Pa does not exceed 20 m3/h for a single leaf door-set, or 30 m3/h for a double leaf door-set;
b) Smoke leakage Sa - when the maximum leakage rate measured at ambient temperature, and at a pressure of up to 25 Pa only, does not exceed 3 m3/h per metre length of gap between the fixed and moveable components of the door-set (e.g. between the door leaf and door frame), excluding leakage at the threshold.
Most manufacturing companies do not consider the smoke leakage criteria in their certification tests. Moreover, those tests are conducted for a 25 Pa pressure rather than the required 50 Pa. This poses a major issue when using fire and smoke curtains to replace fire doors.
Guidance requirements – British Standard BS9999
Another problem is when architects are attempting to use fire curtains to substitute walls of fire fighting shafts. Clause 20.3.2 of BS 9999 specifies a series of requirements for fire fighting shafts that a fire curtain cannot achieve. This clause and its adjacent table (table 18) stipulates that when the walls are not constructed of non-combustible sturdy materials such as brick or concrete, they would need to tested to achieve certain levels of stiffness, low surface damage upon impact of small and large bodies, as well as resilience to crowd pressure. These requirements are almost impossible to realize when using fire and smoke curtains.
Though fire and smoke curtains can be employed to realize the architect’s vision and design concept, the fire engineer should be very careful when specifying those elements in their fire strategy. Considering that they require continuous maintenance, do not achieve all regulation requirements in a lot of cases, and can pose a danger to people evacuating in their proximity, fire engineers need to tread very lightly around this choice. Lots of research for each specific case should be undertaken to determine whether using such features can solve a problem at hand or create a new one.