Clear learning

THE Government guidance, BB 100: Design for fire safety in schools, issued by the Department for Children, Schools and Families in England and Wales in 2007, reiterates the well-known fact that smoke inhalation is the biggest cause of fire deaths, and places much emphasis on controlling smoke spread and preventing it from entering escape routes, particularly through the use of physical barriers.

Physical barriers, such as walls and doors, are the easiest method of stopping smoke from spreading, as long as the barrier is of impermeable construction, properly sealed at all penetrations, and that doors are kept closed. However, these are not conditions that are always met, due to building construction, use, occupancy or management.

As a result, there are many situations, particularly in modern schools and colleges, where additional measures are needed, such as smoke ventilation or pressurisation systems, to maintain reasonable provision for life safety. In addition, as recognised in BB 100, protection of buildings and contents should also be a consideration, and can be aided by smoke ventilation. Automatic smoke control systems (triggered by an automatic fire detection system) can limit smoke spread and keep it at high level. Systems can protect escape routes, reduce smoke and heat damage, and assist fire service operations.

Modern designs

In a traditional school design, there is often a series of small rooms, and one or two larger spaces (typically, sports and assembly halls) linked by single-storey corridors. In this type of school, compartmentation provides all the smoke control needed in almost every case. Exceptions would be basement areas, fire fighting stairs in tall school buildings, and buildings where smoke ventilation is chosen as an option to avoid the discounting of an escape stair.

For schools that do not conform to traditional design, smoke ventilation is generally provided as part of a fire engineering strategy. Modern designs, for example, can contain atria, voids through circulation spaces, and other challenges for fire engineers. Even small retail areas are not unknown.

Many of the innovations in school design are partly provided with effective day-to-day natural (comfort) ventilation in mind. It is important to note that most natural ventilation solutions can be utilised for dual-purpose smoke control with little design change, which also offers a good opportunity to reduce costs.

Typical example

A typical example might involve a school with classrooms on several storeys, linked by an atrium, with open balconies for classroom access. Comfort ventilation would likely be provided using windows and ventilators on the classroom façade for air inlet, with vitiated air passing through a high-level grille and damper into the balcony and atrium, finally exhausting at high level in the atrium. This strategy takes maximum advantage of the stack effect in the atrium and effectively ventilates classrooms, balconies and the atrium.

In the event of a fire, BB 100 refers to BS 5588-7: 1997: Fire precautions in the design, construction and use of buildings. Code of practice for the incorporation of atria in buildings, but makes it clear that the standard’s recommended fire precautions for atria only need to be applied if compartmentation is breached.

Continuing with the school example, at first glance BS 5588-7 need not be applied and no smoke ventilation required, since the top floor is well below the recommended 30m limit for the height of a single multi-floor compartment in BB100 figure 26. However, if there is no alternative escape route, BB 100 recommends that the balcony escape routes should be at least 4.5m from the atrium void (mainly to protect escaping people from smoke flowing through the void) – something that is unlikely to be practical.

The BB 100 guidance offers an option for sprinklered schools, in order to bypass the 4.5m recommendation, as long as travel is available in two directions and the travel distance along the balcony does not exceed 18m. However, providing sprinklers throughout the school simply to meet this recommendation could be a difficult and expensive exercise. Smoke control could be an effective alternative solution. If, for instance, the school has in place a natural ventilation system, it could be adapted to also provide smoke control, perhaps with automatic smoke curtains positioned at the balcony edge.

Factors to consider

Regardless of the reasons for installing a smoke control system, the basic recommendations are fairly constant. It is important to follow Parts 4 and 5 of BS 7346: 2003: Components for smoke and heat control systems, which outline the functional recommendations and calculation methods for smoke and heat exhaust ventilation systems employing steady-state design fires and time-dependent design fires, and/or the BRE guidance, BR 368: Design methodologies for smoke and heat exhaust ventilation. In addition, the Regulatory Reform (Fire Safety) Order 2005, for England and Wales, requires life safety systems to be subject to a suitable system of maintenance and be maintained in an efficient state, in efficient working order and in good repair, so maintenance access should be carefully considered for any design.

The area of ventilation required for smoke ventilation is often similar to that needed for comfort ventilation, but both should be calculated and the larger area provided.

The atrium roof vents in our school example would need to conform to BS EN 12101-2: 2003: Specification for natural smoke and heat exhaust ventilators, and there is a wide range of conforming dual-purpose ventilators available. In addition, the vents would need to be located where they are unlikely to be adversely affected by wind. A gently sloping atrium roof (<30°) is ideal, but clerestory windows are not and require additional measures. Inlet air could be drawn through the lower classrooms, if sufficient low-level openings or vents are not available in the atrium.

Smoke curtains to provide a physical barrier for additional balcony protection may be necessary in some locations, if the design and layout of the building leaves a residual risk after taking benefit of the smoke ventilation. Automatic smoke curtains, in line with BS EN 12101-1: Specification for smoke barriers, can be hidden away at the balcony edge and be unrolled in the event of fire.

System control

Control would usually be linked to the building’s fire detection and alarm system, arranged so that the smoke ventilation system operates as soon as a fire is detected in the atrium or any room posing a risk to the atrium. Early detection and operation of smoke ventilation is critical for maximum benefit; it is much easier to maintain a high-level smoke layer than to clear a smoke-logged space.

The level of sophistication of the controls and of fire resistance to the cabling depends upon the ventilator choice. If ventilators and smoke curtains fail to the fire (open/down) position on loss of power or signal, then simple controls may be suitable. If ventilation equipment needs to be driven open or down, additional protection is required to provide an equivalent level of safety.

A new European standard, EN 12101-9, is currently being developed for control panels and related equipment supporting smoke control systems. This will set the requirements for control panels to provide an equivalent level of safety whatever ventilation equipment is installed. From a fire protection viewpoint, fail-safe equipment is always preferred, but other factors, such as vandalism risk and security, may override this. In any case, a backup power supply is recommended, if only to avoid nuisance opening in the event of a power cut.

One advantage of natural ventilation is the low power required, which allows use of extra low v
oltage equipment and simple battery backup power supplies. Linkage to building management systems (BMS) is quite common, often allowing the BMS to take day-to-day control of the ventilation system until overridden in case of fire. Alternatively, the controls can be autonomous for day-to-day comfort ventilation, and be linked directly to temperature or other sensors to provide fully automatic control.

In addition to assisting with safe evacuation, such a smoke control system will help fire and rescue service personnel to enter the building, and identify and control the fire. This, in turn, can reduce the damage and costs involved in recovering from fire. Not bad for a system which can have a fairly modest marginal cost over the original comfort ventilation system.

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