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Time to smokeout a new solution

technical & practice

Smoke shafts can play a crucial fire safety role in apartment blocks, but design guidance for their use remains sketchy

Designing fire safety into residential apartment blocks has always been problematical for architects, usually due to the conflict between the spatial and functional objectives, and the fire safety requirements of compartmentation and separation.

One area which has probably caused more heartache for designers than any other is the need to provide ventilation to 'common' corridors or lobbies in residential blocks. The move away from designing apartments either side of long corridors spanning the length of the building, to designing central cores surrounded by apartments, has further exacerbated this problem.

The technical standards detailed in Building Regulations Approved Document B: Fire Safety (ADB) make recommendations for the ventilation of common corridors or lobbies. The guidance includes diagrams indicating a simple method of ventilating these common spaces by using windows direct to open air, which may be manual or automatic in operation, depending on the layout of the building.

This simple approach may suit some designs, but it takes up valuable wall space, which obviously affects the fenestration of the building. It also affects the proportion of lettable space provided in a building and the layout of apartments, especially where central cores are used. The net effect is often to reduce the building's value as a result of the need to ventilate the common corridors and lobbies.

Sit tight

The main means for protecting common access areas from an apartment fire is the structural fire protection built into the apartments themselves.

Typically, each apartment forms a single fire compartment with a fireresisting, self-closing front door. In the event of a fire it is not unreasonable to presume that fire and smoke would be mainly contained within the apartment where the fire started, provided that the front door remains closed or closes behind the escaping occupants.

If the front door to the apartment is prevented from closing - say after the occupants have left - then the adjacent apartments are also provided with the same level of fire protection, including fire-resisting doors to the common spaces. A basic premise of the design of an apartment, and the fire strategy adopted, is that, at the very least, the occupants of unaffected apartments can sit out any fire which has started in an adjacent dwelling. Experience within the UK suggests that this approach works well.

Because it is often difficult to provide satisfactory ventilation, many designers have been forced to consider alternative approaches to achieve the desired aim. The Building Regulations are functional, 'goal-based' requirements and although ADB only provides guidance, it is difficult, if not impossible, to design an apartment block without some form of ventilation to the common corridor or lobby once the height of the building is above 11m.

Since ADB represents the 'benchmark' solution for fire safety design, it is often used as a basis for demonstrating 'equivalence' when an alternative design is submitted.

To help designers to solve this problem, over recent years fire consultants have developed a number of methods for ventilating common corridors or lobbies. None of these are more controversial than the 'natural smoke shaft vent' approach.

Shaft: can you dig it?

A smoke shaft is simply a chimney rising through a building with openings at each level, through which smoke can be vented at the top. Until recently, the only design guidance for smoke vent shafts was provided by the British Standard BS5588: Part 5, which deals with the design of firefighting shafts.

In essence, this standard calls for separate shafts to serve both the stairs and associated lobbies. Both shafts require an inlet direct from open air at the base, and another discharging to fresh air at the head.

The inlet required at the base of the shaft has traditionally caused designers and engineers difficulty due to the amount of room it requires. Typically, the size of the duct and its aspect ratio intrudes unacceptably into the design of apartment buildings, given the restricted storey heights generally available. If used within a central core, the shafts have to be routed horizontally at ground level to the outside of the building. With stair shafts typically 1.5m 2in area and lobby shafts of 3m 2, this wastes a lot of valuable space.

The British Standard for smoke shafts and ADB do not give much in terms of real design advice. As a consequence, designers and fire safety practitioners have been fairly ignorant of the precise benefits of a smoke shaft. Until now.

Smoke gets in your eyes

A recent study into the benefits of smoke shafts in firefighting stairs and lobbies, completed by the Building Research Establishment, has given some impetus to the use of smoke shafts in a residential setting and also called into question the effectiveness of the current technical standards.

The study included practical assessments using a scale model, supported by computer modelling of a variety of smoke shafts.

The research raised as many questions as it answered, but its main conclusions were: that the standard openable-window type vent was of little, if any, benefit in ventilating firefighting lobbies or stairs; and second, that the use of smoke shafts could provide sufficiently improved ventilation.

Openable window type vents are inefficient at removing smoke and are particularly subject to the vagaries of wind and weather. If smoke from an apartment fire enters common spaces in significant quantities, the simple window form of ventilation would be unlikely to produce conditions within the corridors which would be considered tenable for escape purposes.

The BRE acknowledged that smoke shafts are also not perfect. But, if properly designed, they are less prone to the vagaries of the weather and so, in terms of removing smoke, are more likely to provide better protection to the space affected.

Although the BRE research was primarily aimed at commercial and industrial applications, its results and recommendations seem equally appropriate to the residential setting.

I have applied this approach to several residential apartment blocks and have successfully achieved the necessary statutory approval in all cases.

Among the main benefits of applying this approach to residential buildings is the removal of the need to provide an inlet at the base of the single smoke shaft serving the common corridor or lobby, as well as removing the requirement for a separate smoke shaft to the staircase. The basic premise is that the revised smoke shaft design will provide conditions within the lobby or corridor which are at least as good as the standard openable windows and will most likely provide better conditions for escape and firefighting in those spaces.

While smoke shafts designed in accordance to the British Standard have been, and still are, typically approved by the regulatory authorities without a second thought, the recent innovations and suggested improvements to the design of smoke shafts have raised a number of questions.

Size isn't everything One of the main concerns about the use of various sizes of smoke shafts is the height over which they may be effective. The recent research by BRE looked into the use of 1.5m 2 shafts. The use of 3m 2 shafts was modelled for buildings up to 100 storeys, but for the 1.5m 2 shafts the modelling was restricted to buildings up to 10 storeys. Therefore the precise limits to the effectiveness of such limited area smoke shafts are still in question.

Hand in hand with these innovations comes the requirement for firefighting shafts in some of our taller apartment blocks. Recent changes to the relevant guidance mean that the common corridor can provide sufficient protection to firefighters so that a separate firefighting lobby is no longer required.

Because of the way the guidance is written, the issue of staircase ventilation is unclear. In some cases regulators have continued to require separate ventilation to the firefighting stair, either by openable windows or smoke shafts. One of the advantages of the revised guidance is that staircase ventilation is not required, other than to provide an automatic opening vent of 1m 2 area at the head of the stairs. But there are still questions about the use of such vents in residential developments - primarily the untested impact that the height of the shaft might have on their effectiveness.

As it stands, it is still not known how applicable the smaller-area shafts are to buildings of more than 10 storeys. This leaves a number of issues to be investigated:

ln order to provide detailed design guidance, we need to understand more clearly what conditions are necessary for safe means of escape through the common spaces of residential buildings - and what requirements should apply for firefighters.

We need to determine ways in which the identified conditions can be achieved. The research by the BRE suggested that current levels of fire safety provision are unlikely to provide conditions normally considered tenable under anything but ideal ambient conditions. In the UK it would be extremely unusual for a person to be injured in the common parts of an apartment block while escaping from a fire.

The degree of smoke ventilation required for apartment block common access corridors and lobbies needs to be reassessed. Any suggestion that a higher degree of ventilation is necessary should be seriously questioned, since at this stage, under the current guidance, we cannot even be certain what the ventilation is for, who it is designed to protect and what sort of conditions should be provided.

Glenn Horton is technical director at Locke Carey Consulting, Dartford, Kent. Contact 01322 272530 or visit www. lockecarey. co. uk

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