The LWF Blog

Thinking outside the box – Part One – Escape stair and common corridor – fire engineering solutions

July 2, 2013 10:03 am

Historically, residential buildings have been restricted in design by prescriptive building standards and codes of practice. These stereotypical layouts have the advantage of adhering to fire safety measures, but stifle creativity and design flair.

Developers are now being tasked with larger, more complex designs and more traditional designs are seen as restrictive and inappropriate for high profile, modern developments.

Fire safety cannot be ignored during the design process and so the challenge of how to design a fire safe and innovative build becomes paramount to architects, developers and fire safety consultants, such as LWF.

Escape stair and common corridor

In the UK, it is accepted that egress without the use of a protected escape stair for any floor over 4.5m above access level is not practical.

Approved Document B of the Building Regulations states that the maximum travel distance within the common corridor is 7.5m, when measured from the furthest flat entrance door to the escape stair, for example. Opening vents measuring 1.5sq m are typically installed at each end of the common corridor and an open-able vent to the stair.

These tenets mean that if the length of the common corridor exceeds 7.5m, a second stairway will be required. Economy and efficiency in many modern builds means that the construction of a second stairway is an unattractive proposition.

Fire engineering solutions can enable extensions that would not otherwise be possible. While minor extensions can be agreed without additional fire safety measures being implemented on occasion; a significant extension or an extended single direction travel distance would necessitate a fire-engineered design. One such design could be a ‘push-pull’ system; mechanical extract and natural supply ducts at opposite ends of the corridor, coupled with backup equipment and smoke detectors to activate the system in case of fire. This would effectively remove limitations on the distance to travel and travel distances up to 20m in a single direction become a possibility.

During the evacuation stage while occupants from the fire origin flat are evacuating, air is supplied from the inlet point at one end of the common corridor and extracted at the other end. This has the effect of preventing smoke from entering neighbouring flats and the single stairway. However, the system alters (i.e. both fans extract smoke) during fire service intervention and the principal is to keep the corridor smoke free so that fire service can approach the flat of fire origin. In order to achieve this, the supply fan is put into reverse by the fire brigade and smoke is extracted from the common corridor. Make up air comes from the open door from the single escape stairway. To assist this arrangement, the door from the stair needs to be designed to open into the common corridor at each level, without impeding occupant egress or fire brigade access.    

In comparison to a code compliant layout for common corridors, the equipment cost associated with the fire engineered solution will impact on the building cost. However, a cost/benefit analysis should be undertaken and the cost of the system is often outweighed by the additional space provision achieved within the building due to the provision of a single escape stair. 

Any fire engineered solution is subject to acceptance by the approving authority. One difficulty with this approach is that there is no performance requirement given for the code compliant layout so there are no defined acceptance criteria for the performance of an alternative method. In order to demonstrate the effectiveness of a fire engineered solution there are different approaches that can be followed:

• A comparative approach attempts to compare the conditions achieved using a code compliant solution and show that the engineered solution can achieve the same or better conditions. 
• A deterministic approach sets out acceptance criteria based upon physical properties required for life safety such as temperature, toxicity of smoke or visibility and then attempts to show that these criteria are met. 
• A probabilistic approach attempts to show that the probability of a defined hazard is so low that the risk can be considered to be acceptable. 
• In all of the above approaches some degree of computational modelling is likely to be required to quantify results.   

Part two of the ‘Thinking outside the box’ series will look at the internal planning of flats.