Mechanical smoke extract systems in residential corridors

IN THE article, Smoke effects, in the July 2008 edition, Allan Meek questions the use of mechanical smoke extract systems to extend travel distance in residential corridors and suggests that fire engineers: ignore standard travel distances; pay only lip service to firefighting conditions; pressurise statutory authorities who do not have the expertise to understand the systems; and over-engineer smoke control so the strategy is not cost-effective. Mr Meek does not appear to have fully appreciated the ‘deemed to satisfy’ solution with which the comparison is made.

Traditional approach

Provision of smoke venting in residential blocks is in Approved Document B for sections of corridor providing stair access. However, up to 7.5m in an unvented dead end or 30m in an unvented middle section is permitted before reaching a vented section of corridor.

Traditional venting methods are the wall-mounted ventilator, whose effective operation is subject to favourable wind conditions and has no specified means of providing supply air, or a 1.5m2 vertical shaft which, although it deals with the wind direction, relies on the buoyancy of smoke that might be relatively cool, especially in a sprinkler-protected tower, and which has no dedicated air replenishment. As a prescriptive approach, the provision of either of these methods requires no further justification.

Ignoring travel distances

Conversely, where a fire engineered mechanical extract system is proposed, the entire length of the corridor from apartment to stair will be vented.

Ventilation does not rely on buoyancy as the smoke is drawn into the shaft by a fan, so even cool smoke is cleared, and air inlet is via a route through the stair or a dedicated supply shaft. The whole system will then be modelled to demonstrate its effectiveness.

Given this enhanced system, it is entirely reasonable to allow extension of the travel distance from 7.5m up to, say, 20m, compared with the potential alternative of a second stair, and up to 30m in an unvented corridor.

Firefighting

The concession for firefighting lobbies in residential buildings is made on the proviso of adopting the means of escape and compartmentation relevant to residential use.

Although the travel distance could be as much as 30m in an unvented corridor, the same concessions still apply. It is the distance from the fire tender or fire riser which is key to firefighting, which will remain unaffected by relaxations on travel distance.

Engineering a system that replaces an unvented 30m corridor or 7.5m dead end with 20m mechanical extract is doing more than, as Mr Meek says, ‘only paying lip service to the conditions encountered by firefighting personnel’.

Statutory authority expertise

The competency of the statutory authority to assess an engineered proposal is not the responsibility of the design team. However, we should not underestimate the investment made by building control authorities in educating their staff. Where this is not the case, they can refuse the appointment or subcontract to a third party.

Commonality of CFD packages might present difficulties in standardisation. But, assuming that a recognised and otherwise accredited program is used, then validation will be dependent on input which is agreed at the outset.

Only after extensive consultation with the statutory authorities will any extension of travel distance be approved.

Cost-effectiveness

These systems are rarely used for simple layouts. The designer is often faced with a scheme that cannot meet the 7.5m distance or where the incorporation of a second stair results in loss of a residency and associated income over every floor. Extending the travel distance by introducing mechanical extract might be the solution that makes a scheme viable.

Coupled with the reduction in shaft area to about 0.6-0.7m2 and the associated gain in floor area, the advantages offered by these systems are clear.

UK government statistics quoted relate to all dwelling fires, including houses. Some 88% of these were contained to the room of origin. There is no indication that fatalities occur in the common corridor. Of 491 fire-related deaths, 74% were in dwellings, 7.5% were in other buildings – the remainder comprising car and outdoor fires.

Therefore, residential buildings should benefit from engineered solutions, just as offices and shopping complexes have, and, I would suggest, it is the risk associated with a compliant design that should be eliminated.

James Lane
Principal fire consultant
Hilson

Promoting the passive message

A NUMBER of major fires across England have made the headlines recently. The most memorable was the blaze at the Grand Pier at Weston-super-Mare, but other incidents include: a garden centre near Luton that was completely destroyed; an Essex branch of a DIY retailer that suffered a loss; a historic brewery in Bolton; and a former church in Shepherd’s Bush in London. The causes of these fires have yet to be identified but there are some common themes.

Within minutes, the fires at each of these locations spread from their point of origin and rapidly developed, destroying the properties. In some cases, the structures collapsed, leaving little hope for restoration of the building or the businesses. A lack of sprinklers was remarked upon in two or three cases – but no mention was made of the passive or built-in fire protection that could have played a major part in saving these buildings.

Few people outside the fire industry have even heard of passive fire protection, or its key principle of compartmentation, where a building is designed with fire-secure compartments to stop the spread of fire and maintain structural stability.

Already, some consultants are asking if the fire assessments now required by legislation were adequate in these recent cases. Do the owners or those responsible for a building consider passive fire protection in their risk assessment? It seems unlikely. Yet, passive fire protection provides the backbone to a building and the first line of defence in preventing the spread of fire.

Complacency is the greatest threat. Luckily, no lives were lost in these fires – but many lives and communities have been destroyed. What will the ongoing cost be to Weston now that its number one tourist attraction is gone? We should be trying to save buildings and businesses, as well as lives.

Fire protection should be based on integrating active and built-in passive measures. Skimping on the overall package by picking and choosing selected parts of it is simply not good enough.

It is up to the fire industry as a whole to make sure everyone gets the message.

Mike Wood
Vice-chairman
Passive Fire Protection Federation (PFPF)