Living it up

Building ever-taller apartment blocks is one way to create a strong architectural statement. But it doesn’t make life easy for the fire engineer who needs to construct the ventilation strategy for spiralling stairways and escape routes.

So it was with one of the newest additions to London Docklands, now taking shape close to Canary Wharf.

The two-tower scheme, called Pan Peninsula, is one of the highest apartment buildings under construction in Europe. When completed in 2009, this focus for the Millennium Quarter regeneration masterplan will be the tallest residential building in the UK.

The developer, Ballymore, appointed Hoare Lea Consulting as the mechanical, electrical and specialist building services provider. We had our work cut out giving design advice, including fire engineering, on the giant towers, designed by architect Skidmore Owings & Merrill. The West Tower will stand 40 storeys and the East Tower will climb to an impressive 50 storeys. Together they will house more than 700 apartments.

Linking the towers on the ground and first floors will be a reception lobby, health club and spa. The scheme also includes valet parking, sky lobbies, a private cinema, restaurants, retail units and a top-floor cocktail bar.

This article describes the ventilation strategy for the West Tower.

Ventilation strategy

When Pan Peninsula was at the design stage, the 2000 edition of Approved Document B (ADB) was current. It was not until ADB 2006 that the now more common methods of smoke ventilation of common corridors were acknowledged.

The 2000 edition of ADB only makes reference to openable vents and automatic opening vents for natural ventilation of the common corridors of a residential building. The basic requirements were:

• an openable vent with a minimum free area of 1m2 to be provided to the corridor where escape is in more than one direction;
• an automatic opening vent with a minimum free area of 1.5m2 to be provided in corridors where travel is in a single direction.

In addition, single-stair buildings were considered acceptable only if the travel distance in one direction were limited to 7.5m.

At Pan, all apartments were planned around a central core to maximise the riverside views. The resulting design meant there was no external wall to the common corridor and travel distances were in excess of 7.5m. This would not have met the code-compliant requirement for corridor ventilation, so a fire-engineered ventilation solution was developed.

Several options were considered. One idea was to separate the corridor into sections, with a maximum travel distance of 7.5m, and ventilate each section with a natural smoke shaft with a cross-sectional area of 1.5m2. An alternative was mechanically-assisted natural ventilation, where a natural ventilation smoke shaft would be located at each end of the corridor with a mechanical smoke shaft, providing supply air, located directly adjacent or opposite the stair.

We also looked at putting mechanical extract shafts at either end of the corridor with a natural supply duct located directly adjacent or opposite the stair. Another option was having two mechanical smoke shafts, one providing supply and the other providing extract.

A review of the ventilation systems concluded, however, that none of these systems could resolve the unique requirements of the giant tower with a single stair. We therefore developed a double reversible mechanical extraction system, which relies on dilution rather than pressurisation to maintain conditions in the corridor. The resulting conditions are expected to be better than those that would exist under a code-compliant design.

The proposed smoke ventilation system is based on two 0.5m2 smoke shafts, providing 2m3/s mechanical inlet or extract to the common corridor. The fans will be reversible, which will allow each shaft to be either extract or supply. The system will be activated by two detectors located at either end of the corridor.

On activation of the smoke detection alarm within the common corridor, the system will start providing extraction from the duct closest to the smoke detector and supply from the duct farthest from the smoke detector. Both fans can be manually reversed by the fire and rescue service to form two mechanical extract ducts after the initial evacuation phase.

By providing an intelligent system that will extract locally, the spread of smoke along the length of the corridor is restricted. This improves conditions within the corridor and stair for means of escape and fire-fighting access.

CFD assessment

The system was assessed using computational fluid dynamics (CFD) modelling to demonstrate that the means of escape and ventilation system would be equivalent to, if not better, than that required by Building Regulations and ADB 2000. A code-compliant means-of-escape CFD model, with single direction means of escape and 1.5m2 of automatic ventilation, was constructed for comparison to the fire-engineered solution.

The results of the CFD modelling demonstrated that visibility in the corridors is far better than that provided with a code-compliant natural solution.

Pan Peninsula is set to be one of the most significant residential schemes in the UK, providing distinctive, upmarket accommodation. It will have a safe, innovative and economical fire-engineering solution to match those lofty aspirations.