Smoke, stacks, and second stairs
SMAY Expert Radek Sikorski examines two different, and not mutually exclusive, methods of enabling safe exit routes from high-rise buildings. How does the Safety Way – Pressure Differential System ensure a safe evacuation?
Fire safety in the design, management and use of residential buildings. Code of practice
Last month marked six years since the Grenfell tragedy. It was a deeply painful blow for British society and the trauma of it continues to resonate. It was also a shock to the fire safety and construction industry – a wake-up call. It was obvious that action had to be taken and it had to be done quickly. Therefore, in 2021, a draft update to BS 9991 (Fire safety in the design, management and use of residential buildings. Code of practice) was released. It regulates fire safety in residential building design, management, and use, and provides specifications and practices for appropriate fire safety measures. The previous update took place in 2015, while the current version is out for consultation.
Two escape staircases for buildings higher than 18m
One of the new requirements in the draft document is for buildings higher than 18m (10.1) to have at least two escape staircases or to meet additional conditions, one of which is overpressure protection of the escape route. This is a change to the previous provision 14.1.3 in the 2015 document, which allowed for a single staircase, giving a very wide range of measures for buildings higher than 11 m (natural ventilation or mechanical extraction or pressurisation).
Second staircase – is this a solution?
Banning single-staircase high-rise residential buildings was not one of Dame Judith Hackitt’s report recommendations, but it was a consequence of a lack of trust in the stay-put strategy after Grenfell. This is hardly surprising as the incorrect compartmentation, the rapid fire’s development along the façade, and the resulting smoke in the stairwell meant that evacuating as quickly as possible was the only solution that allowed people to survive.
The opinion of the National Fire Chiefs Council
Following the opinion of the National Fire Chiefs Council “a correctly designed second staircase removes the risk of a single point of failure, buying critical time for firefighting activities, and providing residents with multiple escape routes.” This is true. The second staircase gives an alternative. It doubles the capacity of people flow, so makes it easier for firefighters to reach the fire floor uninterrupted.
Still, it is unclear what height of building makes the use of two staircases necessary – 30m, 18m, or 11m? The economic cost of this solution (on an investment and economic scale) is also being estimated. This is not a problem exclusive to developers – this cost will be passed onto residents, will be reflected in savings on other elements (smoke control systems for instance), or will result in buildings being built as tall as possible to compensate for the loss.
Moreover, it is also still unclear whether such a prescriptive solution is the most efficient way to ensure the safety of evacuees – whether this passive solution shouldn’t be complemented by active measures, such as a pressurisation system.
International Business Development Manager – SMAY
Pressurisation systems – is this a solution?
There is a solution that can keep smoke out of escape routes, whether in one or two stairwells. It is widely used in continental Europe, the US, the UAE, and many other places. However, pressure differential systems are not highly trusted in the UK and for good reason. It is true that popular, simple systems have been failing to cope with the stack effect in tall buildings (and the BS EN 12101-6:2005 standard helped to cover this up) and that the passivity of constant-rate systems meant that they were unable to respond to dynamically changing conditions, such as the impact of temperature, the influence of wind, or the changing airtightness of the space over time.