Safety in residential blocks has won a victory with the lowering of the threshold to 18m for second stairs. Now the industry must tackle the practicalities
In July, the government confirmed that it will adopt an 18m height threshold for second staircases in all new residential buildings. The announcement of the lower threshold was a huge win for the RIBA-led coalition of built-environment and fire safety groups and a vital step towards safer residential buildings.
A transition period has now been announced of 30 months, that will start from the date the Building Safety Regulator confirms Approved Document B. In a statement on 24th October 2023 Michael Gove, Secretary of State for Levelling Up, Housing and Communities, said: 'Any approved applications that do not follow the new guidance will have 18 months for construction to get underway in earnest. If it does not, they will have to submit a new building regulations application, following the new guidance'. He added: 'The Building Safety Regulator is working to agree these rapidly, and I will make a further announcement soon.'
A second staircase in residential buildings over six storeys gives residents alternative escape routes and fire-fighters the option of a dedicated fire-fighting stair.
While nobody would argue that putting in a correctly designed second staircase would make a building more unsafe, there are concerns that it will reduce the area of the building available for sale, effectively cutting the value and viability of a development.
So some see the requirement for a second stair as a hinderance to development, but others have embraced it as an opportunity to benefit a scheme.
Matt Brook Architects is one such practice. Working with fire engineer OFR Consultants, it has developed a two-stair core design for a series of residential towers in Greater Manchester which maximises safety and accessibility benefits while increasing the useable floor-plate by 5% over a typical two-stair design. ‘It is quite a significant saving in a tall building when you multiply it by, say, 50 floors,’ says Matt Brook, founder of the practice.
Its design is based on two interlocking but independent staircases arranged in a double helix around a central, fire-rated wall that separates the two spirals, so that in plan it looks like a single stair. ‘We’ve got two stairs pretty much within the footprint of a single stair,’ says Brook.
With lift access to every floor, the fire escape stairs have a ‘utility’ classification under the Building Regulations. This enables the flights of concrete stairs to be run in a straight line floor-to-floor.
Lifts and separation
Stair flights are incorporated in a square concrete core which, on a 50-storey scheme, holds four fire fighting and evacuation lifts, arranged in pairs either side of a central lift lobby. Concealed twin doors in the centre of the lift lobby separate the pairs of lifts. The doors close in a fire, dividing the lobby in two – each half having access to both a fire and evacuation lift and an escape stair.
‘The design provides two independent stairs and at least two lifts for use during an evacuation,’ says Richard Rankin, design director of fire engineer OFR Consultants. ‘The fire service also has access via the stairs and fire-fighting lifts.’
Crucially, each half of the core is served by a dedicated smoke ventilation system to ensure complete separation. ‘The smoke control systems are designed not just to prevent smoke getting into the staircases but to protect the route to the staircase and the route to the evacuation lift in the affected corridor,’ explains Rankin. ‘
Notably, no refuge is located in any escape stair, but with two protected lobbies formed once the central doors close, if one is compromised, wheelchair users can move across to the other lobby – a lobby being a commonly accepted place of relative safety. Brook adds: ‘Each lobby has a lift for self-evacuation before the fire service arrives, with an emergency call point for communication. This means all residents can self-evacuate and no one need wait in a stairwell refuge for rescue,’ he adds.’
The scheme also incorporates two vertical wet risers, located at the top and bottom of each flight of stairs. ‘Because the stairs interlock, you have access to two wet risers within the same fire fighting stair, when normally you’d have access only to one,’ explains Brook. ‘It means one fire team can be connected to the wet riser on the lower level and one team on the upper level, so crews can protect each other while keeping the remaining stair free for occupant evacuation.’
Rankin says the ‘slight complexity’ with the interlocking stair arrangement is that while each staircase has access to all floor levels, they do so in an alternating position on each floor; so if you enter the stair on the south side of the core you will exit on the north side on the floors above and below.
While this arrangement will have no impact on the occupants leaving the building in a fire, it will affect how the fire brigade approaches the fire. According to Rankin, to fight a fire in a tall building, fire fighters typically take a lift to the floor below the fire and then proceed to the fire-floor via a staircase. ‘It means they will exit in the opposite corridor to the one from which they entered,’ he says. ‘However, the level of protection afforded to the fire-fighters is identical and all apartments can be reached with standard hose distances regardless of which stair is used,’ he adds. To aid fire-fighter wayfinding, a discussion with Greater Manchester Fire Brigade suggested colour coding each staircase to help with identification.
Two into one
Despite the stairs’ alternating entrance/exit arrangement, Brook and Rankin are insistent that this space-saving two-stair solution is not an iteration on a scissor stair. ‘Often, a scissor stair has no separation, so the two flights are intertwined in one core without a dividing wall; it is used to get twice the number of people down a staircase,’ explains Rankin. ‘Here we’ve provided two separate staircases with identical levels of fire protection. It’s like two single staircase buildings merged and arranged so that a single fire cannot compromise both escape routes at once.’
Because this core is being developed for a tall residential tower it will come under the remit of the Building Safety Regulator at the HSE when it is submitted for formal Gateway One planning application. ‘We’ve done a lot of work using CFD analysis to show this design works,’ says Rankin.
In advance of this submission, the design team has undertaken a pre-application submission to the HSE which Brook says ‘had no concerns’. He says that in their response in support of this solution the HSE makes it clear that ‘scissor stairs are not allowable but that this design is not considered to be one’.
This space-saving, interlocking stair is highly adaptable, depending on building height and type, says Brook. ‘The number of lifts will vary, but there is always access to fire-fighting and evacuation lifts in each half of the lobby. We’ve developed the four-lift version for towers of around 50 storeys; and similar solutions for a range of tower heights and building typologies.’
In its twin-stair design for Urban Vision’s 100 Broad Street, Birmingham, Howells has used more conventional ‘return’ staircases for this 32-storey, 294 apartment, build-for-rent scheme.
Howells’ design exploits the need to accommodate a larger core to establish the building’s massing. ‘We’ve placed the tallest element in the middle of the site and stepped the building down on both sides to give it its identity, along with other benefits such as the creation of roof gardens and dual aspect apartments,’ says Howells director Rob King.
As with Matt Brook Architects’ solution, a fire door in the lift lobby is used to separate the two staircases to create separate escape routes, each with its own fire fighting lift and smoke extraction system.
‘Our approach was to wrap two-bed apartments around the core, to give us the square form – then you start to incorporate other apartments that don’t need to step out as far as you pull away,’ King explains. ‘That allowed us to step the massing while limiting travel distances on the lower floors to a maximum of 15m.’
CONSIDERATIONS AND COMPLICATIONS
Paul Bussey, architect, fellow of the Institute of Fire Engineers and member of the RIBA expert advisory group on fire safety
The case for a second staircase in new residential buildings over 18m has been won, but of course the total picture is more complex. Existing buildings which cannot accommodate a second stair will need remediation work, with additional layers of safety to compensate.
It is helpful to remind ourselves of the circumstances under which emergency evacuations take place. To tackle a fire, firefighters will use the primary firefighting staircase even if there’s a fire-fighting lift, wearing their bulky fire retardant protective suits and carrying large hoses, breathing apparatus and other important equipment. When they charge these lengthy multiple hoses, attached to dry rising mains in the staircase, a dynamic force is generated that can flip and twist on the stair , additionally leaking water everywhere. Passing though the corridor access door, hoses can allow smoke onto the staircase.
Watching fire-fighting practice drills it’s clear that with a single staircase, firefighting and evacuation are unavoidably entwined. Evacuation is hard enough for the able-bodied on a smoke contaminated stair, but for the semi-ambulant, children, elderly or those recovering from an operation it is nigh-on impossible. And getting ‘all people to a place of safety’ has to be the plan – as is written into the new ‘Secretary of state’s intentions’ of the functional requirements in the Approved Documents (ADB 2019 Vol 1 B1 with 2022 amendments, p 8). Everyone is looking for a one size fits all solution for the second stair – but that is impossible. The RIBA has identified and explored possible layers of safety.
In Australia interlocked stairs that are fire separated have been adopted, with colour coding. This would help residents but does not save as much area as imagined and can be complicated for both residents and firefighters.
Two stairs allow one for firefighting, and the other for escape. But the size of the floor plate is critical. With a big plan area hose lengths must reach any fire and both stairs may need to be fire cores with dry risers. Placing wet or dry risers in a protected lobby off the staircase can reduce the problem of dynamic hoses, trip hazards and water leaks on the stairs, and prevent smoke ingress.
Existing tall buildings with a single staircase ideally need a second, escape, stair plus a firefighting staircase. There are now bespoke and prefabricated products offering this, erected by crane two storeys at a time off a low loader lorry.
Fire protection within the building with sprinklers in flats and a temporary place of safety – typically a protected lobby next to the staircase with ventilation in the form of smoke extract or opening windows – can also help.
Depending on dynamic circumstances during a fire, it is sometimes impossible to escape down a staircase at one point; later the smoke clears, permitting escape. Voice connection capability within the fire alarm system to each flat and refuge can allow a fire leader on site to direct residents to the best course of action.
Financial and commercial ways to encourage the second stair could include planning area exemption, zero VAT, and it being protected from weather but outside the thermal envelope – which could all soften the perceived loss of area and additional cost. This will need government help to implement, and could help amend the regulatory ambiguity of combustible cladding at Grenfell. Contractors could also use the second stair for future refurbishment and fit-out works access.
The RIBA fire expert panel has discussed and developed a document that explores all these and other layers in more detail – for instance smoke control, compartmentalisation, refuge lobbies and travel distance issues. Now that the fundamental human right of an alternative means of escape for all has been established, further development of these layers could be considered.