It was shortly after midnight on 14 June 2017 when a fire broke out on the fourth floor of the Grenfell Tower in London leading to 72 deaths and many more injured. This fire shone a global spotlight on building standards in residential high-rise buildings and also on fire safety standards.
“Too often, fire engineers get dropped in at the end of the project,” Sarnia Rusbridge, Principal at Holmes Australia LP, told create. “They should be involved from the start of the design phase, and at every stage of the design process thereafter.”
The key is compartmentalisation, according to Professor Jose Torero Cullen, Head of University College London’s Department of Civil, Environmental & Geomatic Engineering.
“The key is to limit any fire to the floor on which it started,” he said. “The building must perform as if it has no height, then everyone below and above and in the stairs are protected during a fire.”
Understanding vertical fire spread
Tragic consequences occur when a fire runs vertically up a tall building.
“The February 2024 Valencia fire and the 2017 Grenfell disaster in London are two such tragedies,” Andrew Johnson, Arup’s Principal Structures Leader NSW and ACT, explained.
In both cases, the facade was made from combustible materials, which when on fire, created a chimney effect that pulled flames and hot gases upward rapidly. During the Valencia fire, the difference between temperature inside and outside the building created air pressure variations leading to quicker air movement through stairwells and ventilation shafts. This enabled smoke and heat to rise more quickly.
Fire safety engineers call this the stack effect. Johnson explained that one way of counteracting it is by pressurising internal stairwells.
“Pressurised stairwells force air out of the vertical spaces, preventing smoke from being pulled inside,” he said. “On Salesforce Tower in Sydney, we also used zoned mechanical smoke extraction systems and smoke reservoirs, which are zones in the ceiling that can trap smoke prior to extraction.
“On Sydney’s 8 Chifley Square, we used external fire stairs protected by fire walls.”
Sarnia Rusbridge added that stair shafts could be segmented with transfer floors to elevators and corridors.
“Pressurisation of very tall shafts becomes unwieldy, and all occupants can’t be expected to descend via staircase only.”
Designing for fire containment
Room design can also help control a fire to a single floor, Johnson explained.
“Concrete floors must be designed with enough thickness, so you don’t get sufficient heat transfer though the concrete to be combustible in the floor above,” he said. “On the 2IFC and ICC buildings in Hong Kong, we designed a refuge floor, which are high-performance reinforced concrete rooms, every 25 floors.”
External facades can also be designed to resist the vertical spread of fire.
“Solutions here include reinforced concrete downturn or upturn beams, and solid fire-rated facade panels at a minimum of 900-1000 mm below or above the floor level. Another approach is to extend the floor slab outside the facade line, so it acts as a flame barrier and stops the flame accessing the fire load in the floor above.”
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Ensuring structural integrity during fires
The structure of a tall building must be able to withstand a fire long enough for people to get out and firefighters to get in.
“Generally, that’s two hours – although we’d usually design for more resilience if we want the building to keep functioning immediately after or during a fire,” Johnson said. “That could be for post-disaster or key infrastructure use, or if we want it to be easy to repair and bring back to service.”
Johnson added that steel frames require passive protection, as steel starts to lose strength at around 250°C and loses around 30 per cent by 400°C.
“An office fire can reach temperatures of 1000°C, so traditionally we protect the steel frame with concrete encasement, vermiculate spray, intumescent paint or fire-rated board. On Salesforce Tower, we used steel as a jacket on the outside with the internal concrete core providing additional insulation.”
Tailoring fire safety to each building
No tall building is the same though; according to engineer Jose Torero Cullen, this means engineers must “treat each building as a prototype that needs to be designed in a bespoke way”.
Torero Cullen was a third-party reviewer for 25 Kings Street in Queensland, Australia’s tallest timber-engineered office building.
“Here, we found that if the facade failed, there would be more than one floor on fire. We could get people out but there wasn’t enough time for fire fighters to get in safely, so we decided that they would have to fight fire from outside.”
In central Asia, Torero Cullen worked on a fire investigation where the building caught fire during construction, damaging the concrete.
“Winters are cold [there], so they had to insulate the concrete to avoid it freezing,” he said. “The insulation material was combustible, though, and they had to use heaters because the insulation was not enough. They were operating on edge. You must be massively meticulous when managing a site.”
Torero Cullen advises tall building designers to strive for simplicity.
“The problem is already intricate enough, so don’t add unnecessary complexity.”
Safety completing projects starts with effective early project planning. Learn more at this Engineers Australia event.
