Ensuring the fire safety of a building is not just about testing the combustibility and fire performance of individual components and materials. You must address and test the structure as a system and the interplay between different elements.
That’s the view of Øystein Elgsaas of Voll Arkitekter, project architect on Mjøstjårnet, which is currently the world’s tallest residential wood building.
“The totality is very important, particularly when you are using combustible materials,” he said.
This holistic approach was demonstrably the one used on Mjøstjårnet, which stands on the banks of lake Mjøsa in Brumunddal, Norway. Part apartments, part hotel, the building is 85.4m high and comprises 3,500m3 of timber (glulam and CLT), or about 14,000 trees-worth.
It’s designed to “withstand a complete fire”. “That means a fire that’s allowed to develop freely without active firefighting effort,” said Mr Elgsaas.
“The fire strategy report states that the main load bearing system must be designed to withstand 120 minutes of fire and secondary load bearing, such as floors, 90 minutes. This is principally achieved through the large dimensions of the timber and particularly the glulam frame – the fire resistance can be obtained by calculating the remaining cross-section after charring according to the Eurocode – and through design of the structural elements so that they do not compromise each other in the event of fire.”
This approach ensures Mjøstjårnet can withstand the loss of the horizontal stiffness of one floor and the impact load of a timber deck falling down on the floor below.
“There are also two separate sprinkler systems, specially designed to deliver large amounts of water and with two separate water supplies, and each room is constructed as a separate fire cell so flames can’t spread to the next,” said Mr Elgsaas. “There is a fire control room on the ground floor with graphical overview of the building to aid fast and efficient firefighting and an alarm that has direct transfer to the fire service.”
Visible wood in escape routes and internal walls in the main staircase and elevators are fire retardant treated and the escape staircase walls are additionally clad with plasterboard. Firestop is used in the façade to prevent fire spreading upwards and steel connector plates and dowels are embedded deep into the timber (85+mm), with gaps and slots between beams, columns and plates fitted with an expanding fire strip.
Holes for dowels are not plugged, as fire evaluation at the SP Firetech test centre showed this did not critically affect the temperature of the internal steel.
The full height façade of Mjøstjårnet comprises pine cladding from Woodify AS, which is fire-treatment impregnated by Woodsafe Timber Protection AB in Sweden and mounted on retardant impregnated structural timber beneath.
“The fire requirements apply not only to the surface of the cladding, but also to the back of the woodwork and to the underlying construction,” said Mr Elgsaas.
“And, as the fire protection treatment of the cladding impregnates the cells of the wood, it lasts the life of the material.”
Moreover, he added, ventilation of the façade is separate for each floor, every window has an intumescent strip, which expands to close the ventilation gap in a fire and all insulation is incombustible.
Clearly, residents of Mjøstjarnet have no fire safety concerns about living in a multistorey wood building.
“In fact, most buyers wanted specifically to know in which apartments the timber beams and columns were visible, as those are the ones they wanted,” said Mr Elgsaas.
His view is that Mjøstjårnet is not only a safe timber building, but one of the safest buildings on the banks of the Mjøsa full stop. He also sees it as a contributor to further development in sustainable wood construction.
“According to consultants from our engineered wood supplier Moelven Limitre, the Mjøstjårnet construction system could easily deliver a building twice as high,” he said.
