How the world's tallest timber building withstands fire
Mjøstårnet will be the world's tallest timber building. But what if a fire breaks out? "The fire safety measures that have been implemented in Mjøstårnet make the timber building far safer than a corresponding building with a traditional steel and concrete structure. Mjøstårnet is one of the safest buildings along lake Mjøsa, and can withstand even an extensive fire," says Even Andersen. He's is a fire consultant with Sweco Norge AS, the company responsible for fire safety in the building. Mjøstårnet will be 81 metres tall across 18 storeys. When completed in 2019, it will be the world's tallest timber building. Wood is a flammable material, and up to 1997 it was prohibited to build timber houses taller than three storeys in Norway. This was due to the old "Brick law," which came into effect after the Ålesund city fire in 1904.
"Much has changed since Ålesund burned, and at Moelven we have long experience building tall and fireproof timber buildings. The key is in the structure and the use of glulam beams," says CEO Rune Abrahamsen of Moelven Limtre.
Glulam structures don't burn by themselves
Many people may nod in agreement after placing a huge log in the fireplace. It won't keep burning unless replenished with kindling. The large log will quickly develop a layer of charcoal, but the fire will die out by itself.
The world record rising next to Mjøsa has been designed to withstand a burnout fire. This means a fire that is allowed to develop freely without active extinguishing.
"The glulam structures retain their load-bearing ability in a burnout fire. Our fire tests show that glulam acquires a protective layer of coal that denies the fire the ability to keep going, and it dies out. Even after long exposure to fire, a glulam structure with such sturdy dimensions as used in Mjøstårnet, will have a sufficiently great load-bearing core of fresh wood. The structures are also positioned at a sufficient distance from each other so that a fire cannot keep going by itself when only the structures are on fire. This prevents the building from collapsing, even in the event of a burnout fire in fittings and furnishing," says fire consultant Andersen.
Additional measures against fire
In addition to Mjøstårnet being designed to withstand a burnout fire, a number of additional measures have been implemented that one ordinarily wouldn't have in a regular steel and concrete building (see separate fact box). For example, the façade is protected against fire spreading, the building has an upscaled sprinkler system, and each individual room on each floor is designed as an individual fire cell. This has been done to prevent fire from easily spreading to the next room.
6 reasons Mjøstårnet is fireproof
1. The glulam structures: The glulam structures have such huge dimensions that they retain their load-bearing ability in the event of a burnout fire. The glulam structures are positioned such that they won't affect each other in the event of a fire.
2. Sprinkler system: A state-of-the-art sprinkler system covers the building inside and out. It differs from a traditional sprinkler system in that it is scaled to provide a greater water volume, it has an independent extra water supply, as well as a greater degree of monitoring.
3. Facilitation for the fire brigade: Dedicated control room with graphical overview that enables fast and efficient extinguishing. The fire alarm is linked directly to the fire brigade, and the fire brigade's water supply is doubled.
4. Fire strips: The weakest point in a timber structure in the event of fire is the steel used for the actual connections in the structure. The glulam structure therefore has fire strips to protect the steel sheets and dowels in the junctions and joints. The fire strips consist of a material that expands 20 times at 150 degrees Celsius. The strips protect the steel from the increase in temperature, close openings and prevent fire from spreading.
5. Fire cells: Each individual floor, apartment, and each hotel room are designed as separate fire cells that will restrict fire from spreading.
6. Protection against fire spreading in the façade: The outer wall elements have been treated with a fire retardant material, and cavities in the façade are broken on each floor.
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