Mjøstårnet is revolutionising Norwegians’ understanding of timber construction.

In February, TTN featured Mjøstårnet, the (currently under construction) tallest all-timber tower in the world. Now, as it nears its completion, the building is changing the ways Norwegians consider timber as a building material.

In 1904, a devastating fire in Ålesund, a port city on the west coast of Norway, destroyed 850 of the city’s 1100 or so timber houses, leaving 10,000 people displaced. It led to the country’s reformed building laws emphasising fire safety, which in practice restricted the use of timber – one of Norway’s main building materials – in multi-storey buildings

However, the advent of engineered timbers is a game changer for the industry. “Much has changed since Ålesund burned,” says CEO Rune Abrahamsen of Moelven Limtre, the company behind Mjøstårnet’s construction. “At Moelven we have long experience building tall and fireproof timber buildings. The key is in the structure and the use of glulam beams.”

The glulam structures that make up the frame of the building have such massive dimensions that they retain their load-bearing ability in the event of a burnout fire.

“Our fire tests show that glulam acquires a protective layer of char that denies the fire the ability to keep going, and it dies out,” says Even Andersen of Sweco Norge AS, the company responsible for fire safety in the building.

Andersen, the main fire safety engineer on the project, says, “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.”

Structural safety

The weakest point in the structure in the event of fire is the steel used for the connections. 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.

In effect, each individual floor, apartment and hotel room is designed as a separate fire cell that will restrict fire from spreading

In burnout tests that were performed by SP Firetech in Trondheim, Norway, large glulam columns were put in a furnace to undergo an ISO-fire for 90 minutes. When the burners were shut off the glulam continued to char a little more. After several hours, the temperature in all columns was declining and the burning stopped. This supports the design premise that the large glulam columns will self-extinguish and prevent a building collapse.

Additionally, Mjøstårnet has a state-of-the-art sprinkler system that covers the building inside and out. It differs from a traditional sprinkler system in that it is scaled to provide a greater water volume and has an independent extra water supply, as well as a greater degree of monitoring.

Visible wood in escape routes as well as internal walls in the main staircase and elevators all have fire retardant painting. Exposed walls in the escape staircase are covered with plasterboard to increase their fire safety. There is Firestop in the façade to prevent fire from spreading upwards, and cavities in the façade are broken on each floor.

When it comes to robustness the structure is designed to sustain the loss of the horizontal stiffness of one timber floor. It can also carry the impact load of a timber deck falling down on the floor below.

“Our tests and calculations show that Mjøstårnet will not collapse, even after a severe fire,” says Andersen.

He adds, “In Norway, it’s rare that a fire safety engineer is included throughout the entire process of constructing a building.” But on this project, he has been involved since an early planning stage and still frequently takes calls from the site to clarify details on fire safety. “I always tell them to call if they’re unsure and that I’m happy to answer. It helps me sleep better at night.”

The success of Mjøstårnet will change the future of the timber industry in Norway as more tall buildings make use of this abundant natural resource and thriving industry. Andersen is certain the memories of Ålesund can now be laid to rest. He says, “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.”

For more details, visit www.moelven.com/Products-and-services/Mjostarnet/