Following our coverage of a tall wooden tower that might get built, here's one that is being built. Named Mjøstårnet, it's currently under construction in Norway and will rise to a height of 81 m (265 ft) once complete, making it the new world's tallest wooden building. Kind of. Maybe. That depends on rule changes which may or may not come into force. Either way, the project offers some insight into the challenges of tall timber construction.

As things stand, the world's tallest timber tower will be Vienna's 84 m (275 ft) Ho Ho Tower once it's complete. However, the influential Council on Tall Buildings and Urban Habitat, which keeps track of such things, is set to bring in new rules to the effect that buildings like the Ho Ho, which have a concrete core, be defined as wood-concrete hybrid structures.

Mjøstårnet has no such concrete core, instead relying on glulam beams for structural support, thereby making it the tallest wooden building if the changes do indeed come into place. Whatever happens, the record may not stand for long anyway in this fast-moving area of architecture.

Mjøstårnet will comprise over 11,300 sq m (121,632 sq ft) of floorspace, spread over 18 floors. The building will include residential units, a hotel, restaurant, offices, and common areas, plus a large swimming hall.

For those thinking that it'll be a tinder box, Moelven Limtre, the firm in charge of the project along with Voll Arkitekter and Sweco, indicates that such fears are unfounded and that any fire would die out by itself even if the sprinkler system fails, leaving the building structurally sound.

"The requirement has been that the building must remain standing after an eventual fire has gone out by itself – even if the sprinkler system should fail and without extinguishing," says Moelven Limtre's CEO Rune Abrahamsen. "An independent fire test has documented that the strong glulam structures will continue to support the building after a fire has gone out by itself. We have calculated the location of the glulam beams such that they don't mutually affect each other in the event of a fire. The amount of flammable material at any location will be so small that a fire will die out by itself."

The bigger issue in this project actually seems to be swaying in the wind, and the potential for residents in the upper floors to experience the unpleasant sensation of motion sickness.

"Tall wood buildings sway somewhat more than those built from steel and concrete due to a far lower weight," adds Abrahamsen. "This applies in particular to a narrow building such as Mjøstårnet with a width of just 16 m [52 ft]. For this reason we are using concrete slabs for the top seven floors. Swaying at high winds has been calculated to around 14 cm [5.5-in] at the top. The effects are the same with or without concrete slabs, but the greater weight towards the top means that swaying will be slower, thus preventing 'seasickness' among residents."

Work on the project began in April 2017 and it's expected to be completed in March, 2019.

Sources: Moelven, Voll Arkitekter

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