To Build the World's Tallest Timber Skyscraper, a Team Had to Rethink Everything
BY JEN THOMAS
PHOTOS COURTESY OF MOELVEN LIMTRE
THE buzzy building material of the moment isn't a newly discovered metal mined from the Earth's core or a super-strong polymer created in a lab. No, it's actually one of the oldest construction products known to man—wood—and it's popping up in project plans all over the world.
The environmental benefits of timber construction are compelling: While the production of steel and concrete emit carbon dioxide (responsible for about 8 percent and 5 percent of global emissions, respectively), wood actually absorbs more carbon dioxide than it produces. Also appealing is its potential to accelerate construction timelines: Timber doesn't require the setting time of poured materials like concrete, for instance, so wooden-construction projects typically can be finished in a shorter time frame.
These advantages intrigued developer Arthur Buchardt of AB Invest AS, who in 2015 enlisted lumber manufacturer Moelven Limtre to pursue a timber skyscraper project (also known as a “plyscraper”) to serve as a beacon for other building teams in Norway.
“He felt it was time to do something special, to make a change and to inspire others,” says Rune Abrahamsen, CEO, Moelven Limtre, Moelv, Norway. “He actually sketched a building on a napkin and asked me and my colleagues, ‘Can you do this?’ We said, ‘Give us a few weeks to consider whether this is possible or not.’”
—Rune Abrahamsen, Moelven Limtre, Moelv, Norway
Mr. Abrahamsen, also a structural engineer, took the rudimentary project sketch back to his team, who declared it viable. With the feasibility green light, Mr. Buchardt launched the Mjøstårnet project and enlisted contractor Hent AS. Moelven Limtre was selected to supply and place the glue-laminated timber—or glulam—with which the building would be framed, carrying all of the vertical and horizontal load. (Timber hybrid buildings, by comparison, rely on a steel or concrete core to carry the load.) Construction on the mixed-use Mjøstårnet began in April 2017 in Brumunddal, Norway.
REACHING NEW HEIGHTS
At the outset, the NOK700 million structure was slated to be 67.891 meters (223 feet) tall—a highly specific (and sequential) request from Mr. Buchardt. That number was later rounded up to 81 meters (266 feet). But when Mr. Buchardt realized that adding a bit more height would mean he could nab the title of “world's tallest timber building” from the currently under-construction HoHo Tower in Vienna, Austria, he put in a change order to increase the height to its final measurement of 85.4 meters (280 feet).
February 2015: Planning meetings between builder Arthur Buchardt and timber supplier Moelven Limtre begin.
April 2016: Nearly 200 countries sign the Paris Agreement, which aims to reduce global warming and serves as a point of inspiration for Mr. Buchardt's new green project.
January 2017: Mr. Buchardt's AB Invest AS and contractor Hent AS sign a contract.
April 2017: Construction at the project site begins.
March 2019: Construction is completed, and Mjøstårnet is designated the world's tallest timber building by the Council on Tall Buildings and Urban Habitat.
Adjusting the project plans during the execution phase required some ingenuity—the foundations and structures had already been utilized to nearly 100 percent capacity—but Mr. Abrahamsen and his team found some wiggle room in the building's pergola roof structure.
“This structure had to be redesigned because originally the project plans called for the glue-laminated timber beams to be square in shape,” Mr. Abrahamsen says. By rounding those edges, the team improved the pergola's aerodynamics. “In that way, it catches less wind, and the load on the building is reduced back to what we originally designed, despite the added height,” he says.
The timber structures at the top were made hollow instead of dense, which helped lessen cracking. And the less-expensive material at the top meant the change barely impacted the project's budget.
“When large cross-sections are outdoors, they're vulnerable to changes in humidity and temperature,” says Mr. Abrahamsen. “We wanted to prevent cracks as much as possible, and by making the cross-sections hollow, these cracks will be inside the cross-section.”
Mr. Abrahamsen and his team also further mitigated the effects of wind—focusing on what he calls the “comfort criteria” of residents in the upper floors not feeling the swaying of the building—by replacing the wooden floors in the top six stories with concrete slabs, further buoying the surrounding timber framework.
“That decision has nothing to do with structural safety or deflections of the building,” he says. “It has to do with the fact that, if the building is light and you are at the top when the wind is blowing, you can get a tendency of seasickness.”
EMBRACING THE LEARNING CURVE
While Moelven Limtre's team is well-versed in the intricacies of timber construction, contractor Hent AS had never built a wooden structure to the scale and complexity of the 18-story Mjøstårnet. That meant fewer lessons learned to pull from for insights and more time upfront to devote to meticulous project planning.
According to Hent AS project manager Erik Tveit, scheduling was a top concern. The team found efficiencies through constructing four to five floors at a time—instead of the typical two to three—with the glulam. This helped keep the schedule brisk despite other processes that took longer, like installing the wood structures that separate each floor.
Still, the glulam presented its own set of challenges. The large columns and beams that allowed the team to build five floors at a time proved to be an issue when it came to transporting them to the project site. “The roads in Norway aren't always the best for transportation of long items, so we needed to find different routes for the trucks so that they could get to the site,” says Mr. Tveit, Oslo, Norway. “That's a big difference from a steel and concrete building.”
Weather proved another risk to the project materials. “Normally, you just construct the concrete and steel, and it doesn't matter how wet it's going to get,” he says. Not with wood. Although rain wouldn't destroy the wood, it needed to be dealt with swiftly. So the team developed a plan to keep more of the material covered during transport and construction, with frequent checks for moisture.
With all the learnings the Hent AS team gleaned from its work on Mjøstårnet, Mr. Tveit is confident their next plyscraper can be built more rapidly. “Early on, we established that we wanted to make a manual for future wooden-building projects,” he says. “As we learned new things and how to do certain tasks more efficiently, we put that in the manual.” Consulting that lessons learned guide is the first step for project teams working on timber projects to come, he says.
—Erik Tveit, Hent AS, Oslo, Norway
TRIAL BY FIRE
With wind and water issues handled, Mjøstårnet had one more element to contend with: fire. Wooden structures have long had a bad reputation for not being able to withstand blazes. Centuries ago, cities like London, England and Chicago, Illinois, USA were nearly leveled because of fires that spread rapidly, gobbling up entire buildings like kindling. But today's construction methods and materials are designed to resist the burn.
For Mjøstårnet, the team met with local and national authorities to develop a fire mitigation strategy. That included running test demonstrations on the glulam products in a lab, using several different specimens of columns and beams to prove that the project material naturally prevents the spread of fire throughout buildings.
“If there is a fire inside one of these fire cells— meaning an apartment, hotel room or meeting room—everything inside that fire cell can burn completely,” says Mr. Abrahamsen. “The only thing left that can burn is actually the glulam structure. But when there is no more fuel to that glulam structure, the fire dies out, and the glulam is merely charred.”
Local agencies were satisfied by the lab test results, and construction continued apace. In March, 23 months after the project sponsor first sketched a timber skyscraper on the back of a napkin, Mjøstårnet opened its doors.
“We're proud of the building, obviously, but we also hope that with this project we can inspire other clients in Norway and anywhere to consider timber as a viable option for their tall buildings,” says Mr. Abrahamsen. “Replacing some of the concrete and steel with timber helps our climate and improves sustainability. So, we want our record to be broken. We look forward to that.” PM
Norway isn't the only country investing in more environmentally friendly timber structures. Here are three other wooden-construction projects in the works.
IMAGE COURTESY OF SHIGERU BAN ARCHITECTS
Vancouver, British Columbia, Canada
Slated for completion in 2020, this 233-foot (71-meter) timber hybrid structure designed by Japanese architect Shigeru Ban will house 20 luxury residences overlooking Vancouver's idyllic waterfront. When finished, it will be the world's tallest timber hybrid skyscraper.
IMAGE COURTESY OF SUMITOMO FORESTRY
With project plans unveiled in 2018 and completion slated to coincide with Sumitomo Forestry's 350th anniversary, the 1,148-foot (350-meter) W350 tower will be Japan's tallest building. The JPY600 billion skyscraper project will be made from 90 percent wood materials.
IMAGE COURTESY OF PLP ARCHITECTURE
Oakwood Timber Tower
Still in the research proposal phase, this mixed-use building designed by PLP Architecture would add 1,000 new residential units to the city and clock in at 984 feet (300 meters) tall, making it the second-tallest building in London, after the 95-story Shard.