Currently in contention for the title of “world’s tallest wooden building” is “Ascent,” a 25-story mass timber tower under construction right now in downtown Milwaukee, Wisconsin. Standing 285 feet tall, the residential complex by Korb + Associates Architects benefits from the plans, approvals and fire tests completed for the never-built “Framework” tower previously planned for Portland, Oregon, funded by a $1.5-million U.S. Tall Wood Building Prize.
Ascent, set to open in 2021, will offer 493,000 square feet of mixed-use space, including 258 apartments, a pool on the sixth floor and a top floor amenity level. It will also have a six-story reinforced concrete parking podium with post-tensioned slabs and a full-height reinforced concrete core near the end of each leg.
The record-breaking design “exposes the mass timber construction whenever possible to display its natural qualities,” says structural engineering firm Thornton Tomasetti, which worked on the project.
That much is apparent fro the photos. Developer New Land Enterprises wanted the building to have a natural look with a modern edge, and it definitely achieves that, inside and out. The apartments will be made almost entirely from cross laminated timber (CLT), which is made by layering kiln-dried lumber in alternating directions and pressing it together with structural adhesive.
Tall timber buildings always rise concerns about fire.
“Any building, whether mass timber, concrete or steel, has to meet certain fire codes … that’s in the zoning code,” says Tim Gokhman, managing director of New Land Enterprise, in an interview with OnMilwaukee. “No one in the United States met it yet (with mass timber) because no one’s tried. And so in that sense it hasn’t been done.”
So, in order to get the building approved, the engineers “did the world’s first three-hour fire test,” he explains. The CLT panels were sent to a U.S. Forest Service laboratory on the UW-Madison campus. That’s an hour longer than required to meet the fire codes. “They’re not able to release the official results yet, but the initial feedback is very positive,” says Gokhman.
“Steel will fail at about 1100 degrees, and during our fire test, the temperature exceeded that. They had a rod inside the timber (near) the core of the wood, and it was below 100 degrees. Wood is not a good conductor; as a result it’s a great insulator. If you look at any forest fire and at a cross section of the tree, it right away tells you the same story.”
“The inside of the tree is always intact. It has to do with the density and that outside char layer. But the code already knows this. It’s not like the construction industry doesn’t know this. We can’t have in our building any exposed steel. It has to be protected. So either you spray it or you bury it inside the wood. So the steel is protected by the wood.”
When it’s finished, Ascent will beat out the current world’s tallest mass timber building, Mjøstärnet in Norway, by four feet. It’ll also best North America’s tallest timber tower, which is currently Vancouver’s 174-foot Brock Commons Tallwood House.