View course on architecturalrecord.com »

BEING the “tallest” was never the goal for the Ascent, a 284-foot-high timber-framed residential tower in Milwaukee, says Jason Korb, principal at Korb + Associates Archi­tects, the building’s design firm. Neverthe­less, over the course of the project’s design phase, the scheme grew several times, starting at 19 stories and eventually reaching 25, as codevelopers New Land Enterprises and Wiechmann Enterprises sought economies of scale. Now complete, it was designated, by the Council on Tall Buildings and Urban Habitat (CTBUH), a Chicago-based nonprofit, the world’s tallest timber tower in July, just days after the first residents started moving into the 259 one- to three-bedroom market-rate apartments. The Milwaukee project supersedes the previous record holder, the mixed-use 280-foot, 18-story Mjøstårnet, in Bru­munddal, Norway, completed in 2019, by only 4 feet.

Photo courtesy of DIALOG

DIALOG'S SUPERTALL tower concept includes a floor plate system using mass timber.

The Ascent and Mjøstårnet have not been built using traditional light-frame wood construction. Instead, they rely on “mass timber,” a term that encompasses a group of engineered wood products made from small-dimension lumber, wood plies, or other types of wood fibers assembled under pressure and fixed with adhesives, producing construction components of exceptional strength. These include glue-laminated (glulam) columns and beams, cross-laminated timber (CLT) panels, and laminated-veneer lumber (LVL). And there are more and more buildings using these types of structural elements worldwide. In its State of Tall Timber 2022 report, released this past February, CTBUH counts 139 mass-timber buildings, eight stories or higher, that are complete, under construction, or proposed, globally. The number might sound small, but the total has more than tripled in the past five years since the last time the council published a similar accounting, when there were only 39 such buildings.

What is behind the rise of mass timber? One factor is the growth in off-site construction, which pairs well with these products, as the components are typically fabricated in a factory and brought to the site for just-in-time installation. Many architects and their clients are, of course, drawn to wood’s natural warmth and beauty. However, the chief attraction is environmental. When the forest from which they are obtained is responsibly managed, the trees perform important ecological functions, such as filtering water and air and providing habitat for wildlife. And, most critically, timber stores carbon, even after the tree is cut down: when wood is trans­formed into structural components, or other durable products, it sequesters carbon for the life of those elements—only releasing it back into the atmosphere when the wood decays or burns. The amount of this so-called “biogenic carbon” varies by species, but for many of the softwoods used in mass timber, the rate of sequestration is approximately 1 metric ton of CO₂ equivalents per cubic meter.

Photo courtesy of KORB + ASSOCIATES ARCHITECTS

The ASECENT, now the world's tallest timber tower, comprises 19 mass-timber floors over a six-story concrete podium. Much of the timber is exposed on the interior.

Photo courtesy of THORNTON TOMASETTI

Changing regulations are helping build mass timber’s momentum. Until recently, in most American cities, the height limit for wood buildings was 85 feet, about seven stories. But the latest versions of the Inter­national Building Code (IBC), the model code used by most jurisdictions in the U.S., permit mass-timber structures up to 18 stories. These tall mass-timber provisions have been adopted, either in whole or in part, by eight states, as well as Maryland’s Howard County, Denver, and seven cities in Texas.

In 2018, when the Ascent started permitting-process discussions, Milwaukee’s prescriptive height limit of 85 feet for wood structures was—and is still to this day—in place. So, in order to satisfy code officials and build a timber tower that exceeded this mark by nearly 200 feet, consultants conducted extensive simulations, studies, and evaluations. They needed to demonstrate that the structure—19 stories of glulam columns and beams, floor slabs of CLT, and a pair of reinforced-concrete cores over a six-story post-tensioned concrete parking podium—would not compromise the safety of occupants. “We had to show that the building meets the intent of the code,” says John Peronto, principal at Thornton Tomasetti, the project’s structural engineer.

Fire, of course, was a particular concern. When heavy timber burns, it develops a protective char that then insulates the member’s interior from damage. Though this phenomenon is well understood, physical tests were needed to validate that the Ascent’s wood elements—about 40 percent of which are exposed on the building’s interior—were properly oversized to account for the sacrificial layer. Testing for the project involved a three-hour controlled burn on exposed glue-laminated timber. Other safety measures included the installation of a temporary standpipe in the elevator core to protect the structure during construction and until the building’s permanent fire-suppression system could be activated.

Photo courtesy of THE OFFICE GROUP

THE TALLEST timber office building in Central London will be Waugh Thistleton's Black & White Building, though it will be only 58 feet high.

Photo courtesy of THE OFFICE GROUP

Korb estimates that the premium for the wood structure was about 30 percent when compared to an equivalent one in concrete. But there were savings too—in the foundations, for instance—since the superstructure weighs about a quarter of what a concrete one would. And there were other cost offsets: the factory fabrication of the timber components and their steel connectors, aided by a highly detailed digital model, shaved two to three months off the construction schedule, says Korb. “The model included every last screw.”

Ascent could soon be superseded by any number of proposed timber buildings, including one designed by Korb + Associates, a 313-foot-tall, 29-story apartment tower planned for St. Louis. Other, even taller, on-the-boards timber towers are a 600-foot-high, 42-story headquarters for software company Atlassian, in Sydney, by SHoP Archi­tects; a 427-foot-tall, 37-story mixed-use tower in Rotterdam, by PLP Architecture; and a 328-foot-high, 32-story residential building, in Winterthur, Switzerland, by Schmidt Hammer Lassen Architects.

Although tallness grabs attention, for some mass-timber proponents, all the talk about height is a distraction. “Tall buildings should be the exception rather than the rule,” says Andrew Waugh, director of UK-based Waugh Thistleton Architects. Waugh cites studies showing that, compared to shorter structures, taller buildings have increased operating energy and embodied carbon (the emissions associated with materials manufacturing and construction) relative to their floor area. Instead, he advocates for a high-density but low-rise approach. “If the primary reason to build in timber is to reduce the impact of carbon, we have to ask ourselves, ‘Why we are building tall?’ ”

Waugh’s firm is nearing completion on the Black & White Building, which will be Central London’s tallest engineered-