Cross-Laminated Timber is the Most Advanced Building Material
Cross-Laminated Timber Continues to Impress Builders and Engineers
A Wooden Skyline
On a cloudy day in early October, the architect Andrew Waugh circles the base of a nondescript apartment tower in Shoreditch, a neighborhood in East London. Shoreditch suffered heavily during the blitz of World War II—”urban renewal, compliments of the Luftwaffe,” Waugh says—and then spent decades in neglected decay. Recently, though, the neighborhood has come roaring back. Nightclubs and tech start-ups arrived first on the promise of cheap rent, and residents followed. Along with them came architects, urban planners, and engineers, many of whom make a pilgrimage to the same tower that Waugh now circumambulates.
From the outside, there is nothing particularly flashy about the nine-story building, called Stadthaus, that Waugh designed with his partner, Anthony Thistleton. Its gray and white facade blends almost seamlessly into the overcast London skies. It’s what’s inside that makes Stadthaus stand out. Instead of steel and concrete, the floors, ceilings, elevator shafts, and stairwells are made entirely of wood.
But not just any wood. The tower’s strength and mass rely on a highly engineered material called cross-laminated timber (CLT). The enormous panels are up to half a foot thick. They’re made by placing layers of parallel beams atop one another perpendicularly, then gluing them together to create material with steel-like strength. “This construction has more in common with precast concrete than traditional timber frame design,” Thistleton says. Many engineers like to call it “plywood on steroids.”
When it opened in 2009, Stadthaus was by far the world’s tallest modern timber building. Since then, CLT towers have sprouted up everywhere. Waugh Thistleton built a seven-story apartment tower near Stadthaus in 2011, and construction is under way on a 90-foot-tall wood building in Prince George, British Columbia. In 2012, Stadthaus lost the height record to a 10-story apartment building in Melbourne called Forté.
Wood is both renewable and a carbon sink.
There are plans to go even higher. Swedish authorities have approved a 34-story wood tower in Stockholm, while Michael Green, a Vancouver architect, is seeking approval for a 30-story tower in his city. And the Chicago architecture mega-firm Skidmore, Owings & Merrill recently published a feasibility study for a 42-story tower made predominantly of Cross-Laminated Timber. It’s become a competition among architects to see who can build the next tallest wood high-rise, says Frank Lam, a professor of wood building design and construction at the University of British Columbia.
Why the sudden interest in wood? Compared with steel or concrete, CLT, also known as mass timber, is cheaper, easier to assemble, and more fire resistant, thanks to the way wood chars. It’s also more sustainable. Wood is renewable like any crop, and it’s a carbon sink, sequestering the carbon dioxide it absorbed during growth even after it’s been turned into lumber. Waugh Thistleton estimates that the wood in Stadthaus stores 186 tons of carbon while the steel and concrete for a similar, conventionally built tower would have generated 137 tons of carbon dioxide during production. Wood nets a savings of 323 tons.
Demographers predict that the planet’s urban citizenry will double in 36 years, increasing the demand for ever-taller structures in ever-denser cities. Whether architects and construction firms build those towers from unsustainable materials like steel and concrete or employ new materials like CLT could make a huge difference in the Earth’s health. Put differently, the world’s urban future may just lie in its oldest building material.
Cut & Assemble
When most people think of wood architecture, they imagine a balloon—or, rather, a balloon frame, the lightweight but sturdy residential-building system of thin wood beams introduced during the mid–19th century (so light, people said, that it might just float away). The frames, also known as “Chicago construction,” for the city where they first became popular, are cheap and easy to build. But while they are strong enough for a few floors of residential construction, balloon frames buckle quickly under more weight.
That became a problem in the late 19th century, as cities began to grow up as well as out. Fortunately, at around the same time, engineers and architects discovered how to use steel and concrete to build high-rise structures that could climb far above the tallest balloon frames. Chicago’s 138-foot Home Insurance Building, which opened in 1885, was the first to employ a steel skeleton, and thousands followed in quick succession.
It didn’t help wood’s case that in the late-19th and early-20th centuries a series of horrible urban fires swept through square mile after square mile of wooden houses and apartment blocks in cities such as Baltimore, Chicago, and San Francisco. These disasters led to strict local construction codes that limited the height of residential wood buildings to as low as five floors.
The rest is architectural history. The great forests of skyscrapers that grew across the world’s cities in the 20th century were made almost entirely of steel and concrete. “There was a long period where people forgot how to use wood,” says Alex de Rijke, a partner in the London architecture firm of dRMM, which has worked extensively with mass-timber design.
But over the last two decades, architects and engineers have begun to rethink the possibilities of wood as a structural building material. First came the technology itself. In the mid-1990s, the Austrian government funded a joint industry-academic research program to develop new, stronger forms of “engineered” wood to soak up the country’s oversupply of timber. The result was CLT—a lightweight, extremely robust material that could be prefabricated and custom cut.
The simple beauty of CLT is its orthotropic quality. Normal wood is strong in the direction of the grain but weak in the cross direction. CLT’s perpendicular layers make it strong in two directions. And because it relies on layers of smaller beams, it can reduce waste by using odd-shaped, knotty timber that lumber mills would otherwise reject.
CLT came about just as architecture was going through its own technological revolution. In the past, an architect would draft schematics by hand and send them to an engineer, who would convert the documents into specifications for each wood beam or steel plate. The components would then be cut at a mill and assembled, piece by piece, on-site—an expensive, time-consuming and often imprecise process.
Today, that’s all done by computer. An architect designs a building using 3-D AutoCAD software, and the program generates the material specs and sends them to robotic wood or steel routers, which shape panels with millimeter precision. The result is a set of building blocks that a small crew of workers can screw together in a matter of weeks. It took just 27 days for four men, working three days a week, to erect the timber portion of Stadthaus, about 30 percent faster than a comparable steel-and-concrete structure. Instead of building the tower from scratch on-site, Waugh said, it was more like assembling a piece of furniture. “The instructions are like Ikea but a little more straightforward, and the names are more pleasant,” he says.
For all its benefits, CLT has been a tough sell until recently. After employing the material to build a small arts club in 2003, Waugh and Thistleton spent years trying—and failing—to convince more clients to use it. “Whatever client came in, timber came on the table,” says Waugh, “and after an hour, timber all too often came off.”
The resistance arose from assumptions about wood as a material: Clients believed that any wood structure would behave like a balloon frame, with its structural weaknesses and vulnerability to fire. “We found the journey at times frustrating,” Thistleton says. “One thing we found was the inability of anyone to distinguish between mass timber and a timber frame.”
Fire is, of course, the first concern that comes to mind with wood construction. And yet, mass timber is actually safer in a fire than steel. A thick plank of wood will char on the outside, sealing the wood inside from damage. Metal, on the other hand, begins to melt. “Steel, when it burns, it’s like spaghetti,” says B.J. Yeh, the technical services director for APA—the Engineered Wood Association.
Slowly, though, developers are coming around, particularly those that grasp the economic benefits of building with CLT. When the Australian arm of Lend Lease, a global project management and construction company, began to design Forté, a 10-story apartment building in the docklands neighborhood of Melbourne, its engineers were not considering mass timber. “We originally looked for a lightweight construction solution that could work on relatively poor soil conditions,” says Andrew Nieland, who oversees timber construction projects for the company. CLT, they found, made the most sense financially. “We did our due diligence and came across engineered timber,” Neiland says. Generally speaking, CLT construction is about 15 percent cheaper than conventional steel and concrete, according to research by Waugh Thistleton.
Tenants are getting on board too. Despite fears that some may be turned off by safety concerns surrounding life in a wood tower, Forté proved to be a huge commercial success, with all the units sold out. “It was on the news in China,” says Nieland. “A colleague’s mother called and said, ‘What is this building?’ ” Going forward, he says that Lend Lease Australia is committed to building 30 to 50 percent of its projects with CLT.
But the biggest driving force behind the turn toward wood is a growing awareness among architects and developers about their field’s contribution to climate change. “Our industry leads all others in terms of its impact on the planet and human health,” Waugh says. Concrete and steel require enormous amounts of energy to produce and transport, generating more than a ton of carbon dioxide per ton of steel or concrete.
Wood, on the other hand—even engineered wood like CLT, which requires additional energy to cut and press into sections—is far more environmentally friendly. According to Wood for Good, an organization that advocates for sustainable wood construction, a ton of bricks requires four times the amount of energy to produce as a ton of sawn softwood; concrete requires five times, steel 24 times, and aluminum 126 times. Wood also performs better: It is, for example, five times more insulative than concrete and 350 times more so than steel. That means less energy is needed to heat and cool a wood building.
When CLT is used to build high-rise towers, the carbon savings can be enormous. The 186 tons of carbon locked into Stadthaus are enough to offset 20 years of its daily operations, meaning that for the first two decades of its life, the building isn’t carbon neutral—it is actually carbon negative. Rather than producing greenhouse gases, Stadthaus is fighting them.
While firms like Waugh Thistleton have focused on the lower end of the high-rise scale, others are designing radically taller buildings, up to 40 or more stories. The most recent proposal comes from Skidmore, Owings & Merrill, the firm behind some of the world’s tallest skyscrapers, including 1 World Trade Center and the Burj Khalifa. Called the Timber Tower Research Project, it reimagines Chicago’s 42-story Dewitt Chestnut apartment tower, which Skidmore designed in 1966, as a structure built primarily with CLT. Overall, the proposed building is about 80 percent wood with steel and concrete at the joints to provide added stiffness.
So far, the study is just that: a thought experiment. But for a blue-chip firm like Skidmore to embrace high-rise wood construction is a sign of how rapidly the technology is moving from the engineering vanguard to the mainstream.
It is unlikely that we’ll see wood towers rising as high as today’s supertall skyscrapers. But that leaves plenty of opportunity. Even in the world’s largest cities, only a handful of buildings are taller than 40 floors. “A huge chunk of the market is viable. New York is a high-rise city, but it’s not that tall,” says William F. Baker, who oversaw the Skidmore study with project engineer Benton Johnson. “We could handle most of Manhattan.”
Which brings us back to Stadthaus. If that unassuming building on a street corner in Shoreditch is actually a trap for hundreds of tons of carbon, imagine an entire city of Stadthauses. Structures that were once a major source of greenhouse gases could instead scrub them from the atmosphere. “Wood is the new concrete,” says de Rijke, of dRMM. “Concrete is a 20th-century material. Steel is a 19th-century material. Wood is a 21st-century material.”
The New Wood: Making CLT
The process for producing cross-laminated timber makes clear why architects call it “plywood on steroids.” Its layered structure gives it immense strength in two directions, producing a lightweight alternative to steel or concrete.
The New Wood
Beams of wood, usually spruce, are set down side by side in layers, with each layer perpendicular to the one beneath it, creating a wood board up to a foot thick. A thin layer of glue is placed between each layer.
The wood boards are placed in a massive press, which squeezes them together.
The edges of the boards are sanded down. If longer sections are needed, the edges are fingerboarded to create a serrated interlocking end. They are then glued to the matching end of another panel to create sections up to 78 feet long.
The boards are cut to custom specification, incorporating spaces for windows and utilities, using 3-D files sent by the architects or construction team.
Anatomy Of A Timber Tower
Anatomy Of A Timber Tower
1) Whereas steel or concrete structures are skeletal, using columns to carry loads, CLT towers distribute weight over the entire, solid vertical panel.
2) Steel or concrete L-brackets fix the horizontal and vertical CLT panels together.
3) The horizontal spans between vertical CLT elements can be significantly longer than with steel or concrete beams.
4) Interior walls are usually fireproofed by applying a layer of gypsum paneling on top of the mass timber panels.
5) A two-inch layer of concrete typically covers two two-inch layers of insulation (separated by a three-inch void) to reduce acoustic vibration between floors.
6) Panels come made to order with windows cut out and sometimes piping and electrical installed. Construction is as easy as screwing the panels together.
7) Elevators have double walls with insulation sandwiched between them for fire safety and soundproofing.
This article originally appeared in the March 2014 issue of Popular Science.