Could a 35-story, all-wood skyscraper cut housing costs?

One of the centerpiece ideas in Sidewalk Labs’ plan to build a high-tech neighborhood in Toronto is for dozens of mass-produced, mass-timber towers to rise above the former industrial waterfront. Now the project’s design team is claiming it can build an all-wood structure that’s 35 stories tall—almost double the height of any mass-timber building under construction today.

Mass timber is a major component of the Google-affiliated $1.3 billion vision for Sidewalk Toronto, a brand-new affordable, sustainable community that’s meant to test innovations that can be used in other cities. In addition to using the renewable, attractive, climate-friendly material in many of the structures in the 12-acre development, the proposal includes building a factory nearby to kick-start the manufacturing process.

Since last year, a Sidewalk Labs team that includes well-known mass-timber architect Michael Green, the global architecture firm Gensler, and other engineers and consultants, has been working on how exactly those wooden buildings would manifest themselves in the Toronto development. The team has come up with a “digital proof-of-concept” they call Proto-Model X, or PMX.

While the spread of mass-timber construction has resulted in many handsome towers built in Europe, the material has been slower to catch on in North America. A handful of mostly mid-rise towers have been built in the Northwest U.S. and Vancouver, but many that have been proposed have never materialized. Some of that has to do with local building codes that haven’t caught up to construction innovations. (In 2018, Oregon was the first state to allow mass-timber towers over six stories without special permission.)

But one major challenge is that mass timber is thought to have height limitations—namely, the material is so light and so flexible that architects of taller mass-timber buildings often opt for steel or concrete for key structural elements. The Shigeru Ban-designed Terrace House in Vancouver, billed as the “world’s tallest” mass-timber building at 19 stories, has a concrete and steel core. This drives up cost as well as the carbon footprint.

For mass-timber towers taller than 20 stories or so, this hybridization seemed inevitable. SOM, for example, released a study in 2013 for a theoretical 40-story mass-timber building using reinforced concrete joints. A 23-story mass-timber tower under construction in Milwaukee is actually being built atop a five-story conventionally built concrete podium.

But the Sidewalk Labs team wanted to explore a “pure timber” solution, and ended up borrowing a technique from traditional tall-building construction—a cross-brace frame made from beams of cross-laminated timber (or CLT), which would create the diamond-shaped support system seen on the exterior of many skyscrapers. Because all the supports would be external, this would also free up more space for living or working on the inside of the building. Perkins and Will’s River Beech research project proposes a similar diagrid structure in exploring the feasibility of a 80-story mass-timber tower for Chicago.

A second challenge—stabilizing a tall wooden building—was more tricky. All tall buildings sway, but PMX’s wood-frame construction could make it sway enough to make people sick on the upper floors. The tower would then need another traditional skyscraper element known as a tuned mass damper, a giant counterbalance suspended on springs atop the structure that keeps a building from rocking too much. Unfortunately, wood doesn’t offer the necessary density, so PMX’s mass damper would be made of steel, the only non-wood structural concession the designers made.

As part of the Sidewalk Toronto announcement last June, Sidewalk Labs CEO Dan Doctoroff said that using mass timber could reduce the cost of building the development by 20 percent—savings that could then be passed along to residents. “The impact of that on affordability could be truly profound,” he said. “We do believe this can be a model for inclusive growth.”

Developing a more affordable method for building residential architecture might inform a separate Google project: a $1 billion investment to build 20,000 new homes on or near its Mountain View, California campus to help ease the state’s housing shortage.

Sidewalk Labs

For any cost savings to pencil out, however, Sidewalk Labs must first develop and streamline what’s essentially a brand-new construction method. This would include creating a modular system that’s cheaper and faster than the expensive, carbon-intensive activities of concrete pours and steel framing, as well as addressing a nationwide construction labor shortage that has made building just about every type of housing more expensive.

So designers are thinking about how to most efficiently manufacture prefabricated pieces that can be snapped together like Lego bricks on-site, from floor panels that can hide pipes and wires, to exterior cladding designs that allow for distinctive—and sometimes colorful—customization.

The mass timber industry has made big strides in the U.S. just in the last six months. After Sidewalk Labs made its announcement last year, the startup Katerra opened the largest mass-timber production facility in North America in Washington, giving a good look at how Sidewalk Labs’ own mass-timber operation might work. (Katerra also acquired Michael Green Architecture in 2018.) The Spokane Valley factory sources timber grown in certified local forests and owes much of its efficiency to using artificial intelligence to analyze and sort raw wood—one mass-timber task that sounds perfect for a Google-related company to optimize.