Reach for the Sky—Wood Frame Building Will Be 35 Stories

Look closely—it’s made of wood. Exterior rendering of office and residential levels of Proto-Model X, Sidewalk Labs’ model of a timber frame destined for the Toronto waterfront. (Image courtesy of Michael Green Architecture and Gensler.)

Given that wood is flammable and biodegradable, it may never have been an ideal building material. We have steel for that. However, in many parts of the world, wood is available in abundance, so it is pressed into service for our buildings. Wood framing is common in North America for residential buildings but less so for commercial buildings. Wood framing has largely been unheard for use in high rises—until today, when plans of a 35-story wood frame sky scraper, part of Sidewalk Labs development project in Toronto, popped into my inbox.

No building this tall has ever been built with a wood frame. It’s not even close. The current tallest wood frame building is Norway’s 85.4m-tall Mjøstårnet. The second tallest is the 53m-tall Brock Commons Tallwood House in Vancouver. Both buildings are 18 stories.

Sidewalk Labs has a digital model, a proof of concept it calls the PMX Tower (Proto-Model X). There’s a lot to be worked out when making a wooden building this tall.

Engineering Wood

Looks like the real thing because it mostly is. Mass wood, an engineered structural material, can pass for normal wood—at a distance. (Image courtesy of HGA.)

The PMX plans do not call for using plain, ordinary wood, but “mass wood,” or a wood-mostly material that when glued together is called “glulam” and is used for ultra-long beams and columns. It is called nail laminated timber (NLT), and the plywood-like cross-laminated timber (CLT), which is used for floor and roof decks as well as bearing walls. Mass wood can be made fire resistant with the addition of chemical fire retardants, though this certainly makes the material less green. Mass wood’s manufacturers claim that the carbon emissions produced from making it are far less than the emissions created in making of steel or concrete—though cutting down trees is hardly green. Mass wood looks better than steel or concrete. We cannot argue with that.

Plans for PMX call for a wooden external skeleton. (Image courtesy of medium.com.)

With a much lower strength-to-weight ratio than steel, wood of any type poses special challenges. But with a Sidewalk Labs team dead set on sustainability, a steel frame and concrete curtain walls were a nonstarter. Still, duplicating the same type of frame used in steel and concrete construction with wood would have resulted in ridiculously massive structural elements. A “timber core” design would have walls 5-feet thick. Not only would walls this thick require too many trees, they would also be difficult to manufacture and ship. In addition, they would take up too much floor space. PMX is going with a design that uses a wooden “exoskeleton” consisting of diagonal bracing and vertical columns on the outside of the building that support a 10-inch-thick “lean wood core.”

The BIM was done with Autodesk Revit and is hosted on BIM 360, a cloud-based construction management application.

A Counterintuitive Counterweight

A concrete and steel tower would be 2.5 times as heavy as a wooden skyscraper. But whereas light weight is an asset in aircraft and rockets that seek to escape gravity, it can be a liability in buildings that need to stay put. Preliminary analysis showed the 35-story wood frame construction had as much deflection in the wind as a 40- to 50-story building constructed with a steel frame.

With the addition of a tuned mass damper, the PMX tower model stabilizes quicker. A gust of wind would make the towers bend to one side, then oscillate. Without losses, the tower would oscillate forever, but internal dampening would cause the structure to stop eventually. A tuned mass dampener would add more losses and make the tower stop swaying sooner. Note that in the model shown the dampener would only reduce swaying from right and left, but not back and forth. (Image courtesy of Gensler, Aspect.)

The PMX team found that it had to allow a lot of steel into the design—in the form of a 70-ton steel weight, part of a system that is designed to dampen vibration.

While it may seem counterintuitive—perhaps even dangerous—to have massive weight on top of a building, that is exactly what civil engineers may order for a tall building that is swaying too much or is expected to do so. Tall buildings can have deflections of several feet on their top floors—unsettling and even sickening their occupants. A tuned mass dampener (TMD) system, can be designed in or retrofitted. A TMD with a precisely calculated amount of mass made of concrete, steel, lead or other dense material stays still due to its own inertia when a tall building initially bends— as a result of the ground shaking or a gust of wind. Dampeners attached to the mass absorb the energy and act to limit the number of oscillations.

Chicken Little would not approve. An 18-ft diameter, 660-metric ton steel sphere, which hangs like a pendulum and is visible between the 88th and 92nd floors of Taipei 101 in Taiwan, is a tuned mass damper (TMD) that reduces the swaying of tall buildings. (Image courtesy of Pinterest.)

TMD systems have been around for some time, but the increase in super tall and very thin tall buildings has made them even more sought after. Shanghai, New York and Dubai have several buildings with TMDs. Taiwan’s Taipei 101 tower uses a system that makes its TMD, with a suspended golden ball, a visible design feature.

The Canadian National Tower, at one time the tallest structure in North America at 102m, also in Toronto’s downtown, has two doughnut-shaped steel rings, one at 488m and the other at 503m—each weighing 9 metric tons—that serve as TMDs. They are tuned to the 2nd and 4th mode shape of the tower, while the 1st and 3rd mode are controlled by the prestressed concrete and don’t require additional damping.

Boston’s John Hancock Tower had two 30-ton sliding dampers installed retroactively that were designed to reduce the building’s sway by 40 percent to 50 percent.

TMDs can take several forms, including sliding, rolling or swinging weights.

Not Your Parents Prefab

Mechanical, electrical and plumbing equipment is embedded in a floor panel off-site in a factory. (Image courtesy of Integral.)

As much as possible, the PMX designer sought to make the building off-site in parts, and then assemble the parts on-site. This is the long sought-after advantage manufacturing has enjoyed, while construction has lagged behind. PMX is making staircases, floor panels, walls, and kitchen and bathroom “pods” standard and assembled in assembly lines, transporting them to the waterfront site on trucks, and then snapping them together … like Legos, according to this article. These “cassettes,” as the sub assemblies are called, will be made in 25 steps, with each step estimated to take 25 minutes. It is assembly line techniques at work, rather than the painstaking, laborious, material wasting current practice of laying floors, pouring concrete, joining gigantic steel members, and so on, that is the common conventional construction trade practice.

In addition to busting out of age-old construction practices, the PMX also hopes to bust out of the lowly status that prefab construction can’t seem to shake, like a screw-top wine. The plan’s exoskeleton can be draped in any manner of dress and color—a far cry from the welcome to middle-class, prefab homes in cookie cutter neighborhoods that gave prefab a low-class status.

Sidewalk Toronto

Sidewalk Labs has a $1.3 billion project to develop Quayside, a 12-acre area in Toronto on the banks of Lake Ontario. Sidewalk Labs, part of Alphabet Inc., which also owns Google, was formed to create communities “from the Internet up.” When complete, Sidewalk Toronto would potentially bring 44,000 jobs, many of them tech jobs, to Toronto’s downtown. It was to be a test bed for technology close to city scale, including roads especially designed for autonomous vehicles. But the proposal may have represented too much technology for Toronto’s residents. Sidewalk Labs plans to pool and make public “urban data” gathered from those who were in Sidewalk Toronto. The city will be voting on whether to move forward with the Sidewalk Labs proposal.