A proposed structural system would use approximately 70 percent timber to build tall buildings as an environmentally friendlier alternative to steel and concrete.
The movement to construct tall buildings largely with wood as an environmentally friendlier alternative to steel and concrete has received a boost from an unusual source — a leading architectural firm known for its towers of steel and concrete.
Skidmore, Owings & Merrill, the Chicago-based firm that has designed a long list of skyscrapers, including the new One World Trade Center in lower Manhattan, has developed a structural system that uses so-called mass timber — columns and thick slabs that are laminated from smaller pieces of wood. In a report this year, the firm showed how the system could be used to build a 42-story residential tower that would have a lower carbon footprint than a conventional structure.
“We're tall building engineers,” said William F. Baker, a partner in the firm. “We wanted to see what we can do to help on the sustainability side.” With their system, about 70 percent of the structural material is wood; most of the rest, including the foundation, is concrete.
Benton Johnson, an engineer who worked on the report, said that wooden high-rises could help solve the growing worldwide problem of providing adequate housing to the billions of people who are, or will be, living in cities — while also addressing climate change.
“We know that we need to build a lot more buildings,” Johnson said. “And we know that we need to lower CO2.”
Until now, tall wooden buildings had been championed by a handful of architects and engineers, mostly from smaller firms overseas and in Canada. They welcomed the Skidmore, Owings & Merrill report.
“I'm really thrilled that they're involved with it,” said Michael Green, an architect in Vancouver, British Columbia, who has designed many wooden buildings and, with partners, came up with a different structural system for wooden towers that was detailed in a report last year. “This is the first new way to build in a hundred years. It's going to take a little time to work through the best way of doing it.”
Few modern tall wooden buildings have been built around the world, and only one, an apartment building that was completed this year in Melbourne, Australia, has reached 10 stories. Green's design of a 90-foot-high mixed-use building in Prince George, British Columbia, will make it the tallest wooden building in North America when it is completed next year.
Constructing more and taller towers will require changes in building codes — most of which limit wood structures to four stories or fewer — and construction methods. Architects, engineers, contractors and, crucially, developers will have to be convinced that wooden buildings can be safe, attractive and profitable. (They are generally more expensive than conventional towers, although in some areas of construction there can be savings because the slabs can be erected fairly quickly.) Fire protection is a particular concern, but advocates for wooden buildings say mass timber does not ignite easily and forms a layer of char that slows burning. They say wooden towers can meet fire safety standards for steel or concrete buildings.
Production of steel and concrete produces significant amounts of the greenhouse gas carbon dioxide, while wood holds the carbon from carbon dioxide removed from the atmosphere through photosynthesis. So using wood in the structural elements can help offset the carbon emissions from the other parts of the construction process and from the operation of the finished building. A timber tower uses a lot of wood. This is not conventional frame construction, in which 2-by-4s and other thin elements are nailed together, but more akin to building with concrete slabs. The tower in the Skidmore, Owings & Merrill study, for example, would contain about 3.9 million board-feet of wood; a typical single-family home contains less than 20,000 board-feet of framing lumber.
The use of so much wood raises the issue of the potential impact on forests if wooden buildings were to become prevalent. Baker noted that in the United States and Canada, about 60 billion board feet of lumber was harvested each year, and as long as forests were managed, sustainable wooden buildings should not have much of an impact. There are also millions of fir trees in North American forests killed by a widespread beetle infestation that could be used to produce the timber panels.
The Skidmore, Owings & Merrill system uses a type of engineered wood called glued laminated timber, or glulam, for the building columns, and cross-laminated timber slabs for the central core, floors and shear walls, which provide stiffness against wind loads. But the concept calls for concrete beams along the perimeter of each floor and elsewhere to allow for longer spans and thus more flexibility in floor layouts.
The case study 42-story building in the report is based on an actual Skidmore Owings & Merrill tower, the Dewitt-Chestnut apartments (now called the Plaza on Dewitt) in Chicago, that was built in 1966. That building, made of steel and concrete, had an innovative structural design — it is basically a tube that acts like a vertical cantilever to resist wind loads — that was used in later buildings, including the Sears Tower.