A team of designers has created a digital prototype to identify and address some of the challenges associated with constructing tall timber buildings. © SOM
An architecture and engineering firm identifies and addresses some of the challenges of constructing tall timber buildings.
July 2, 2013—Although timber is often seen as a building material more in keeping with the goals of sustainable development than is the case with concrete or steel, the fact that it is light and combustible has traditionally made it undesirable for tall buildings. But one design firm is seeking to address some of the challenges associated with timber so that the material may one day see wider use.
Skidmore, Owings & Merrill LLP (SOM), an international architecture and engineering firm headquartered in Chicago, recently released the Timber Tower Research Project , a 72-page report that documents its work to study the structural feasibility of a 42-story prototype timber-framed building. The Softwood Lumber Board, an industry-funded organization that promotes the use of lumber products and also is based in Chicago, sponsored the project.
Using computer software, SOM designed the 400 ft tall prototype to resemble a modern version of Chicago’s Dewitt-Chestnut Apartments, a concrete-framed building that SOM designed in 1963. The researchers selected that building as the study benchmark not only because its was a groundbreaking structure in its day, incorporating as it did the first framed tube system, but also because it is highly efficient and has a simple geometry, says Benton Johnson, P.E., S.E., an associate of SOM. “For this study, the focus was on timber and technical feasibility, not on architectural design,” he says. The apartment building has “a simple rectilinear geometry, so we decided to start with that.”
While architectural design was not the focus of the study, SOM’s interior designers, technical architects, and mechanical, electrical, and plumbing team also were involved, the aim being to create apartment layouts that would be marketable in Chicago today. “We wanted to be sure that what we were doing wasn’t just an engineering exercise of no relevance,” says William F. Baker, P.E., S.E., F.ASCE, a partner of SOM. “These are real units in dimensions,” he notes, that would be expected in the current marketplace.
The primary challenge of any tower design is reducing the effect of lateral forces, but SOM knew that challenge would be significantly greater for a tall timber building, particularly with respect to wind. That’s because timber is much lighter than other building materials, making overturning moments harder to resist. “A building that is twice as tall, say a 20-story to a 40-story building, that 40-story building is actually going to have to resist about four times more overturning moments,” Johnson explains. “So to avoid uplift, the building would need to be [many] times heavier if it used the same system. And the problem with that, when we go to timber versus a concrete building, is the issue of weight. . . . Timber is really, really light.”
To reduce the uplift caused by wind, the team eliminated as many interior columns as possible and instead concentrated the gravity load at the building’s core and perimeter. As a result, the building’s floor spans are exceptionally long for wood, reaching 28 ft. “We actually have longer spans than we would have in some concrete buildings, which gives the interior designer a great deal of flexibility,” Baker says. But the long spans also accentuated another challenge with timber: strength. “Concrete is about 3 times stiffer and stronger than timber, and steel is about 20 times stiffer and stronger than timber,” Johnson says. “So right off the bat, you’re trying to span farther because you’re avoiding these interior columns, and you’re doing it with a material that doesn’t have as much stiffness.”
The researchers knew they would need a special type of timber for the flooring system, so they immediately turned to cross-laminated timber, which is formed by gluing together layers of wood in such a way that the members of each layer are perpendicular to those of the layers above and below it. The only problem was that the thickest off-the-shelf panels they could find (1 ft) were recommended for spans of no more than 24 ft. “We’re trying to make these big floor spans to minimize the amount of gravity load that’s taken away from the core. And we’re also trying to get these marketable units,” Johnson says. “So you have these two competing interests: how do you make the long spans and use the least amount of material possible so that it’s still competitive in price?”
To reduce uplift caused by wind, the team concentrated the gravity
load at the tower’s core and perimeter. © SOM
One of the things SOM is known for is the design of tall buildings, so the firm drew upon that expertise to resolve the issue. In a concrete building, everything is formed together, creating monolithic, rigid connections over the entire structure, which in turn makes it possible for the floor plates to be thinner. With that in mind, SOM developed what it calls a concrete joint timber frame to strengthen the timber connections. Forming the joint entails drilling rebar into the ends of the timber panels and then joining the panels together in much the same way that precast-concrete planks are assembled. Concrete is then pumped into the joints at select locations to hold everything together. “We get the benefits of monolithic behavior and fixity, similar to concrete, so we can drive the thickness of those panels down and make something that we think could be economical,” Johnson says.
The concrete also adds weight to the building, helping to reduce the uplift from overturning moments. “There’s a little bit of conflict there: we’re trying to do a timber building, but then we use concrete in select places,” says David Horos, P.E., S.E., LEED-AP, M.ASCE, an associate director at SOM. “But we’re trying to use each of the materials in locations where their strengths can be taken advantage of to the greatest extent.” The building is 80 percent timber and just 20 percent concrete by volume, but on a typical floor it is 40 percent timber and 60 percent concrete by weight. “It’s interesting how a little bit of concrete makes up for a lot of weight compared to the wood,” Horos says.
While SOM has highlighted several issues associated with tall timber buildings, many more challenges would have to be resolved before timber towers could be constructed, a formidable one being that building codes in the United States prohibit the use of combustible building materials at heights above 65 ft. “The study tried to identify the roadblocks associated with tall timber construction so that then perhaps further research and/or further code development can remove these roadblocks,” Baker says. “Our ultimate goal is that this would be a viable alternative for tall construction.”
One of the primary benefits of wood is that it is viewed as a material that lends itself to sustainable design. Timber takes less energy to produce than do concrete and steel, and the prototype timber tower structure has a 60 to 75 percent smaller carbon footprint than the benchmark building structure, Johnson says. As the need for buildings that adhere to the principles of sustainable development increases, timber may be the key to achieving carbon neutrality. “We would like to see wood as a tall-building option going forward because, depending on what happens with sustainability and carbon emissions, it may become much more important in the future to be able to do these kinds of buildings,” Horos says. “Right now there’s not a screaming need to do tall wood buildings, but you don’t know. In 10 to 20 years a lot of the dynamics may change, and this may be the best way to go.”