For its prototypical tower, SOM proposes a concrete jointed timber frame structural system. Solid, 8-inch-thick mass timber floor panels span between the timber shear walls at the building core and the perimeter columns. The wood primary structural members are connected with steel reinforcing through the concrete joints, such as the spandrel beams.
The growing interest in designing wood high-rises moved a step closer to feasibility when Skidmore, Owings & Merrill recently published its Timber Tower Research Project. Credited with designing many of the world’s tallest buildings, including the Cayan Tower in Dubai, SOM is no stranger to venturing into unchartered territory.
In its report, SOM develops the design for a timber structural system for a prototypical building with the same parameters of the Dewitt-Chestnut Apartments, a 395-foot-tall, 42-story building in Chicago by SOM erected in 1966. Currently the tallest modern buildings that utilize wood construction hover around nine stories tall. Talks are under way to build 20- and 30-story buildings in Canada and Austria.
Rather than proposing an all-timber structural system, SOM developed a “concrete jointed timber frame” option that combines the structural behavior of reinforced concrete with the carbon-sequestering benefits of cross-laminated timber (CLT). The design’s material use and efficiency could reduce the carbon footprint of a conventional reinforced-concrete and steel building by 60 to 75 percent, SOM estimated.
Benton Johnson, an SOM associate who helped lead the study, talked with ARCHITECT about the next steps to taking a timber skyscraper into reality.
Why did SOM become involved in this project?
The number of people living in cities is projected to double from 3.5 billion to 7 billion by 2050. We’re going to have to add a lot of buildings to our cities, and we wanted to figure out the most sustainable way to do that. We know that structure is a key part of the embodied (carbon) footprint [of buildings]. After some research, we found that wood is hands down the most sustainable material.
Benton Johnson, SOM associate
When was the study performed?
In June 2012, we [began to explore] how we would research the subject. The actual period of work happened over four months, between November and February.
How does SOM’s study differ from the work of other designers interested in tall timber structures, such as the Vancouver-based designer Michael Green?
SOM has done a fair amount of wood and timber construction, such as the Atlantico Pavilion in Lisbon and the Cathedral of Christ the Light in Oakland, Calif. We’re coming from the perspective as tall building experts, while a lot of the other work that has been performed has been from wood experts who are trying to go tall. We've been designing tall buildings for about 75 years. The amount of senior leadership who reviewed this project was quite large [and included] John Zils, who helped design the Sears (now Willis) Tower.
What are some of the most notable findings in your study?
The thing that most people find interesting is the combination of materials in the system. A lot of people expected it would be a timber structure supplemented with structural steel. We chose reinforced concrete to supplement the timber instead.
As tall building designers … one of the first things we struggle with is the floor-to-floor height of a building. In a high-rise building, we want to minimize that height … because it reduces the wind and lateral loads. We also had to achieve these long spans that go 28 to 29 feet from the core to the perimeter, with the least amount of material. [The span length] was set by the existing (Dewitt-Chestnut Apartments) building, but we found it is still compatible with today’s market.
Perspective of timber tower prototype
If you open up the CLT handbook, and you look for what you need to span 28 feet, you see “N/A.” No panels go that far; even a 1-foot-thick panel runs out of gas because it’s too flexible. This led to a whole series of studies on how we can achieve this rigid end connection with a timber floor plate. … The great thing about concrete is that you can drill in the reinforcing, pour the concrete over everything, and the concrete holds everything together [without allowing the floor ends to rotate]. It’s a much easier rigid connection.
The report lists several issues that require further study including fire testing, seismic loading, acoustics, uplift, and differential shortening.
The biggest one people ask about is fire. You need to apply or look at performance-based design in order to design these structures. If you have a typical residential construction with 2x10 floor joists, it doesn't take long for it to burn because of the large exposed surface. When timber, which is massive, starts to burn, it forms a char layer, and the char layer is basically insulation. The path to designing these structures to resist fire is to account for a certain amount of sacrificial charring of the members. And through that sacrificial amount, you can actually design the structure to support load for a certain amount of time.
The amount of time … will need to be investigated by a fire engineer … to evaluate the necessary fire loads and the amount of [sacrificial] structure needed to allow for the fire to burn out before the structure loses strength. If we can do that, we can design the structure to meet the intent of the code. There are many ways to do it, through dry wall protection and so forth. At the end of the day, there is a path to making these buildings fire safe.
For the other issues, such as differential shortening … we already have strategies in place for other building systems, such as building the floors camber. Things will need to be fine-tuned by the engineer as we learn more by doing them in practice.
What are the biggest selling points of a timber tower?
The first and foremost is that by choosing a timber structure, it shows that you’re committed to sustainability. There will be challenges because it hasn't been done yet. The second benefit is that in time, we hope, these buildings can be constructed quite quickly. The developer who chooses [a wood structure] now can become more comfortable with it. As the technology becomes mainstream, they’re going to be more ahead of the game.
How much would a timber tower cost in comparison with other systems?
It’s not a mistake that you can’t find cost information in the report. The reason we didn't want to touch cost was because we’d need to hire a cost consultant. Because we’re using systems that haven’t been tested yet, it’s even difficult for them to estimate. What we focused on instead is reducing the amount of materials [required in construction]. We published quantities for people to make their own judgments about how efficient the structure is … knowing that’s a direct link to cost. Beyond that, we tried to develop the system to be constructed as fast as possible, understanding that [time] is another big cost associated with building.
What are your next steps?
The next thing is to engage the rest of the community that’s involved with these types of buildings. We need to move forward together, identify the big steps, and pool resources for research and development. We’re trying to whittle down our list of recommendations to specific tasks to move this type of building forward. It’s a structural testing of components and joints to better understand their behavior and doing in-depth research of its fire performance, and then using the results of those things together to move to larger scale mock-ups. Our next steps are about making sure people are comfortable with the technology that we proposed.