Timber is gaining ground as a viable structural system for high-rise buildings. Several tall-timber projects have been completed in Canada and Europe, and two domestic tall-timber projects are expected to begin construction soon, incentivized by prize money from the U.S. Department of Agriculture’s (USDA’s) U.S. Tall Wood Building Prize Competition.
But Skidmore, Owings & Merrill (SOM) thinks timber can go further—and higher—if it is used in a composite system that combines the structural properties of wood with reinforced concrete. In a multiyear research effort led by Chicago-based associate Benton Johnson, the firm behind many of the world’s tallest skyscrapers proposes a Concrete Jointed Timber Frame, which employs mass timber for structural elements—beams, columns, shear walls, and floor planks—and reinforced concrete for connections. The system would reduce the embodied energy of a comparable tower with a reinforced-concrete frame by 60 to 75 percent, according to the firm’s press release. (The firm’s ongoing study of tall timber received an honorable mention in ARCHITECT’s 2014 R+D Awards.)
This spring, SOM and Oregon State University embarked on a physical testing program that ran 14 different timber specimens through 20 different structural tests. The study and testing has received support from the Softwood Lumber Board, which also provided funding for the USDA’s competition.
On Aug. 8, the team reached a major milestone in its proposal with an ultimate load test on a full-size structural floor element that measured 36 feet long and 8 feet wide. Comprising a cross-laminated timber (CLT) deck with a structural reinforced concrete topping slab (more on that below), the floor element supported an ultimate applied load of 82,000 pounds, which translates to approximately 640 pounds per square foot (psf), Johnson says. (For comparison, a 10-foot-deep swimming pool produces a dead load of 624 psf.) This ultimate load is about eight times the design load that would be required by building code for the floor structure.
ARCHITECT talked to Johnson about the significance of the load tests and its proposed timber composite system.
ARCHITECT: Tall timber projects currently use CLT and other mass-timber products. What are the advantages of the hybrid timber–reinforced concrete system that SOM is proposing?
Johnson: The biggest advantage is that we're trying to tap into an existing cost of a project and use it for additional benefit. Almost every timber building utilizes a thin concrete topping slab for acoustic control. We think that you might as well make that slab structural and a composite, and then you can get longer spans that are more [typical] in construction. If you compare what we’re proposing to [the other tall-timber construction projects], it doesn’t have more or less concrete. We're just using it in a different way.
You say that your system can achieve longer spans. Can you also reduce the amount of timber used by having structural concrete?
You get a choice. You can either take the same amount of materials and span longer or, if you want to stay with the same column bay, you can make everything thinner.
Is the concrete slab reinforced with rebar or welded wire fabric?
It could be either. For the test, we used rebar because it was deemed to be the most reliable. It's only about 15 percent of what you'd be using [in a conventional concrete floor system]. It's like a #3 rebar—the smallest rebar at the widest possible spacing. It’s our expectation that in the future, we could eliminate most of the reinforcing by using fiber reinforcing that's mixed in with the concrete.
How is the topping slab bonded to the timber?
You can do it with glued-in or screwed-in steel plates, or you can use specially designed screws. We tested all three types on smaller specimens [to understand the appropriate conditions for their use]. Something that's highly loaded may require the glued-in solution, while the screwed-in [option would work] for a more typical load condition.
What types of tests did you conduct?
We had a control test to study bare CLT. We also studied concrete toppings and different types of shear connectors for the topping slab. We tested samples in positive bending and negative bending, meaning the concrete was in compression or the concrete reinforcing was in tension. We ran 3D warping tests. We also conducted movement testing, both short term and long term; the long-term test has been [ongoing] for about four months. Our last test was the big full-scale mock-up.
What span can you achieve?
We modeled a 24-foot span, which is consistent with typical residential modules. Now that we understand exactly how the system works, we can hit any span necessary for a project. If you needed a 30-foot span, we could achieve that. If you only needed 20 feet, that's OK too—it would just be more economical.
Did the testing go as you had predicted?
For the most part, but there were some benefits that we weren't accounting for. We designed the specimen as a partial composite [meaning the timber and concrete are not fully bonded], which reduced its [inherent] stiffness. But this more economical design [ended up providing] additional stiffness because we weren't cracking the concrete as much. It's good when all the unexpected things go in your favor.
We were trying to satisfy code requirements in terms of movement and vibration. And when we looked at the results, the system was about 30 percent stiffer than we it needed to be. That means we can make [the system] more economical by reducing the number of shear connectors or reinforcing, or the grade of timber.
Any preliminary structural design guidelines to report?
We're still trying to develop those criteria for the final report. [For example,] you may say that for a concrete flat slab, the thickness has to be 1/36 of the span. But we wanted to make people aware of the successful test now because we know we're successful.
Besides a reduction in embodied energy, what other environmental advantages does a timber structure offer?
Timber is a natural insulator and will improve the [thermal performance] of a building. Timber also functions as a moisture sink that regulates the air quality. It takes a while for timber to reach its saturation [point], so when you have seasonal variability between summer and winter, it can take the edge off of how much conditioning the air needs. Those are just two ideas.
Would the system require an exterior finish?
All timber on the outside would be protected. We tried to use the right material in the right place so the timber in our proposal is inside the building and protected from the elements. But there's no reason why you couldn’t use any standard exterior wall system for this construction type.
Any other construction benefits?
You get a lot of the same benefits of a traditional concrete system but without all the waste: sound, acoustics, and a system that's highly fire resistant through the reinforced topping slab. At the same time, you maintain the lightweight nature of timber structure: The overall mass is about one-half to two-thirds of what you’d see in a concrete frame. It’s similar in mass to a steel structure using a lightweight topping slab. Any time that weight is your enemy—if you have a project with a complicated foundation condition, or located in a high-seismic zone—a lightweight structure is good thing.
The system plugs in very well to how contractors think about building. A concrete building and a timber composite building [are essentially] the same. Instead of the cost of formwork and shoring, you have the cost of timber and frame. And then all the concrete pieces get cheaper—there's less concrete, less rebar, and less foundation.
What’s next for the project?
In our 2014 system report, we had a whole host of recommendations, so we’re going down the list, knocking them down one by one to eliminate barriers to entry. We know enough about the system to design some high-rises now. But we have to do some fire testing before it will be appropriate for use. We're working with a number of different fire professionals and laboratories to understand the right methodology.
This interview has been edited and condensed for clarity. This story has been updated since first publication to correct the size of the rebar used in the concrete slab.