Skidmore, Owings & Merrill (SOM), the global firm responsible for many of the world’s highest and most high-tech skyscrapers, knows a few things about building high-rises. While their architects have created many of the most iconic steel-and-glass structures in the world, the firm has also made recent research investments in elevating another material, wood, as a structural backbone for future tall towers.
With the general push toward greener construction over the last few decades, the idea of tall timber towers, supported by wooden frames, has become a sought-after goal, offering a more sustainable way to build skyscrapers. SOM itself was a leader in promoting the idea, conducting a Tall Timber Research Project in 2014 that concluded that tall wood structures of 40-plus stories could “compete with reinforced concrete and steel while reducing the carbon footprint by 60 percent to 75 percent.”
While many have been built around the globe, cross-laminated timber projects still produce a degree of uncertainty, especially around issues of fire safety and structural stability. Earlier this month, SOM, along with Oregon State University (OSU), may have put some of the doubter’s fears to rest.
As part of a series of structural tests, a group of engineers tested a concrete-reinforced wooden floor system under the weight of a hydraulic press. When the test floor was eventually crushed, researchers determined it could support 82,000 pounds, approximately eight times the required design load. That’s the equivalent of 640 pounds per square foot, strong enough to support a 10-foot-deep swimming pool and more than tough enough to pass muster with building inspectors.
Including concrete may sound like cheating. But according to Benton Johnson, a structural engineer at SOM, it’s a key part of the Concrete Jointed Timber Frame system, a hybrid design which utilizes concrete joints. SOM believes it’s reliable and sustainable enough to make tall timber a standard building system. To put it in perspective, Johnson noted that the Willis Tower, which is called a steel tower, has as a steel-to-concrete ratio equivalent to the wood-to-concrete ratio of a proposed tall timber building.
This test was a big step forward, but more tests are required to fine-tune the system, including measuring how the floors shift (and preventing future movement), testing acoustic properties (wood floors aren’t great at stopping heavy machine noises, such as air conditioners), and measuring fire rating. Reinforced timber is a tough material, but to help it reach its full potential—Johnson believes in a decade, tall timber buildings between 10-20 stories will be much more commonplace—they need to be diligent, expand upon the data, and devise even stronger systems.
“If you want to make a difference with this type of construction, it’s all about making it universal,” says Johnson. “We’re not concerned about going tall for tall’s sake. It’s about building as many buildings as we want. Building more 10-15 story structures just allows for many more to be made with timber.”