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Building a Solid Case for Green Timber Towers

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Rendering courtesy of SOM
Study shows that a concrete-jointed timber frame can drastically reduce a tower's embodied carbon footprint over concrete alone.
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Benton Johnson does much more than walk the green talk—he bikes it. The 30-year-old structural engineer has pedaled more than 15,000 miles through rain, snow and frigid Chicago winters to his office at Skidmore, Owings & Merrill LLP. He calculates that, over the four years of his SOM commutes, he has reduced his carbon footprint by 2.1 tons by not taking the train.

Benton Johnson
JOHNSON

On his arrival at the office, Johnson checks his bicycle at the door. But that doesn't stop him from moving toward greater sustainability in tall structures, which is no easy task. A high-rise frame's embodied carbon is significantly higher per sq ft than a low-rise building's, due to greater imposed loads.

Thanks to Johnson, SOM is on a course to ameliorate that with mass-timber towers, which are greener than concrete, steel or masonry structures. Last May, SOM published the 72-page "Timber Tower Research Project," initiated by Johnson. The young associate's pet project is the first to consider timber as a tower's main structural material. SOM says its concrete-jointed timber system, developed for a 42-story residential high-rise, could reduce a tower's embodied carbon footprint by 60% to 75% over structural concrete.

Though SOM has a handful of clients who are intrigued by timber towers, the architect-engineer has a way to go before one is built. Codes restrict timber frames to four or five stories, mostly because wood is combustible.

"There is a need to do physical fire testing for local approvals," says Johnson. It is also important to garner more detailed knowledge to adapt the SOM system to buildings of all geometries and heights, he adds.

To that end, SOM recently kicked off a follow-up study, again with funding from the Softwood Lumber Board. Cees de Jager, SLB's chief marketing officer, says, "SOM's research builds on early efforts by thought leaders such as [architect] Michael Green and takes them to the next level."

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