Council on Tall Buildings and Urban Habitat
 

CTBUH Chicago Talks Tall Timber
February 25, 2015
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CHICAGO – The Council on Tall Buildings and Urban Habitat (CTBUH) Chicago Chapter, in association with the Young Professionals Committee (YPC), held a lecture entitled “Tall Timber Buildings: Trends and Research” on February 25. Graciously hosted at the SOM offices, the event drew over 90 attendees, who were all eager to hear about the technologies and strategies that make timber a suitable structural material in tall buildings.

The lecture was given by Benton Johnson, a structural engineer at SOM who has been involved in some of the firm’s explorations into the applications of heavy timber at height. Johnson is also a member of the CTBUH Tall Timber Working Group, which recently held their kickoff meeting in the CTBUH Research Office at University Iuav of Venice. The Working Group was founded in order to investigate the possibility of realizing the first 40-story-tall timber building, and will develop a proposal for a significant research grant later in 2015.

Johnson began his lecture by highlighting some of the recent advancements in the field, of particular note, Cross-Laminated Timber (CLT). Typical heavy timber possesses immense compressive strength when force is applied parallel to the grain of the wood, but is much weaker when force is applied perpendicularly, Johnson explained. CLT solves this problem by laminating planks of wood together in an alternating pattern, thereby imbuing the structural element with compressive strength along every axis. In addition to its structural benefits, CLT is able to develop a “char zone” during a fire. As a timber element burns in a design fire, the char zone develops on the outer layer of the material, burning at a constant rate and keeping the inner layer cool. If timber structures are designed taking the reduced strength of the char zone into account, fire scenarios become predictable, and therefore, solvable engineering problems.
Benton Johnson, SOM, presents on recent advancements in the field of heavy timber at height
Example of Cross-Laminated Timber (CLT), which can be fire-engineered to the standard of other primary building materials
Next, Johnson expounded upon the research that he and his colleagues at SOM have undertaken to understand the innate possibilities of heavy timber. They began by looking at a previously completed steel-framed SOM project, the 42-story Dewitt Chestnut Apartments in Chicago, and designed a structural system that would achieve similar results using heavy timber. Johnson said that by pushing the concept of a timber building to this unprecedented height, findings can be interpolated back to shorter buildings with confidence. The presenters discussed several key challenges to this exercise, one of which was the question of how to achieve large horizontal spans without excessive sagging. Johnson and his team addressed this issue by developing a composite solution, involving concrete floor joints that clamp down on the edges of the floor panels to reduce movement.
Self tapping screws, used to connect heavy timber elements
Attendees networking following the presentation
Chief among the concerns of this research was the carbon footprint of a tall timber building compared to a typical steel or concrete building. As a material, timber has a negative embodied carbon value, made possible by the carbon that is captured by the tree over its lifetime. This fact, coupled with the regenerative timber harvesting practices that are prevalent today, make timber an easy win in terms of sustainability. Johnson emphasized that in order to make tall timber buildings possible, it is critical that they are marketable, serviceable, economical, and sustainable.

“We always talk about these things in this order, because if you can’t show that it’s marketable, serviceable, and economical, it doesn’t matter how sustainable it is.”

After the 90-minute lecture, the engaged audience asked challenging questions for over 30 minutes. This was followed by further discussion during the networking session.

The event was organized and moderated by YPC Chair Sasha Zeljic, Design Director, Gensler; YPC Co-Chair Nathaniel Hollister, Intern Architect, Goettsch Partners; and YPC Secretary Saul Moreno, Designer, Thornton Tomasetti.