Timber Construction Doesn’t Have to Be “All or Nothing”

How Hybrid Curtain Walls Can Drive Sustainable Innovation in Architecture

December 5, 2018

Principal, NBBJ

Editor’s Note: This post was adapted from the white paper “Hybrid Timber: Performative, biophilic and beautiful” [PDF].

The increased use of timber in construction is a growing and robust opportunity. Wood evokes deep passion and motivation, but why? For one, it’s exciting to have technological and structural advancement within an industry that has been fairly constant since wood balloon framing was invented.

In addition, the prospect of managing our forests sustainably is the future. It supports the use of wood while avoiding the use of old growth species, instead using young saplings or beetle kill forests. It creates sustained carbon capture by circumventing the carbon release that occurs at the end of a tree’s life through decomposition, thereby reducing greenhouse gas emissions. Other benefits include low embodied energy, low thermal conductivity compared to aluminum or steel, better indoor air quality (IAQ), biophilic connections supporting a sense of well-being and health, and the outperformance of other building materials “cradle-to-cradle.”

The use of wood in curtain wall construction, in particular, is an emerging trend. A typical approach to long-span exterior curtain wall design is an aluminum curtain wall framing with secondary steel support—but this convention is being challenged by the use of wood as the primary structural support of the glazing.

Given the industry’s unfamiliarity incorporating wood within typical curtain wall assemblies, this proves to be a challenge, for several reasons:

  • Interest in bidding: The curtain wall market has been busy, making it difficult to draw interest in bidding, especially for smaller scale work.
  • Atypicality: The use of wood is not familiar to most large-scale builders.
  • Cost: The prior two variables drive cost upwards, even though the cost of glue-laminated timber is more cost-effective than steel at similar spans.
  • Engineering: Wood does not possess the same properties as steel, and in fact its strength varies by species.

However, the appropriate application of wood is not a matter of “all or nothing.” Hybrid options using wood as the lateral supporting system or as a dead load support, combined with more conventional aluminum systems or a semi-unitized curtain wall system, can yield a more conventional and familiar system design, making wood a more viable option for cost and schedule.

In one example I worked on, the curtain wall subcontractor provided the engineering of the curtain wall and attachments to the glue-laminated timber, and the structural engineer of record provided the engineering of glue-laminated timber and its attachment to the primary structure of the building, similar to the use of a more conventional secondary steel system.

In another example, the curtain wall subcontractor provided the entire engineering of the composite system, including the wood dead load supports, which transfer the window system loads to the primary structure.

With both of these options, the curtain wall consultants worked closely with the full engineering team as the point of intersection and peer-review for the system as a whole. Wood suppliers provided design information on the wood and glue-laminated timbers available, and communicated their unique strength characteristics by species to the design team.

Essential to the success of these projects was our strategic and proactive planning toward connecting markets and suppliers and building consensus between them, defining engineering roles and responsibilities, and effectively addressing fire and combustibility concerns.

Photo © Lawrence Anderson

Building a proper team with supportive and knowledgeable industry partnerships is paramount in being able to meet these challenges with clarity. Therefore, it is critical to partner with both an experienced timber/curtain wall engineer and forestry partners that have an in-depth knowledge of the process and the fluency to ask the right questions at the right time to support success and mitigate risk. I also recommend partnering with local fire authorities early in the process, onboarding them to the use of timber prior to permit submission.

Our hope is to create a ripple effect for the imperative change needed at a larger, industry-wide scale. Similar to code related energy requirements, only larger-scale demand will propel cross-industry advancement and expertise. This will drive innovation towards higher performance, reductions in our carbon footprint, less harmful chemical dependency and beautiful biophilic outcomes. The ultimate outcome will enhance our human experience with respect for our planet.

For more on timber construction, please read my white paper “Hybrid Timber: Performative, biophilic and beautiful” [PDF].

Banner photo courtesy of NBBJ/Sean Airhart.

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