A 26-foot-tall, load-bearing black willow tree replaces a steel column in the atrium of the rennovated Myrick Hixon EcoPark in La Crosse, Wis.
WholeTrees Architecture & Structures A 26-foot-tall, load-bearing black willow tree replaces a steel column in the atrium of the rennovated Myrick Hixon EcoPark in La Crosse, Wis.

A tree’s natural strength is often compromised in wood-frame construction, in which logs are stripped and milled into boards that are rejoined into man-made systems. But fabricator WholeTrees Architecture & Structures, in Madison, Wis., is recouping it by engineering tree trunks and small-diameter (4 to 21 inches), round timber—removed as scrap during routine forest thinning—as columns, beams, and trusses. The firm says the wood rivals steel’s strength-to-weight ratio and costs slightly less.

The trick is to preserve the naturally occurring pre-tensioned structural fibers that allow trees to grow upwards against axial loads while withstanding lateral ones, say Roald Gundersen, AIA, and Amelia Baxter, who founded WholeTrees in 2007. Their 14-person team sources species such as ash, black locust, oak, and pine from public and private forests in the upper Midwest. It visually grades wood that has uniform dimensions based on ASTM standards for structural poles while the non-uniform pieces are scanned as a 3D mesh and analyzed digitally to identify structural weak spots. In application, systems using the load-bearing trusses, beams, and columns are engineered to accommodate these weak spots. The wood is then peeled, treated to repel insects and fire, and stained and sealed. 

A rendering of WholeTrees' steel–wood connection.
WholeTrees Architecture & Structures A rendering of WholeTrees' steel–wood connection.

“The superior strength of the round timber comes from the way the fibers are uninterrupted as they move through the tree,” Baxter says. She likens a tree to a tube of straws. “If you were to slice off the edges … suddenly the strength starts to fray; it loses the integrity.”

In 2013, the firm generated more than $1 million in revenue and, since its founding, has received $1.1 million in grants and $915,000 in equity investments. Since 2010, WholeTrees has worked with the U.S. Department of Agriculture’s (USDA’s) Forest Products Lab in Madison to refine its analysis algorithm and to engineer a steel–wood connection (left) for its truss products that replicates tree branches’ innate joinery and can be used in commercial applications. 

“The question we’ve spent years testing, retesting, and testing again is how to intersect that tube of straws that is a tree with steel and bring loads through steel and through [truss] webs … to intersect the fibers of the tree without compressing them or pulling too many of them out,” she says.

Now, the firm wants to take the material mainstream with licensable software that processes 3D tree scans to improve ASTM grading, help assign design loads to non-uniform elements, and create an online model library of engineered trees. A $504,000 USDA grant is supporting a pilot of the software at two timber staging yards with the hopes of licensing it by 2016. WholeTrees also plans to add the imperfect round timber material as a BIM element, joining materials such as steel, concrete, and engineered wood as a structural option.

“We came out of left field in so many ways,” Baxter says. “It’s been so many years of people thinking [that this material] couldn’t be graded and the technologies don’t exist for this to become mainstream. Because we were able to build enough of both federal funding and individual investors and early clients willing to build with us, we just kept marching forward.”