I’ve been surrounded by wood all my life. At camp as a kid, I learned the constellations and how to identify trees. In my 20s, I did a lot of bushwhacking in New England, and learned to read its dynamic forested landscape. Over the past 15 years, I’ve developed into a structural engineer at a timber framing company.
Trees are an essential part of a sustainable future. Hermann Hesse may have captured it best when he wrote, “Trees are sanctuaries. Whoever knows how to speak to them, whoever knows how to listen to them, can learn the truth. They do not preach learning and precepts, they preach, undeterred by particulars, the ancient law of life.”
Humans first learned to use trees structurally for water wells 7,000 years ago. The East and West traditions of timber framing have been passed down through generations and have been blended together. We now have the knowledge and ability to construct most of our buildings nearly completely out of wood. To do so, we must know the material properties and design capacities, leveraging inherent strengths while accommodating weaknesses.
Wood is relatively lightweight compared with other structural materials and has good strength in tension and compression. It is naturally tough, insulates, and is readily worked—cut, shaped, connected, and finished. Its grain, color, and growth patterns across the roughly 100,000 different species naturally bring unparalleled beauty to wood buildings. Some species also have inherent bio-chemical resistance to rot and decay. With all this variation, we are fortunate that engineers have studied and tested most of the principal species harvested for structural use. We have safe structural values for engineering analysis, and a good understanding of the issues when working with this organic material.
The rings a tree builds each year are made of woven cellulose fibers running in the axial direction of the trunk. As a result, woods are much stronger along the grain. Understanding this directionality is key to engineering, designing, and building well with wood.
When alive, a tree is filled with water; often there’s more water than wood fiber at the time of harvest. This “free water” between the cells is released as the wood dries until it reaches fiber saturation. This is when the free water is gone, but water within the cell walls remains, typically at a moisture content around 30% of the weight of the wood fiber. As wood loses its free water, its geometry doesn’t change. Once it arrives at fiber saturation it begins its journey to its final equilibrium moisture content.
The three axes of a piece of wood are axial (trunk direction), tangential (ring direction), and radial (from the pith out to the bark; perpendicular to the rings). Uneven shrinkage can lead to performance problems in structures when the equilibrium moisture content is several percent different than the installed moisture content, or when the seasonal variation of moisture content is big enough to open air paths through the building envelope. This shrinkage can be accommodated in design details, or wood should be dried to its equilibrium moisture content prior to installation. This later approach often is enabled by using modern engineered wood products.
Modern engineered wood products start with forestry practices where trees are harvested in a responsible, sustainable way. As demand for wood products expands globally, we need to ensure that forests—and the communities that depend on them—are kept healthy. We can do this by using certified wood.
After growing and harvesting sustainable wood, modern mills use saws and evaluation machines to determine the stress grade of fibers in each piece. Then the wood is sorted and put to use in structural members in its optimal location. In engineered wood, the wood is dried to moisture contents from 8% to 15%, which allows the material to be glued together into larger elements having great structural capacities. Zero VOC, formaldehyde free, polyurethane adhesives produce bonds stronger than the natural bond between wood fibers. Modern adhesives allow for stress-rated products like glued-laminated beams and columns or cross-laminated timber panels. These massive timber beams, columns, and panels are the building blocks for the skyscrapers of the future.
We now know that using wood substitutes for other construction materials can save up to 31% of global CO2 emissions. As such, if we’re able, we are morally obligated to use more wood in creating our built environment.
With bio materials in place of fossil fuels, we have the ability to help transform our planet into a healthy environment. Whether it is cellulose insulation, homes, bridges, wooden skyscrapers, or cellulosic jet fuel, our future depends on growing our understanding and sustainable use of the global forest resource.
Chris Carbone is a company steward and the head of engineering at Walpole, N.H.–based Bensonwood.
