Cross Laminated Timber: What Can it Do for Our Communities and Environment?
Rendering Courtesy of VIA
The construction industry is responsible for approximately 40% of the world’s carbon emissions. Of this 40% the fabrication, transportation, and assembly of concrete and steel are major contributing factors. Climate policy, climate action, and climate awareness has us all asking ‘what can we do’ to combat these emissions and support our collective future? Enter: Mass Timber.
Mass Timber is known industry-wide as a catch-all category for environmentally sourced Engineered Wood Products (EWPs) manufactured by bonding multiple layers of wood together. The strength of traditional lumber depends on the direction of it’s grain in relation to an applied load. Mass timber products are engineered to maximize the wood’s inherent strength for specific load applications.
CLT: A Mass Timber Product
Mass Timber products are widely accepted as sustainable building materials. This is due in part to the fact that wood is a renewable resource, and that forests and wood-made-products are considered carbon-sinks that capture and remove carbon from the atmosphere. Beyond their comparably low carbon footprint, EWPs are durable, versatile, and are designed to reach strength ratings comparable to non-wood materials (e.g. concrete and steel) while simultaneously maintaining lighter self-weights. Common varieties of mass timber include Glulam (Glued-Laminated Timber or GLT), Nail Laminated Timber (NLT), Dowel Laminated Timber (DLT), and Cross Laminated Timber (CLT).
CLT is an engineered wood product that is pre-manufactured by gluing together layers (a.k.a. plies) of 2x lumber with alternating grain orientations and sealing the lot together with a press. Typically, the CLT panel’s two outer plies are oriented to span between primary supporting elements (whether vertical or horizontal), and inner plies are oriented perpendicular (and parallel) to outer plies – depending on how many plies each CLT panel is. (Note that odd-number-ply panels are most common). The more plies, the stronger the panel, and the further it can span.
This unique alternating grain/ply-pattern enables the CLT wood panel to achieve multi-directional strength ratings (unlike traditional lumber with just one). The effect is that CLT panels act like elevated, reinforced concrete slabs which explains why these panels are frequently used as structural walls, floors, and roofs. So, what’s the catch? CLT is both sustainable and strong, but is it a headache to construct? Nope! In fact, because CLT is pre-manufactured, it minimizes on-site construction waste.
Vermont’s First Ever Commercial Building to Utilize CLT
An example of a mass timber success story is one of Vermont’s first-ever commercial buildings that utilizes CLT – the Tang Science Annex at the Fairbanks Museum in St. Johnsbury – designed and detailed by our very own structural team. Here, by using CLT panels for the floors and glulam for the beams and columns, in lieu of more traditional materials like concrete/metal deck floors, steel beams and columns – the new building’s structural system material carbon footprint is reduced by approximately one third.
A structure’s carbon footprint is measured by assessing the environmental impact of manufacturing the materials used in a building’s construction (Environmental Product Declarations – i.e. EPDs) as well as by considering the life-span and performance of these materials over time (Life Cycle Assessment – i.e. LCAs). This helps to answer questions like: ‘How much carbon was emitted while manufacturing this product and transporting it to site?’ and ‘How long does this product last and how much carbon does it remove from or emit into the atmosphere during its lifespan? ’ These measurements enable stakeholders to evaluate and compare the impact that certain products have on both environmental and human health and to make informed decisions that align with their own climate agendas.
Choosing the Right Species
Most EWP and CLT manufacturers have their own pre-selected species with pre-determined EPDs. The CLT panels used at the Tang Science Annex at the Fairbanks Museum utilized CLT made from local species (Eastern Hemlock) that was recently tested at UMass Amherst with support from the Massachusetts Executive Office of Energy and Environmental Affairs. This process allowed panels made from locally sourced lumber to be certified through the American National Standards Institute (ANSI) and the American Plywood Association (APA) to meet the industry standard PRG 320 performance requirements. (2).
The feat of this certification is twofold. Not only does it (theoretically) reduce carbon emissions by locally sourcing lumber (that doesn’t have to travel as far to get to site), but it also appropriates low-grade wood that is generally overlooked by the lumber industry for structural applications. The result creates a market for such material and allows room for other species to regrow thereby promoting our forests’ health. (3).
Engineered Wood Products are the rising stars of the sustainability-minded Structural, Architectural, and Construction industries. But despite their potential, various obstacles to their successes persist. The shortage of Mass Timber manufacturers located here in the Northeast requires EWPs to travel further distances from the source to the site – thereby increasing its carbon footprint. Added travel distances increase shipping and other logistical costs which disproportionately affects smaller projects and can make their use uneconomical.
Nonetheless, designing and constructing with CLT and other EWPs is another tool in our climate-change-fighting-toolbox that has great potential to positively impact our collective climate future – on both local and universal levels. We at Engineering Ventures remain hopeful that projects like the Tang Science Annex at the Fairbanks Museum will continue to set an example – and a new standard – for sustainable, economical, and innovative design practices here in the Northeast.
World Green Building Council. Bringing embodied carbon upfront. Coordinated action for the building and construction sector to tackle embodied carbon; World Green Building Council: London, UK, 2019. https://search.informit.org/doi/epdf/10.3316/informit.985354490427057
Kaboli, H; Clouston, P; Lawrence, S; Feasibility of Two Northeastern Species in Three-Layer ANSI-Approved Cross-Laminated Timber; ASCE, 2020.
Hellman, R. The Fairbanks Museum Tests Innovative Wood product in its new addition. Seven Days. https://www.sevendaysvt.com/vermont/the-fairbanks-museum-tests-innovative-wood-product-in-its-new-addition/Content?oid=36008211 (2022, July 28).