Wednesday, April 27th, 2016

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The Possibilities of Mass Timber Use

by Michael Poulos

The idea of tall wood buildings has captured the imagination of architects and engineers for some time now.  With a heightened awareness of the environmental impacts of construction and advances in mass timber, these ideas are becoming a reality.

The environmental impacts of mass timber buildings are one of the driving forces behind the push for large scale wood buildings.  Wood is a renewable resource and due to the embodied carbon content, wood has a negative carbon footprint.   Other traditional building materials for large scale structures, such as steel and concrete, have a large carbon footprint due to the amount of energy required to produce the materials.  Cross laminated timber (CLT) has the ability to be made from beetle kill pine.  Beetle kill pine is an issue affecting many areas in both the United States and Canada.  The ability to use the material that was previously wasted to make a high strength construction material will have a huge impact on these infested forest regions.

Along with the environmental advantages, there are also economic advantages to mass timber structures.  Conventional wood construction is typically the most cost effective solution, and this also holds true for mass timber.  Mass timber can often be locally sourced reducing transportation cost.  The increase in wood construction also creates jobs in rural communities.

Considering the environmental and economic benefits of mass timber structures, wood is the natural choice for the building material for your next large scale or tall structure.  However, there are still several technical and code related challenges associated with building structures out of wood. A common concern is how a large scale wood building would withstand fire.  In typical light framed wood construction, fire rating is often achieved by covering the material in non-combustible material, such as gypsum board.  However, the design aesthetic for heavy timber is often to have the wood exposed, making standard fire ratings impossible to achieve.  Large wood members, such as Glu-laminated beams and columns, can resist fire thru charring.  During a fire event, the exterior layer of wood chars creating a protective layer, and insulates the core of the member.  Current codes allow for the charring affect as the fire rating, however there are many limitations.  Additional testing of wood to wood connections and heavy timber floor assemblies (CLT, NLT, etc.) are required and are currently being conducted.  Also revisions to the current building codes that include current mass timber construction standards are required to facilitate the design of large scale and tall wood buildings.

In addition to the required code advancements relating to fire, additional code information is required for the seismic design of large scale and tall wood structures.  Many current codes, such as the International Building code, set height restrictions on wood lateral force resisting systems.  Current wood lateral force resisting systems (plywood shear walls) are limited to 65ft.  The latest testing includes various forms of CLT shear panels, including conventional panels, post-tensioned panels and panels with damping devices.  Other countries allow and implement other forms of wood lateral force resisting systems.  In Europe, such systems include wood braced frames and moment frames. Current research in Europe includes wood buckling restrained braced frames as an option for tall wood structures.  Options for lateral force resisting systems in taller buildings increase these systems and are tested and codified, making taller wood structures a possibility.

With the increased awareness of environmental impacts of construction and a renewed interested in the mass timber design aesthetic, the demand for large scale and tall wood structures is growing.  The current advancements in heavy timber technology and the current research being conducted enable the goal of tall sustainable wood buildings to soon be a reality.

 

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