I was recently introduced to a mass timber construction material called Cross Laminated Timber or CLT. This system basically takes dimensional lumber (2x4, 2x6, 2x12, etc), and glues them together, similar to how a glulam beam is made of stacked boards that are glued and laminated together. CLT is kind of like a horizontal glulam beam, but the primary feature that makes it interesting is that each layer of dimensional lumber is staggered and rotated perpendicular to the layer above/below it. This staggering and rotation creates a much stronger, more stable product (meaning it is less likely to warp, crack, fail, etc.). The following illustration attempts to describe this difference:
You've seen glulam beams all over the place. Often times they are used as horzontal beams, spanning between columns and holding up roofs, but you often see them as columns as well. One of my favorite examples of glulams is at the Credit Valley Hospital in Ontario, Canada, where they used glulam beams in a very sculptural way to make the structural elements resemble trees.
Credit Valley Hospital Lobby in Ontario, Canada by Farrow Partnership Architects
While CLT can be used in similar ways, it is often used as a flat panels that can span longer distances between beams which reduces the number of columns/beams required, thus opening more floor area to programmable space.
According to WoodWorks.org's article "Solid Advantages", CLT panel sizes vary by manufacturer but are available in panels up to 19.5 in thick x 18 ft wide x 98 feet long. Complex geometries can be cut into the panels with large CNC machines so the penetrations (such as window and door openings) can take on a whole new dimension. CLT panels weigh less than steel or concrete so foundation sizes and costs can be reduced. Because these panels are manufactured in a controlled facility (rather than on the project site), the process maintains a high level of quality control, minimizes waste, and maximizes efficiency, which, in turn, makes the product much more environmentally friendly and sustainable than standard steel and concrete practices. Also, because of it's consistent stability, engineers can trust that it will perform as calculated.
This material is surprisingly versatile because it can be used in just about any structural and finish application in a building, but when necessary, it can be further manipulated in the field with simple tools.
How much does CLT cost? A 2010 study by FPInnovations, compared CLT cost with concrete, masonry and steel building types. The study did not take into account the quicker construction or reduced foundation costs, however it was estimated that cost of a U.S.-built CLT structure was particularly competitive for mid-rise residential (15 percent less), mid-rise non-residential (15 to 50 percent less), low-rise educational (15 to 50 percent less), low-rise commercial (25 percent less), and one-story industrial buildings (10 percent less). (SOURCE: Cross Laminated Timber: A Primer, Crespell, P., S. Gagnon, FPInnovations, 2010)
Wood framed construction has historically been limited to about 5 stories of building height, but CLT buildings have potential to reach much higher. Also with traditional wood framed construction, openings (windows, doors, etc) can only be so large or they have to be placed certain distances apart to comply with code, but CLT frees up designers from many of those constraints due to it's inherent rigidity and strength. This material has potential to be very versatile and can help a wood building be so much more than they have been in the past, allowing longer spans, larger openings, and much more interesting designs.
Here are some exciting uses of CLT:
Metropol Parasol in Seville, Spain by Jürgen Mayer