Barriers to the design and use of cross-laminated timber structures in high-rise multi-family housing in the United States

Wood structures not only have a significantly lower embodied energy and associated carbon emissions than equivalent steel and concrete structural systems, wood is a carbon sink, removing carbon dioxide from the atmosphere and storing it in building components. The reduction of carbon dioxide is critical to addressing related energy and climate issues. Unfortunately, the structural properties and life-safety concerns have limited the use of wood to the structural systems to low-rise buildings in the United States. This paper uses the hypothetical design of a high-rise multi-family housing building using Cross-Laminated Timber (CLT) to address these barriers and highlight the potential of CLT for use in the United States. CLT adapts itself more naturally to multi-family housing than other building typologies due to the solid panel nature and limitation in spans. It has been chosen to use a high-rise design since there are stricter building codes and to understand if professionals feel CLT is capable to reach high-rise limits safely. Since post-tensioned concrete is the typical structural system currently for high-rise multi-family housing, CLT will be compared to this system. CLT is an engineered wood product consisting of glue laminated wood boards, approximately 20-60mm in thickness, with each layer set at right angles to the next layer. This cross lamination creates panels, ranging from a 3-layer 57 mm (2.24 in) panel to as thick as an 11-layer 300 mm (11.8 in) panel, capable of spanning in two directions and being used for load-bearing walls and spans. CLT was first developed in the early 1990s in Austria and Germany and has been gaining popularity in residential and non-residential applications, mainly in Europe. Currently, panels are being manufactured in a limited number of places in North America, which allows CLT to be used in a few projects while trade organizations and governmental agencies adopt specifications and codes for its use. While CLT is being used in Europe as the structural system for eight-story buildings and proposals up to seventeen stories, the barriers to the adoption of CLT for high-rise construction in the United States needs to be exposed and understood. Through the use of semi-structured interviews and surveys of design, engineering and construction professionals, this paper analyzes these barriers that include systems integration (fire-safety, acoustics, plumbing, electrical), aesthetics and information gaps. SFI like FSC has different standards and certifications depending on the stage of the manufacturing process. There are three different types of certifications under SFI which include the forest, chain-of-custody, and sourcing while each area requires a third-party audit to achieve certification. The three different types of certification have their own criteria needed to be met in order to use the SFI stamp on their products. Though both programs are aiming for the end result of responsible forest management, FSC is the only type of wood to be recognized in LEED ratings. The programs are trying to ensure forests continue to contribute to their additional environmental benefits. Trees do not require the use of a fossil fuel for their creation because they use the sun’s energy to convert carbon dioxide to oxygen while storing the carbon. The fossil fuel use takes affect when the trees are harvested to be used as wood products. Companies have been using the bio-waste from the harvesting process as the primary energy source for their operation. After a tree has been harvested, 50% of the weight of the wood is carbon (Woodworks 2012). Wood is a “carbon sink” because it stores carbon until the point it begins to decompose meaning the more structures built out of wood, the more carbon is stored which in turn reduces the greenhouse gases in the atmosphere. With the depletion of non-renewable sources of energy, and global warming, the investigation of conserving these resources has become more pertinent. Engineered wood products are being looked at more seriously as a sustainable alternative to common structural systems. Engineered wood products are able to be built taller and denser than previously thought. The restrictions of the building code and the perceived fire potential have been restricting the use of wood as a primary structural material. Wood has been used by humas for many millennia whether it has been used for a building material or to create a useful item. Among the oldest structures are the ancient Chinese timber frames dating back to approximately 7000 years ago (Liu 2002). Newer building materials emerged becoming stronger and more durable. The perceived fire danger of wood structures increased after a series of large fires that wiped out large portions of cities. The most well-known being the great Chicago fire of 1871 and the fire resulting from the 1906 San Francisco earthquake. These events have led to stricter building codes limiting the use of wood as a structural material. With new innovations in wood technology and safety testing, wood could be the primary structural material in larger and taller buildings than permitted by code today. 1.2 Cross-laminated timber Cross-Laminated Timber (CLT) is an engineered wood product composed through laminating smaller pieces, approximately 20-60 mm (.75-2.5 in) in thickness, of wood together alternating the direction of the grain between layers in order for the panel to resist loading in both directions. CLT was first developed in the early 1990s in Austria and Germany. Recently it has been gaining popularity in residential and non-residential applications, mainly in Europe (Gagnon & Pirvu 2011). The panels currently on the market are being manufactured in a limited number of places in North America and Europe that allows CLT to be used in a few projects while experts are examining its performance. The project type which CLT has been used in is mainly multi-family housing including towers up to nine stories tall (Gagnon & Pirvu 2011). Using CLT as the floor system being supported by glue laminated columns has also been used to construct parking garages (Crespell & Gagnon 2010). Since CLT is an engineered wood product it is restricted by building codes such as the International Building Code (IBC) that limits the height and square footage it can be used in, however there are proposals for using CLT to create high rise buildings up to seventeen stories. These will be held back until the building codes are adapted to allow for the use of CLT in larger structures. In order to change the building codes, CLT will need pass safety tests including fire resistant assemblies and seismic performance. These tests will need to show the material is capable and performing to a high standard of ratings. 1.3 Strengths and limitation of cross-laminated timber CLT has multiple strengths including the ability to become its own seismic support/bracing, its ability to self-protect against fire, its lessened environmental impact, its renewable material source, but there are certain limitations to it as well. One of the primary misconceptions is since wood burns, unlike steel and concrete, it may not hold up as well in the event of a fire even though it is known steel bend in a fire and thus required additional fireproofing. Heavy timber assemblies or solid wood designs are able to reach the necessary fire resistance ratings for buildings types made out of non-combustible materials. The char rate of CLT has been tested at .67 mm per min (.02 in per min) (Gagnon & Pirvu 2011). Over the course of being exposed to fire for two hours the CLT panels will have experienced a loss of 80 mm (2.4 in) in panel thickness. If CLT is to be left exposed, adding an extra layer or two to the panel could result in having the equivalent to a two-hour fire rating. CLT is capable of having fire protection built into the aesthetic qualities of the panel. Seismic performance is extremely important in regions of high seismic activity. Traditional structural systems need additional members and connections to integrate seismic design. Seismic design with CLT is completely controlled by the connections of the panels. Large scale tests have been performed by IVALSA (Trees and Timber Research Institute of Italy) on a seven story CLT structure in Japan (Crespell & Gagnon 2010). The structure was exposed to record earthquake simulations including the devastating Kobe earthquake (magnitude of 7.2 and accelerations of 0.8 to 1.2 g) with the result of moderate damage. The extent of the damage was found to be located around the connections. A few connections had failed, but overall the structure withstood the simulations and needed minor repairs for re-occupancy. The CLT structure showed ductile behavior and good energy dissipation mainly influenced by the mechanical connections used. CLT requires fewer materials for seismic design compared to traditional structural systems and performed well during seismic testing. CLT acts similar to older load-bearing masonry structures that start wider at the base and when loads lessen begins to taper to a thinner wall at the top. Each panel thickness is capable of supporting up to five stories of structure. Keeping load bearing masonry design in mind, the thickest panel will be at the bottom of the overall assembly and every five stories gained in height, the panel thickness will reduce by one size. For a twenty-story tower, the panel size at the bottom will be a 9-layer panel, at floor six it would change to a 7-layer panel, at floor 11 it would change to a 5-layer panel and finally at floor 16 it would change to a 3-layer panel for the remainder of the floors. Even though columns tend to mimic this idea, they do not take up nearly as much floor space, nor do they begin to divide a building by having repeated structural walls. CLT’s repetitive nature and modular pieces do make for a shorten construction time and require less labor. Since there are two trades required to assemble a CLT system versus approximately 12 for a post-tensioned concrete system, it leads to less confusion on site and a speedier assembly, which in some cases shorten the construction schedule by 50% (Crespell & Gagnon 2010). The installation of a single panel, measuring up to an approximate size of ten feet wide by sixty feet long, can be as little as 15-20 minutes. Murray Grove, a CLT housing development in London, was constructed by four carpenters in 27 days averaging 3 days per floor. The distance CLT is able to span is one of its primary limitations. Using a five layer floor panel approximately 150 mm (6 in) thick has the capability of spanning 5.5 m (18 ft), while jumping up to the seven layer floor panel which is approximately 235 mm (9.25 in) thick has the capability of spanning 8.5 m (28 ft). CLT being a solid panel system and the limited spans lends this structural system towards multi-family housing rather than office or warehouse space. Wood transfers sound easily which leads to poor acoustical performance and another limitation of CLT. The acoustical performance needs to be increased through the addition of acoustical insulation, sleeper studs, and a wall covering. This increases the overall system thickness which leads to an assembly which is approximately 150 mm (6 in) thicker than concrete for an 8.5 m (28 ft) span.