Cross-laminated timber (CLT) (a sub-category of mass timber) is a wood panel product made from gluing layers of solid-sawn lumber together. Each layer of boards is usually oriented perpendicular to adjacent layers and glued on the wide faces of each board, usually in a symmetric way so that the outer layers have the same orientation. An odd number of layers is most common, but there are configurations with even numbers as well (which are then arranged to give a symmetric configuration). Regular timber is an anisotropic material, meaning that the physical properties change depending on the direction at which the force is applied. By gluing layers of wood at right angles, the panel is able to achieve better structural rigidity in both directions. It is similar to plywood but with distinctively thicker laminations (or lamellae).
CLT was first developed and used in Germany and Austria in the early 1990s. Austrian-born researcher Gerhard Schickhofer presented his PhD thesis research on CLT in 1994. Austria published the first national CLT guidelines in 2002, based on Schickhofer's extensive research. These national guidelines, "Holzmassivbauweise", are accredited with paving a path for the acceptance of engineered elements in multistory buildings. Gerhard Schickhofer was awarded the 2019 Marcus Wallenberg Prize for their groundbreaking contributions in the field of CLT research.
By the 2000s CLT saw much wider usage in Europe, being used in various building systems such as single-family and multi-story housing. As old growth timber become more difficult to source, CLT and other engineered wood products appeared on the market.
Building code requirements (United States)
In 2015, CLT was incorporated into the National Design Specification for wood construction. This specification was used as a reference for the 2015 International Building Code, in turn allowing CLT to be recognized as a code-compliant construction material. These code changes permitted CLT to be used in the assembly of exterior walls, floors, partition walls and roofs. Also included in the 2015 IBC were char rates for fire protection, connection provisions and fastener requirements specific to CLT. To meet structural performance requirements, the code mandated that structural CLT products met the requirements specified by ANSI/APA PRG 320.
The manufacturing of CLT can be split up into nine steps: Primary lumber selection, lumber grouping, lumber planing, lumber cutting, adhesive application, panel lay-up, assembly pressing, quality control and finally marketing and shipping.:77–91
The primary lumber selection consists of two to three parts, moisture content check, visual grading and sometimes depending on the application structural testing. Depending on the results of this selection, the timber fit for CLT will be used to create either construction grade CLT or appearance grade CLT. Timber that cannot fit into either category may be used for different products such as plywood or glued laminated timber.
The grouping step ensures the timber of various categories are grouped together. For construction grade CLT, the timber that has better structural properties will be used in the interior layers of the CLT panel while the two outermost layers will be of higher aesthetic qualities. For aesthetic grade CLT, all layers will be of higher visual qualities.
The planing step improves the surfaces of the timber. The purpose of this is to improve the performance of the adhesive between layers. Approximately 2.5 mm is trimmed off the top and bottom faces and 3.8 mm is trimmed off the sides to ensure a flat surface. There are some cases in which only the top and bottom faces are treated; this is typically the case if the sides do not have to be adhered to another substance. It is possible that this step may change the overall moisture content of the timber; however, this rarely happens.
The timber is then cut to a certain length depending on the application and specific client needs.
The adhesive is then applied to the timber, typically through a machine. It is important to note that application of the adhesive must be airtight to ensure there are no holes or air gaps in the glue and that the adhesive is applied at a constant rate.
A panel lay-up is performed to stick the individual timber layers together. According to section 8.3.1 of the performance standard ANSI/AP PRG 320, at least 80% of surface area between layers must be bound together.
Assembly pressing fully completes the adhering process. There are two main types of pressing methods, vacuum pressing and hydraulic pressing. In vacuum pressing more than one CLT panel can be pressed at one time making the process more time and energy efficient. Another advantage to vacuum pressing is that it can apply pressure to curved shaped CLT panels because of the way the pressure is distributed around the whole structure. As for hydraulic pressing, advantages include higher pressures can be achieved and also the pressures placed on each edge can be specified.
Quality control is then performed on the CLT panels. Typically a sanding machine is used to create a better surface. The CLT panels are also cut to suit their specific design. Often, if the panels need to be conjoined to form longer structures finger joints are used.
CLT has some advantages as a building material, including:
- Design flexibility – CLT has many applications. It can be used in walls, roofs or ceilings. The thickness of the panels can easily be increased by adding more layers and the length of the panels can be increased by joining panels together.
- Eco-friendly – CLT is a renewable, green and sustainable material, since it is made out of wood and does not require the burning of fossil fuels during production.
- Prefabrication – Floors or walls made from CLT can be fully manufactured before reaching the job site, which decreases lead times and could potentially lower overall construction costs.
- Thermal insulation – Being made out of multiple layers of wood, the thermal insulation of CLT can be high depending on the thickness of the panel.
- CLT is a relatively light building material – Foundations do not need to be as large and the machinery required on-site are smaller than those needed to lift heavier buildings materials. These aspects also provide the additional capacity to erect CLT buildings on sites that might otherwise be incapable of supporting heavier projects, and eases infilling projects where construction is especially tight or difficult to access due to the preexisting buildings around the site.
CLT also has some disadvantages, including:
- Higher production costs – Being a relatively new material, CLT is not produced in many locations. Also, the production of CLT panels requires a considerable amount[clarification needed] of raw materials compared to regular stud walls.
- Limited track record – CLT is a relatively new material, so it has not been used in many building projects. A considerable amount[clarification needed] of technical research has been done on CLT but it takes time to integrate new practices and results into the building industry because of the building industry's path-dependent culture which resists deviating from established practices.
- Acoustic performance – In order to achieve acceptable acoustic performance, more CLT panels must be used. According to the CLT handbook, two CLT panels with mineral in-between achieves the international building requirement for sound insulation in walls.:369
In September 2016 the world's first timber mega-tube structure was built at the Chelsea College of Arts in London, using hardwood CLT panels. The 115 feet (35 m) long "Smile" was designed by architect Alison Brooks and engineered by Arup, in collaboration with the American Hardwood Export Council, for the London Design Festival. The structure is a curved tube in a shape of a smile touching the floor at its centre. It has the maximum capacity of 60 people.
Because of CLT's structural properties, its ability to be prefabricated and how light it is compared to other construction materials, CLT is starting to be used in many high-rise buildings (see: Plyscraper). With its 4,649 cubic metres of CLT provided by UK-based B&K Structures, Dalston Lane at Dalston Square is one of the largest CLT projects globally. The project finished in early 2017. Considering the building was built on a brownfield, it was much taller than was thought to be feasible because of how light CLT is.
In the United States, Framework in Portland, Oregon, planned to utilize CLT for its 12-story structure, to become the tallest timber building in North America. This structure was designed by LEVER Architecture, and may have had the first CLT post-tensioned rocking wall core as the lateral seismic system. This project was cancelled due to funding in 2018.
In Australia, a nine-story all-timber office building was due to be completed in Brisbane in late 2018. Because of CLT’s ability to be prefabricated, construction was finished six weeks earlier than predicted. Due to CLT being lighter than traditional construction materials like concrete and steel, 20% more space was able to be reallocated from structural elements to functional space.
The current tallest CLT structure and the first hybrid structure more than 14 stories tall is UBC’s Brock Commons Residence hall. Completed in September 2017, the building is approximately 53 meters tall with 18 stories and houses approximately 400 students. The architect firm for this building was Acton Ostry Architects, while the structural engineering company was Fast + Epp. Seventeen out of the 18 stories use CLT as the floor panels and glue-laminated timber as the columns, 70% of cladding used in the facade is made from wood. It is estimated that the carbon dioxide emissions were reduced by 2432 tonnes when compared to using concrete and steel. The approximate cost of the building was $51.5 million. This project targeted to achieve LEED gold upon its completion.
CLT is also used in a number of bridge projects. The 160-metre-long Mistissini Bridge is located in Québec, Canada, and crosses Uupaachikus Pass. The designer for this bridge was Stantec and it was completed in 2014. Locally sourced CLT panels and glue-laminated timber girders were used as the main structural members of the bridge. This project won numerous awards including the National Award of Excellence in the Transportation category at the 48th annual Association of Consulting Engineering Companies (ACEC) and also the Engineering a Better Canada Award.
The Maicasagi Bridge is located in the north of Québec and spans 68 meters. The bridge was completed in 2011 and uses CLT and glue-laminated timber in combination to create two box girders. This combination of timber was chosen because of the ability to be prefabricated allowing for a short lead time compared to a traditional steel bridge.
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