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CLT-plate with three layers made from spruce

Cross-laminated timber (CLT) is a wood panel product made from solid-sawn lumber. Each layer of boards is orientated 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). The cross gluing allows using the panels with two span directions whereby some differences in stiffness and strength are observed along the two directions. It is similar to plywood but with distinctively thicker laminations.

CLT is distinct to glued laminated timber (GLT), a product with all laminations orientated in the same way.[1]

Contents

HistoryEdit

CLT was first developed and used in Germany and Austria in the early 1990s but it was only after the mid 1990s more extensive research was completed. By the 2000s CLT saw much wider usage in Europe, being used in various building systems such as single-family and multi-story housing. The main reasons behind the rise of CLT were because of its sustainability and lack detrimental effects to the environment, but also improved marketing and availability.[2]

Building Code RequirementsEdit

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 recognised as a code compliant construction material. These code changes permitted CLT to be used in the construction of exterior walls, floors, partition walls and roofs. Also included in IBC 2015 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.[3]

ManufacturingEdit

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.[2]:9-23

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 planning step improves the surfaces of the timber. The purpose of this is to improve the performance of the adhesive between layers. Approximately 2.5mm is trimmed off the top and bottom faces and 3.8mm is trimmed off the sides to ensure a flat surface[4].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 is 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.[5]

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.

UseEdit

PavilionsEdit

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. [6]

High Rise BuildingsEdit

Because of CLT's structural properties, the 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. With its 3,852 cubic metres of CLT, Dalston Lane at Dalston Square is one of the largest CLT projects globally. The project finished in early 2017. Ramboll carried out full structural engineering and the design of the 33m CLT structure. B&K Structures, a specialist hybrid frame contractor, were the structural frame and CLT provider. Considering the building was built on a brownfield, it was much taller than was thought to be feasible because of how light CLT is as a construction material. [7][8]

In the United States, Framework in Portland, Oregon will utilize CLT for its 12-storey structure to become the tallest timber building in North America.[9]This structure will have the first CLT post- tensioned rocking wall core as the lateral seismic system. The construction for this project is expected to start early 2018.[10]

In Australia, a nine-storey all-timber office building is 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, 20% more space was able to be reallocated from structural elements to functional space.[11]

The current tallest CLT structure and the first hybrid structure more than 14 storeys tall is UBC’s Brock Commons Residence hall. Completed in September 2017, the building is approximately 53 meters tall with 18 storeys and houses approximately 400 students. The architect firm for this building was Acton Ostry Architects, while the structural engineering company was Fast + Epp. 17 out of the 18 storeys use CLT as the floor panels and glue laminated timber as the columns, 70% of cladding used in the façade 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 its completion. [12]

See AlsoEdit

ReferencesEdit

  1. ^ "Glulam/CLT Structural Timber Association". Retrieved 2 January 2017. 
  2. ^ a b Karacabeyli, Erol; Douglas, Brad; Forest Products Laboratory (U.S.); FPInnovations (Institute); Binational Softwood Lumber Council (2013). CLT handbook: cross-laminated timber. Pointe-Claire, Québec: FPInnovations. ISBN 9780864885548. 
  3. ^ "What the 2015 International Building Code means for wood construction: Part I - Construction SpecifierConstruction Specifier". www.constructionspecifier.com. Retrieved 2017-11-19. 
  4. ^ Julien, F. (2010) Manufacturing cross-laminated timber (CLT): Technological and economic analysis, report to Quebec Wood Export Bureau. 201001259-3257AAM. Quebec, QC: FPInnovations
  5. ^ Brandner, Reinhard. (2013). Production and Technology of Cross Laminated Timber (CLT): A state-of-the-art Report. Graz.
  6. ^ Himelfarb, Ellen (29 July 2016). "The Smile by Alison Brooks Architects Gives CLT a Boost". Architect Magazine. American Institute of Architects. Retrieved 2 October 2016. 
  7. ^ "Dalston Lane". Retrieved 27 February 2017. 
  8. ^ "World's biggest CLT structure relies on collaboration". Retrieved 20 October 2017. 
  9. ^ "Tallest all-wood building in the U.S. approved for construction". Digital Trends. 2017-06-07. Retrieved 2017-11-24. 
  10. ^ "Framework Portland - 12 Story CLT Timber Building". StructureCraft Builders. Retrieved 2017-11-24. 
  11. ^ http://www.abc.net.au/news/2017-06-22/all-timber-office-building-to-be-built-brisbane/8642424
  12. ^ "Structure of UBC's tall wood building now complete". UBC News. 2016-09-15. Retrieved 2017-11-24. 

External linksEdit