Cross-laminated timber (CLT) (a sub-category of
engineered wood
Engineered wood, also called mass timber, composite wood, man-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of woo ...
) is a wood panel product made from gluing together at least three layers
of solid-sawn lumber, i.e., lumber cut from a single log. 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
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
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
Plywood is a material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured ...
but with distinctively thicker
lamination
Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materials ...
s (or lamellae).
CLT is distinct from
glued laminated timber
Glued laminated timber, commonly referred to as glulam, is a type of structural engineered wood product constituted by layers of dimensioned lumber, dimensional lumber bonded together with durable, moisture-resistant structural adhesives so that ...
(known as glulam), which is a product with all laminations orientated in the same way.
History
The first patent resembling CLT was first developed in the 1920s by Frank J. Walsh and Robert L. Watts in Tacoma, Washington. Many sources however, date the first patent back to 1985 when it was patented in France.
Significant developments were then made in Austria when Gerhard Schickhofer presented his PhD thesis research on CLT in 1994. Utilizing the theories he developed during his research, Schickhofer began working with three small sawmills and the Sawmillers Association, to start production of CLT. With help from some government funding, they were able to hand build a test CLT press and create the first few panels. At the same time, the first press system, utilizing water-based pressure, came on the market enabling Schickhofer and his team to think beyond the capabilities originally thought possible for CLT. After years of extensive research, Schickofer submitted the results to the Austrian and EU government bodies that dealt with the approval of materials for commercial products and in December 1998 it was approved. A period of substantial growth in production and projects soon followed in Germany and other European countries as a push for green buildings became more prominent. CLT was slow to take off in North America, but it has begun to gain momentum in more recent years.
Building codes
In 2002, Austria used Schickhofer's research to create the first national CLT guidelines. The International European Technical Assessments (ETA) began to regulate the properties and design of CLT in 2006. Efforts to standardize CLT in Europe started in 2008 and by 2015 the first European product standard for CLT, EN 16351, was approved. Also in 2015, CLT was incorporated into the
International Building Code
The International Building Code (IBC) is a model building code developed by the International Code Council (ICC). It has been adopted for use as a base code standard by most jurisdictions in the United States. The IBC addresses both health and saf ...
in accordance with ANSI/APA PRG 320 and the
National Fire Protection Association
The National Fire Protection Association (NFPA) is an international nonprofit organization devoted to eliminating death, injury, property and economic loss due to fire, electrical and related hazards. As of 2018, the NFPA claims to have 50,000 mem ...
(NFPA) began to research and develop codes regarding the fire safety of CLT and other engineered woods. The 2021 revision of the IBC included three new construction types for mass timber buildings, Type IV-A, Type IV-B and Type IV-C. These new types enabled buildings using mass timber to be built taller and over greater areas than before.
Manufacturing
The manufacturing of CLT is generally divided into nine steps: primary lumber selection, lumber grouping, lumber planing, lumber cutting, adhesive application, panel lay-up, assembly pressing, quality control and marking and shipping.
During primary lumber selection, lumber will undergo a moisture content (MC) check and visual grading. Depending on the application, structural testing (E-rating) may also be completed. The moisture content check is conducted because the lumber that is typically used, can arrive with a MC of 19% or less, but lumber for CLT needs to have a MC of approximately 12% during manufacturing to avoid internal stress due to shrinkage. This test is also done so that adjacent pieces of lumber do not have an MC difference greater than 5%. In order to conduct an MC check, various hand-held or on the line devices can be used. Some are more accurate than others, as they check moisture content within the wood not just at surface level. Further research and development is ongoing to improve the accuracy of such devices. Temperature in the manufacturing facility is also checked and maintained throughout this process to ensure the quality of the lumber. Visual grading is performed so that any warping in the lumber is prevented from affecting the pressure the bondline can withstand. It also ensures that waning, defects in the wood due to bark or missing wood due to the curvature of the log, does not significantly reduce the available bonding surface. For a product to be considered an E-class CLT, visual grading must be considered for perpendicular layers while parallel layers must be determined by the E-rating (the average stiffness of a piece of lumber). Products are classified as V-class if visual grading is used for both perpendicular and parallel layers.
Using the results from lumber selection, the grouping step ensures the timber of various categories are grouped together. Lumber used for the major and minor strength directions are grouped primarily relying on MC and visual grading. Within the major strength direction, all lumber is required to have the same engineering properties so that panel limitations can be determined. Likewise, all lumber for the minor direction must have a single set of properties. Higher quality lumber can also be grouped so that it is reserved for areas in which fasteners are installed to maximize fastener effectiveness. For aesthetic purposes, some lumber will be set aside so that the outermost layer of a panel is visually appealing.
Timber that does not fit into either category may be used for different products, such as plywood or glued laminated timber.
The planing step improves the surfaces of the timber to reduce oxidation which increases the effectiveness of the adhesives. 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. In some cases, when the lumber edges are not glued because they have an acceptable width tolerance, only the top and bottom faces are planed. It is possible that planing may increase the overall moisture content of the timber due to the drying variations throughout the wood. When this occurs, bonding suitability should be assessed and reconditioning may be necessary.
The timber is then cut to a certain length depending on the application and specific client needs. Cut-offs from the longitudinal layers may be used to create the transverse layers if the same specifications are needed for both the parallel and perpendicular layers.
Application of the adhesive occurs shortly after planing to avoid any issues affecting the surface of the lumber. Applying the adhesive is most often done in one of two ways: a through-feed process or side-by-side nozzles. In the through-feed process extruder heads distribute parallel threads of adhesive along the piece of lumber in an airtight system to avoid air gaps in the glue that could affect bonding strength. This is typically used for phenol
resorcinol formaldehyde (PRF) or polyurethane-reactive (PUR) adhesives. For PUR adhesives, the layers of lumber may be misted to help with curing. The side-by-side nozzle option is commonly reserved for CLT layers that are formed in advance and works by installing the nozzles along a beam that will travel along the length of the lumber and apply the adhesive. To avoid additional manufacturing costs, adhesive is typically only applied to the top and bottom faces of the lumber, but edge-gluing can be done if necessary.
Panel lay-up is performed next and involves laying the individual pieces of lumber together in preparation for assembly pressing. In accordance with ANSI/APA PRG 320, at least 80% of the surface area between layers must be bound together so that the bond is effective. In order to meet this standard, manufacturers are tasked with finding the most efficient way of laying the lumber. This time between spreading the adhesive and applying pressure is known as assembly time and must fall within the time targeted by the specific adhesive used.
Assembly pressing completes the adhering process with either a vacuum press or a hydraulic press. Vacuum pressing generates a pressure of approximately , which is not always enough to address the warping potential or surface irregularities. To accommodate this, lumber shrinkage reliefs can be cut in the longitudinal direction. These reliefs release the stress in the lumber and decrease the risk of cracking due to drying. They must however, have a maximum width and depth so that the bonding area and panel strength are not impacted significantly. Using a vacuum press may be more beneficial in some circumstances, because they can press more than one CLT panel at once and can be used for curved elements. A hydraulic press on the other hand, generates a greater pressure, ranging from , and applies it to specific faces of the panel. For this reason, panels may need to undergo both vertical and side clamping pressing.
Once assembly pressing is complete, the CLT panels undergo quality control machining. Sanders are used to sand each panel down to the desired thickness with a tolerance of 1 mm, or less if specified by a project. The CLT panels are then moved to a multi-axis numerically controlled machine that makes precision cuts for doors, windows, splices, and connections. Any minor repairs that are necessary at this stage are completed manually.
To meet the requirements of ANSI/APA PRG 320 and ensure that the correct product has been specified, delivered, and installed, CLT panels must be marked to identify a variety of information. This includes the grade, thickness, mill name, agency name or logo, ANSI/APA PRG 320 symbol, manufacturer designations, and a top stamp if it is a custom panel. These markings must be stamped at intervals of or less so that when longer pieces are delivered to site and cut, they still display the necessary information. Further markings may be added to demonstrate the main direction of loading and the zones designed to receive connections. During shipping and construction, the CLT panels must be protected from the weather to maintain their structural integrity.
Advantages
As a building material, CLT has numerous advantages:
* ''Design flexibility'' – CLT has many applications in construction as it can be used for walls, floors, and roofs. The size of the panels is also easily varied as it is only limited by the site storage and transportation to the site.
* ''Environmentally conscious'' – CLT is a renewable, green, and sustainable material, if the trees used to make it are sourced from efficiently managed forests.
* ''Carbon capture'' – Because CLT is made from wood, it sequesters carbon. Different factors will affect how much carbon is sequestered, but numerous studies have shown that the use of CLT (in combination with other
engineered wood
Engineered wood, also called mass timber, composite wood, man-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of woo ...
) in construction could significantly reduce our carbon emission output.
* ''Prefabrication'' – CLT panels are fully fabricated before they are transported to site. This enables quicker construction times compared to other materials which can lead to shorter schedules and its subsequent benefits such as cost savings, less risk for accidents, and reduced disruption to the neighborhood.
[Lehmann, Steffen (2012-10-18). "Sustainable Construction for Urban Infill Development Using Engineered Massive Wood Panel Systems". Sustainability. 4 (10): 2707–2742. doi:10.3390/su4102707.]
* ''Thermal insulation'' – CLT panels provide air tightness and great thermal insulation to buildings as the thermal conductivity (U) of a panel is approximately 0.3458 W/m
2K. Other common building materials can have U values ranging from 0.4 to 2.5 W/m
2K. The various layers of wood also serve as a
thermal mass
In building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...
, which can help reduce a building's energy use.
* ''Light weight'' – CLT is significantly lighter than traditional building materials so foundations can be designed to support a smaller load and therefore use less material. The machinery required on-site to move and place the CLT panels are also smaller than those needed to lift heavier building materials.
These aspects enable contractors to erect CLT buildings on sites that might otherwise be incapable of supporting heavier projects. This can help ease infilling projects where construction is especially tight or difficult to access due to other preexisting structures around the site.
* ''Strength and stiffness'' – CLT products have relatively high in and out of plane strength and stiffness due to the perpendicular layers. This reinforcement is comparable to a reinforced concrete slab and increases the panel's resistance to splitting. CLT has also been shown to perform well under seismic forces.
* ''Fire safety'' – Wood is inherently flammable which leads to the D class
fire rating
A fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail other criteria, involving evide ...
CLT receives. Despite this, CLT ranks highly for its ability to withstand a fire once started. It is classified as REI 90, indicating that it can retain the necessary load bearing capacity and meet integrity requirements for 90 minutes during a fire. This leads to a better overall fire safety performance than unprotected steel, which loses its load bearing capacity after it is exposed to a fire for only 15 minutes.
Challenges
There are also some drawbacks associated with CLT:
* ''Costs'' – As CLT is a newer material for North America, it is only produced in a few regions, generally the Pacific Northwest. Transporting CLT panels across potentially vast distances will incur additional upfront costs. Some sources also indicate that the production costs of CLT are greater than other commonly used building materials due to the newness of the system and lack of current demand.
* ''Limited track record'' – CLT is a new venture for many in North America which can limit the number of engineers and contractors willing to take on a CLT project due to their lack of knowledge and experience with the material. The building codes for mass timber projects are also not as developed as those for concrete and steel which can again make developers hesitant to employ CLT. A considerable amount of technical research has been done on CLT, but it takes time to integrate new practices and results into the building industry because of its path-dependent culture which resists deviating from established practices, especially when the research conducted on CLT has not necessarily reached those who have the opportunity to implement it.
* ''Acoustics'' – CLT alone does not meet the necessary sound insulation ratings. In order to meet these requirements, additional elements, such as decoupled
gypsum board
Drywall (also called plasterboard, dry lining, wallboard, sheet rock, gypsum board, buster board, custard board, and gypsum panel) is a panel made of calcium sulfate dihydrate (gypsum), with or without additives, typically extruded between thic ...
, must be used in conjunction with the CLT panels. Changing the effective mass area of the CLT panel by increasing thickness or adding a second panel (creating an air gap) also improved the sound insulation to meet the standards.
* ''Vibrations'' – Current standardized methods for testing the vibration performance of floors are not applicable to CLT floors due to their lightweight nature and natural frequency. Looking at deflection under a uniformly distributed load, we can get some idea of the vibration performance of a CLT floor, but it is heavily reliant on a designer's judgement and neglects the influence of mass characteristics. In order to fully test the vibration performance of a CLT floor, a new testing method will need to be developed.
Applications
CLT is used in a number of various different structures around the world.
Pavilions
In September 2016 the world's first timber mega-tube structure was built at the
Chelsea College of Arts
Chelsea College of Arts is a constituent college of the University of the Arts London based in London, United Kingdom, and is a leading British art and design institution with an international reputation.
It offers further and higher educat ...
in London, using hardwood CLT panels. The "Smile" was designed by architect
Alison Brooks
Alison Brooks, (born 29 December 1962) is a Canadian-British architect. She is the founder and creative director of Alison Brooks Architects, based in London. Her awards include the RIBA Stirling Prize, Manser Medal, Stephen Lawrence Prize, an ...
and engineered by
Arup, in collaboration with the
American Hardwood Export Council, for the
London Design Festival
London Design Festival is a citywide design event that takes place over nine days every September across London. It was conceived by Sir John Sorrell and Ben Evans CBE in 2003 and celebrated its 20th edition in September 2022.
In an article by ...
. The structure is a curved tube in a shape of a smile touching the floor at its centre and has a maximum capacity of 60 people.
Plyscrapers
Stadthaus
Stadthaus is a nine-storey residential building in Hackney, London. At nine stories (30 meters/98 feet), it is thought to be the second tallest timber residential structure in the world, after the Forte apartment complex in Melbourne, Austral ...
, a residential building in
Hackney, London
Hackney is a district in East London, England, forming around two-thirds of the area of the modern London Borough of Hackney, to which it gives its name. It is 4 miles (6.4 km) northeast of Charing Cross and includes part of the Queen E ...
, built in 2009, was the first building constructed using only CLT framing, including for the stairs and elevator shaft. At 9 stories and 30 m tall, at the time of construction, it was the tallest CLT building in the world.
In 2012, Forte Living, an apartment complex in
Melbourne
Melbourne ( ; Boonwurrung/Woiwurrung: ''Narrm'' or ''Naarm'') is the capital and most populous city of the Australian state of Victoria, and the second-most populous city in both Australia and Oceania. Its name generally refers to a met ...
, Australia, became the tallest
plyscraper
A plyscraper, or timber tower is a skyscraper made (at least partly) of wood. They may alternatively be known as mass timber buildings.
Materials
There are four main types of engineered wood used for mass timber including cross-laminated timber ...
framed with CLT alone. The building has 10 stories and stands just over 32 m tall. The 759 CLT panels necessary for the project were manufactured in Austria using European spruce that was grown and harvested there.
![Ascent MKE completed](https://upload.wikimedia.org/wikipedia/commons/9/9e/Ascent_MKE_completed.jpg)
In 2022, the
Ascent MKE
Ascent MKE is a mass timber hybrid high-rise apartment building in Milwaukee, Wisconsin. The 284-foot (87 meter), 25-story high-rise is the world's tallest mass timber structure, edging out Norway's Mjøstårnet. It features 259 luxury apartment ...
building in
Milwaukee
Milwaukee ( ), officially the City of Milwaukee, is both the most populous and most densely populated city in the U.S. state of Wisconsin and the county seat of Milwaukee County. With a population of 577,222 at the 2020 census, Milwaukee is ...
, Wisconsin, became the tallest high-rise to utilize CLT components. Reaching 25 stories and 86.6 m, the Ascent relies on concrete, steel, and mass timber components. The CLT was primarily used to create the slabs for each floor.
Bridges
The Mistissini Bridge in
Mistissini
Mistissini ( cr, ᒥᔅᑎᓯᓃ/Mistisinî meaning Big Rock) is a Cree town located in the south-east corner of the largest natural lake in Quebec, Lake Mistassini. The town is inside the boundaries of the Baie-James Municipality and is the ...
, Quebec, Canada, is a 160-meter-long bridge that crosses the Uupaachikus Pass. Designed by
Stantec
Stantec Inc. is an international professional services company in the design and consulting industry. The company was founded in 1954, as ''D. R. Stanley Associates'' in Edmonton, Alberta, Canada. Stantec provides professional consulting servi ...
and completed in 2014, the Mistissini Bridge employs the use of locally sourced CLT panels and glue-laminated timber girders to serve as the main structural members of the bridge. The bridge 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 Exploded View
''The Exploded View'' is a quartet of stories by Ivan Vladislavic published in 2004. The stories revolve around four very different gauteng residents in Johannesburg: a statistician employed on the national census, an engineer out on the town wi ...
is a fully CLT bridge in the design phase . Originally proposed in 2020 by Paul Cocksedge, this bridge will cross the
Liesbeek River
The Liesbeek River (also spelt Liesbeeck) is a river in Cape Town in South Africa. It is named after a small river in the Netherlands. The first "free burghers" of the Dutch East India Company were granted land to farm along the river in 1657, sh ...
in
Cape Town
Cape Town ( af, Kaapstad; , xh, iKapa) is one of South Africa's three capital cities, serving as the seat of the Parliament of South Africa. It is the legislative capital of the country, the oldest city in the country, and the second largest ...
, South Africa. Cocksedge plans to manufacture the CLT from
Eucalyptus
''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as euca ...
trees, an invasive species in the area.
Parking structures
The Glenwood is a CLT parking garage that is part of a larger redevelopment plan in
Springfield, Oregon
Springfield is a city in Lane County, Oregon, United States. Located in the Southern Willamette Valley, it is within the Eugene-Springfield Metropolitan Statistical Area. Separated from Eugene to the west, mainly by Interstate 5, Springfield ...
. Construction is underway , but once complete, it will stand at 4 stories and have 360 parking spots. In order to help protect the CLT from the rain while keeping it exposed, a
façade
A façade () (also written facade) is generally the front part or exterior of a building. It is a Loanword, loan word from the French language, French (), which means 'frontage' or 'face'.
In architecture, the façade of a building is often t ...
made from overlapping glass panels will be installed.
Open Platform and JAJA Architects won a design competition in 2020 for their plans to create a Park n’ Play garage in
Aarhus
Aarhus (, , ; officially spelled Århus from 1948 until 1 January 2011) is the second-largest city in Denmark and the seat of Aarhus Municipality. It is located on the eastern shore of Jutland in the Kattegat sea and approximately northwest ...
, Denmark. The garage not only employs the use of CLT for the structure but surrounds the garage with planters and other greenery to promote the use of the space as more than just a place to leave a car. There are 6 stories with 700 spots, some designed specifically to promote green transportation, including charging stations and carpool only spots. The facility was designed to help the country achieve its goal of reaching carbon neutrality by 2050.
Modular construction
CLT has also been identified as a suitable candidate for use in
modular construction
Modular construction is a construction technique which involves the prefabrication of 2D panels or 3D volumetric structures in off-site factories and transportation to construction sites for assembly. This process has the potential to be superior ...
. Silicon Valley based modular construction startup
Katerra
Katerra was an American technology-driven off-site construction company. It was founded in 2015 by Michael Marks, former CEO of Flextronics and former Tesla interim CEO, along with Fritz Wolff, the executive chairman of The Wolff Co. Katerra was ...
opened a modular construction CLT factory in Spokane, Washington in 2019 and some politicians are calling for the use of pre-fabricated modular CLT construction to address the housing crisis in cities like Seattle.
The
Dyson Institute Village
Dyson Institute Village was built in 2019 on the outskirts of Malmesbury, Wiltshire, England, to provide on-campus student housing for the Dyson Institute of Engineering and Technology. The village was designed as a number of stacked studio apar ...
was built in 2019 on the outskirts of
Malmesbury
Malmesbury () is a town and civil parish in north Wiltshire, England, which lies approximately west of Swindon, northeast of Bristol, and north of Chippenham. The older part of the town is on a hilltop which is almost surrounded by the upp ...
, England, to provide on-campus student housing for the
Dyson Institute of Engineering and Technology
The Dyson Institute of Engineering and Technology is a private institution of higher education in England, founded in 2017 by James Dyson and based at the Dyson technology campus in Malmesbury, Wiltshire. Students work in a position in the Dyson ...
. The village was designed as a number of stacked studio apartment modules by London architects
WilkinsonEyre
WilkinsonEyre is an international architecture practice based in London, England. In 1983 Chris Wilkinson (architect), Chris Wilkinson founded Chris Wilkinson Architects, he partnered with Jim Eyre (architect), Jim Eyre in 1987 and the practice w ...
, and modeled after Montreal's
Habitat 67
HABITAT 67, or simply Habitat, is a housing complex at Cité du Havre, on the Saint Lawrence River, Montreal, Quebec, Canada, designed by Israeli-Canadian architect Moshe Safdie. It originated in his master's thesis at the School of Architect ...
. The pods are constructed from cross-laminated timber with each pod wrapped in aluminum.
See also
*
Engineered Wood
Engineered wood, also called mass timber, composite wood, man-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of woo ...
*
Brettstapel
*
Glued laminated timber
Glued laminated timber, commonly referred to as glulam, is a type of structural engineered wood product constituted by layers of dimensioned lumber, dimensional lumber bonded together with durable, moisture-resistant structural adhesives so that ...
*
Laminated veneer lumber
Laminated veneer lumber (LVL) is an engineered wood product that uses multiple layers of thin wood assembled with adhesives. It is typically used for headers, beams, rimboard, and edge-forming material. LVL offers several advantages over typi ...
*
Green building
Green building (also known as green construction or sustainable building) refers to both a structure and the application of processes that are environmentally responsible and resource-efficient throughout a building's life-cycle: from planni ...
*
Plyscraper
A plyscraper, or timber tower is a skyscraper made (at least partly) of wood. They may alternatively be known as mass timber buildings.
Materials
There are four main types of engineered wood used for mass timber including cross-laminated timber ...
*
Lumber
Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...
*
Plywood
Plywood is a material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured ...
References
External links
{{Wood products
Engineered wood
Composite materials