**2.4 Dowel-laminated timber**

Dowel-laminated timber (DLT) is another member of MTP family. DLT is similar to NLT regarding laminations, but different in fasteners. Instead of nails, DLT uses hardwood dowels to join laminations, **Figure 13**. The manufacturing of DLT is another example of one-dimension additive process. DLT was developed in the early 1990s in Switzerland [7]. DLT is manufactured with softwood lumber of a thickness of 38 mm and a depth of 89, 140, or 184 mm, stacked on edges just like NLT, and fastened face-to-face with wooden dowels. Unlike NLT, finger-joined lumber is typically used in manufacturing of DLT. The moisture content of laminations is 19% or less at time of manufacturing [7]. The visual or MSR grade of laminations is, if spruce-pine-fir lumber is used for example, SS and No. 2 and Better or 2100f-1.8E [18]. The wooden dowels, which are usually made of high-density hardwood species (such as oak), have typically a diameter of 19 mm and a moisture content of approximately 6–8% [7]. The predrilled holes of a diameter being about the same as dowels are required prior to driving dowels into laminations [18]. The dowels can then be hydraulically pressed in a linear or staggered way with spacing of 300 mm [7], the latter of which could offer additional stiffness DLT panels, **Figure 13**. Dowels

**Figure 13.** *DLT with linear (left) and staggered (right) fastening patterns.*

are commonly penetrated through 7–10 laminations, resulting in a more efficient process of manufacturing DLT than NLT. As the moisture content of both materials used in DLT equilibrate after fabrication, the dowels swell and the lumber shrinks, which forms a strong friction-fit joint between the lumber and the dowels, resulting in a panel that does not require glue or nails [7]. DLT has been gaining interest in both Europe and North America since it is almost made of 100% wood, except those of finger-joined laminations that contains very limited amount adhesive. DLT is ease of being manufactured using computer numerical controlled (CNC) machinery, such as lathes, routers, and mills. The prefabricated DLT panels typically have a length up to 18 m, a width up to 4.3 m in any increment, and a thickness ranging from 76 to 349 mm [18]; however, the panel size is usually limited by transportation restrictions.

DLT panels can readily be milled and routed for preintegrated electrical and other service conduits, which offers a unique feature to DLT, i.e., the flexible design, **Figure 14**. This also allows designers to improve the acoustic performance and visual appeals of a building by making kerfs and curves. For example, acoustical strips can be integrated into the bottom surface of a DLT panel, helping designers reduce sound while keeping the wood exposed and also allowing for a variety of surface finishes [18]. DLT can be also recognized as a type of MTP that can be used in exterior exposure, allowing itself to be used for decks, balconies, and canopies.

DLT performs similarly, in terms of structural performance, to GLT and NLT, because its grains run in one direction. DLT allows a significant flexibility in architectural design, which is well suited for floor and roof applications, but can be used as wall panels as well [18], **Figure 15**. Two-way spans can be achieved with the use of reinforcement such as adding multiple layers of plywood atop the DLT panels [18]. In addition, DLT panels can be used as structural bearing or shear walls, and elevator and stair shafts. The design requirements for DLT may be considered the same as those used for NLT, if the hardwood dowels can adequately connect the laminations [7]. In reality, there is almost nothing that can be referenced in the

**19**

*Lumber-Based Mass Timber Products in Construction DOI: http://dx.doi.org/10.5772/intechopen.85808*

building authority on a case-by-case basis.

**2.5 Cross-laminated timber**

**Figure 15.**

*StructureCraft [18]).*

codes worldwide, except that a few manufacturers provide published design values for their own DLT products [4]. Thus, use of DLT would require approval by the

*Seven-story T3 Atlanta building comprising DLT floor and roof panels in USA (Source: Photos obtained from* 

Cross-laminated timber (CLT) is a new-generation engineered large-size structural panel product, which consists of layers of dimension/MSR lumber (typically three, five, or seven) oriented at right angles to one another and then bonded using adhesives, **Figure 16**—upper. CLT was originally invented in the 1970s in Europe [6] and introduced as an innovative wood product in the early 1990s in Austria and Germany [19]. In the mid-1990s, Austria undertook an industry-academia joint research effort that resulted in the development of modern CLT [19]. In the last 2 decades, the use of CLT has gained interest to both construction and wood industries in North America, featured with the publication of two editions of CLT Handbook [20, 21] and erection of 18-stories CLT building "Brock Commons Tallwood House" in 2017 in Canada. Unlike GLT, NLT, and DLT, the manufacturing of CLT is a kind of three-dimension additive process. The species of wood used depends on the location of a manufacturing plant. For example, black spruce is widely used in Eastern Canada. The commonly used lumber products in manufacturing of CLT are dimension lumber of a grade of No. 1/No. 2 or MSR lumber of a grade of 1200f-1.2E or better in its major strength direction, and dimension lumber of a minimum grade of No. 3 in its minor strength direction [7]. In the major strength direction, the minimum net width of a lamination shall be 1.75 times its thickness, and in the minor strength direction, the net width of a lamination shall not be less than 3.5 times its thickness if the laminations are not edge-glued [7]. The moisture content of lumber at fabrication of CLT is about 12% [7]. The cold-set structural adhesives are preferred to increase the productivity of manufacturing CLT panels, which include emulsion polymer isocyanate (EPI), polyurethane (PUR), and phenol-resorcinol formaldehyde (PRF) [7]. In Canada, the adhesives used in manufacturing process of CLT must comply with the Canadian Standard O112.10 "Evaluation of Adhesives for Structural Wood Products (Limited Moisture Exposure)" and ASTM D7247 "Standard Test Method for Evaluating the Shear Strength of Adhesive Bonds in Laminated Wood Products at Elevated Temperatures" [7]. The finger- or scarf-joined lumber is used to face-toface and/or edge-by-edge laminating as two-dimension components. Use of edgegluing or not slightly differs in the manufacturing of CLT between North America and Europe. In North America, edge-gluing of lumber is not a common practice due

**Figure 14.** *Two sample profiles of DLT (Source: Pictures obtained from StructureCraft [18]).*

*Lumber-Based Mass Timber Products in Construction DOI: http://dx.doi.org/10.5772/intechopen.85808*

**Figure 15.**

*Timber Buildings and Sustainability*

*DLT with linear (left) and staggered (right) fastening patterns.*

**Figure 13.**

are commonly penetrated through 7–10 laminations, resulting in a more efficient process of manufacturing DLT than NLT. As the moisture content of both materials used in DLT equilibrate after fabrication, the dowels swell and the lumber shrinks, which forms a strong friction-fit joint between the lumber and the dowels, resulting in a panel that does not require glue or nails [7]. DLT has been gaining interest in both Europe and North America since it is almost made of 100% wood, except those of finger-joined laminations that contains very limited amount adhesive. DLT is ease of being manufactured using computer numerical controlled (CNC) machinery, such as lathes, routers, and mills. The prefabricated DLT panels typically have a length up to 18 m, a width up to 4.3 m in any increment, and a thickness ranging from 76 to 349 mm [18]; however, the panel size is usually limited by transportation restrictions. DLT panels can readily be milled and routed for preintegrated electrical and other service conduits, which offers a unique feature to DLT, i.e., the flexible design, **Figure 14**. This also allows designers to improve the acoustic performance and visual appeals of a building by making kerfs and curves. For example, acoustical strips can be integrated into the bottom surface of a DLT panel, helping designers reduce sound while keeping the wood exposed and also allowing for a variety of surface finishes [18]. DLT can be also recognized as a type of MTP that can be used in exterior exposure, allowing itself to be used for decks, balconies, and canopies. DLT performs similarly, in terms of structural performance, to GLT and NLT,

because its grains run in one direction. DLT allows a significant flexibility in architectural design, which is well suited for floor and roof applications, but can be used as wall panels as well [18], **Figure 15**. Two-way spans can be achieved with the use of reinforcement such as adding multiple layers of plywood atop the DLT panels [18]. In addition, DLT panels can be used as structural bearing or shear walls, and elevator and stair shafts. The design requirements for DLT may be considered the same as those used for NLT, if the hardwood dowels can adequately connect the laminations [7]. In reality, there is almost nothing that can be referenced in the

*Two sample profiles of DLT (Source: Pictures obtained from StructureCraft [18]).*

**18**

**Figure 14.**

*Seven-story T3 Atlanta building comprising DLT floor and roof panels in USA (Source: Photos obtained from StructureCraft [18]).*

codes worldwide, except that a few manufacturers provide published design values for their own DLT products [4]. Thus, use of DLT would require approval by the building authority on a case-by-case basis.

#### **2.5 Cross-laminated timber**

Cross-laminated timber (CLT) is a new-generation engineered large-size structural panel product, which consists of layers of dimension/MSR lumber (typically three, five, or seven) oriented at right angles to one another and then bonded using adhesives, **Figure 16**—upper. CLT was originally invented in the 1970s in Europe [6] and introduced as an innovative wood product in the early 1990s in Austria and Germany [19]. In the mid-1990s, Austria undertook an industry-academia joint research effort that resulted in the development of modern CLT [19]. In the last 2 decades, the use of CLT has gained interest to both construction and wood industries in North America, featured with the publication of two editions of CLT Handbook [20, 21] and erection of 18-stories CLT building "Brock Commons Tallwood House" in 2017 in Canada. Unlike GLT, NLT, and DLT, the manufacturing of CLT is a kind of three-dimension additive process. The species of wood used depends on the location of a manufacturing plant. For example, black spruce is widely used in Eastern Canada. The commonly used lumber products in manufacturing of CLT are dimension lumber of a grade of No. 1/No. 2 or MSR lumber of a grade of 1200f-1.2E or better in its major strength direction, and dimension lumber of a minimum grade of No. 3 in its minor strength direction [7]. In the major strength direction, the minimum net width of a lamination shall be 1.75 times its thickness, and in the minor strength direction, the net width of a lamination shall not be less than 3.5 times its thickness if the laminations are not edge-glued [7]. The moisture content of lumber at fabrication of CLT is about 12% [7]. The cold-set structural adhesives are preferred to increase the productivity of manufacturing CLT panels, which include emulsion polymer isocyanate (EPI), polyurethane (PUR), and phenol-resorcinol formaldehyde (PRF) [7]. In Canada, the adhesives used in manufacturing process of CLT must comply with the Canadian Standard O112.10 "Evaluation of Adhesives for Structural Wood Products (Limited Moisture Exposure)" and ASTM D7247 "Standard Test Method for Evaluating the Shear Strength of Adhesive Bonds in Laminated Wood Products at Elevated Temperatures" [7]. The finger- or scarf-joined lumber is used to face-toface and/or edge-by-edge laminating as two-dimension components. Use of edgegluing or not slightly differs in the manufacturing of CLT between North America and Europe. In North America, edge-gluing of lumber is not a common practice due

#### **Figure 16.**

*CLT products (Upper: a generic CLT made of lumber only; lower: a hybrid CLT made of dimension lumber in the major strength direction and structural composite lumber in the minor strength direction).*

to the added manufacturing costs. The gaps between lumber could provide some tolerances for wood movement due to the change in moisture in service. However, the European practice appears to widely apply edge-gluing with an aim to offer good stiffness and strength of a CLT panel. Anyhow, as a trade-off between cost and improved panel performance, edge-gluing of selected layers as needed could be adopted [6]. CNC routers are often employed to precisely cut CLT panels to size and openings for windows, doors, connections, ducts, and service channels. A CLT product can be produced in large sizes of a width ranging from 1.2 to 3 m, a length from 5 to 19.5 m, and a thickness from 100 to 500 mm [7]. CLT can be also manufactured in custom dimensions, with panel sizes varying by a manufacturer.

Despite the availability of commercial machines to manufacture construction size CLT using dimension lumber, there are challenges with the existing systems, such as the need to apply pressure to all four sides of a panel to ensure adequate edge-glue bond quality, as well as the out-of-plane pressing. From a product performance perspective, CLT is known to be prone to the so-called rolling shear failure and excessive deflection when subjected to out-of-plane loading. This is particularly critical where the lumber layers are not edge-glued. These performance issues could be addressed by replacing one or more of the layers in a CLT panel with SCL, such as LSL and OSL. Such an innovative hybrid CLT can offer many advantages over the

**21**

**Figure 17.**

*Lumber-Based Mass Timber Products in Construction DOI: http://dx.doi.org/10.5772/intechopen.85808*

generic one that is made of 100% dimension lumber, **Figure 16**—lower. The hybrid CLT products could reduce the production cost because of the reduced efforts to layup of individual lumber pieces and the possible elimination of the need to press the panel on all four sides simultaneously, improve the rolling shear strength and stiffness properties of generic CLT since SCL has relatively high shear strength and rigidity, and improve the fire resistance of CLT due to the elimination of gaps present in generic CLT made with non-edge-glued dimension lumber. The research on three- and five-layer hybrid CLT, recently conducted in the Wood Science and Technology Centre, the University of New Brunswick, Canada, showed that the bending stiffness, moment capacity, and shear capacity of hybrid CLT were

Cross laminating technology provides CLT panels with improved stable dimensions, and relatively high in-plane and out-of-plane stiffness and strength properties in both directions, giving these panels a two-way action capability [6]. It is wellsuited to floors, walls, and roofs, and may be left exposed on the interior for esthetics. The light weight of CLT directly helps reduce the size and cost of foundation. As a prefabricated building component, CLT offers shorter onsite construction time than traditional platform frame construction or steel and concrete construction, minimizes waste and noise during construction, and provides a very competitive cost in comparison to concrete and steel [19]. CLT has also been used to fabricate bridge decks, heavy equipment mats, and platforms for oil rigs, and to construct mid-rise and tall wood buildings of over seven stories, and large industrial structures [4]. In addition, CLT exhibits good seismic and fire performance. The 2015 International Building Code (IBC) and 2015 International Residential Code recognize CLT products manufactured according to the ANSI/APA PRG-320 "Standard for Performance Rated Cross-Laminated Timber." Under the 2015 IBC, CLT at the required size is specifically stated for prescribed use in Type IV buildings, i.e., heavy timber buildings, which hold well under fire conditions due to formation of char layer. However, CLT can be used in all types of combustible construction, i.e.,

increased to a large degree in comparison to generic one [22–24].

wherever combustible framing or heavy timber materials are allowed [4].

CLT is sometimes deemed as a standalone building material and construction system. A kind of post-and-panel construction has emerged, accompanied with many innovative connections. The tallest wood building as of the year of 2018,

*Brock Commons Tallwood House, Vancouver, Canada (Source: Photo obtained from UBC Public Affairs).*

*Lumber-Based Mass Timber Products in Construction DOI: http://dx.doi.org/10.5772/intechopen.85808*

*Timber Buildings and Sustainability*

to the added manufacturing costs. The gaps between lumber could provide some tolerances for wood movement due to the change in moisture in service. However, the European practice appears to widely apply edge-gluing with an aim to offer good stiffness and strength of a CLT panel. Anyhow, as a trade-off between cost and improved panel performance, edge-gluing of selected layers as needed could be adopted [6]. CNC routers are often employed to precisely cut CLT panels to size and openings for windows, doors, connections, ducts, and service channels. A CLT product can be produced in large sizes of a width ranging from 1.2 to 3 m, a length from 5 to 19.5 m, and a thickness from 100 to 500 mm [7]. CLT can be also manufactured in

*the major strength direction and structural composite lumber in the minor strength direction).*

*CLT products (Upper: a generic CLT made of lumber only; lower: a hybrid CLT made of dimension lumber in* 

Despite the availability of commercial machines to manufacture construction size CLT using dimension lumber, there are challenges with the existing systems, such as the need to apply pressure to all four sides of a panel to ensure adequate edge-glue bond quality, as well as the out-of-plane pressing. From a product performance perspective, CLT is known to be prone to the so-called rolling shear failure and excessive deflection when subjected to out-of-plane loading. This is particularly critical where the lumber layers are not edge-glued. These performance issues could be addressed by replacing one or more of the layers in a CLT panel with SCL, such as LSL and OSL. Such an innovative hybrid CLT can offer many advantages over the

custom dimensions, with panel sizes varying by a manufacturer.

**20**

**Figure 16.**

generic one that is made of 100% dimension lumber, **Figure 16**—lower. The hybrid CLT products could reduce the production cost because of the reduced efforts to layup of individual lumber pieces and the possible elimination of the need to press the panel on all four sides simultaneously, improve the rolling shear strength and stiffness properties of generic CLT since SCL has relatively high shear strength and rigidity, and improve the fire resistance of CLT due to the elimination of gaps present in generic CLT made with non-edge-glued dimension lumber. The research on three- and five-layer hybrid CLT, recently conducted in the Wood Science and Technology Centre, the University of New Brunswick, Canada, showed that the bending stiffness, moment capacity, and shear capacity of hybrid CLT were increased to a large degree in comparison to generic one [22–24].

Cross laminating technology provides CLT panels with improved stable dimensions, and relatively high in-plane and out-of-plane stiffness and strength properties in both directions, giving these panels a two-way action capability [6]. It is wellsuited to floors, walls, and roofs, and may be left exposed on the interior for esthetics. The light weight of CLT directly helps reduce the size and cost of foundation. As a prefabricated building component, CLT offers shorter onsite construction time than traditional platform frame construction or steel and concrete construction, minimizes waste and noise during construction, and provides a very competitive cost in comparison to concrete and steel [19]. CLT has also been used to fabricate bridge decks, heavy equipment mats, and platforms for oil rigs, and to construct mid-rise and tall wood buildings of over seven stories, and large industrial structures [4]. In addition, CLT exhibits good seismic and fire performance. The 2015 International Building Code (IBC) and 2015 International Residential Code recognize CLT products manufactured according to the ANSI/APA PRG-320 "Standard for Performance Rated Cross-Laminated Timber." Under the 2015 IBC, CLT at the required size is specifically stated for prescribed use in Type IV buildings, i.e., heavy timber buildings, which hold well under fire conditions due to formation of char layer. However, CLT can be used in all types of combustible construction, i.e., wherever combustible framing or heavy timber materials are allowed [4].

CLT is sometimes deemed as a standalone building material and construction system. A kind of post-and-panel construction has emerged, accompanied with many innovative connections. The tallest wood building as of the year of 2018,

**Figure 17.** *Brock Commons Tallwood House, Vancouver, Canada (Source: Photo obtained from UBC Public Affairs).*

Brock Commons Tallwood House (**Figure 17**), stands in Vancouver, Canada. This building includes 17 stories of CLT floors supported on GLT columns atop a concrete base with two 18-stroy concrete cores. This 53-m-high building is used as student residence providing 404 bed units. Its unique designed column-to-column metal connector makes a column-panel-column connection, minimizing the accumulation of deformations (i.e., the transverse wood movement) generated from each CLT floor. It was reported that 80% of the work for this tall building was prefabricated and 70% alone was gaining code approval [25].

### **3. Endnotes**

Environmental awareness coupled with sustainable design and construction practices are increasingly becoming a requirement for many building projects throughout North America and around the world [7]. Sustainable design aspires to use less energy and material resources in conjunction with lowering the environmental impacts on a building from its cradle to grave [7]. The reasons for using wood in construction are attributed to its environmentally friendly attributes, ease of assembly, reduced noise and waste during construction, natural beauty, and cost-effectiveness. Increasing use of renewable and sustainable building materials in construction, such as wood, is a worldwide move. Wood-based materials, such as MTP, consume less energy and emit fewer greenhouse gasses (GHG) and pollutants over their life cycle than traditional energy-intensive construction materials such as steel and concrete [2]. To spur innovation and certify the performance of wood as a construction material, many countries have made a great effort to support the research and development of wood products such as MTP. In Canada, for example, the 2015 Edition of its National Building Code of Canada (NBC) allows to construct wood frame buildings up to six stories. The Canadian have been working hard to the code revisions with an aim at the 2020 Edition of the NBC to permit tall wood buildings up to 12 stories [2]. Their long-term objective is to establish the performance-based codes for the 2025 Edition of the NBC and beyond, which will eliminate the distinction between building materials. This will give architects and developers freedom of choice in their materials. Ramage et al. illustrated the selection of structural systems for multi-story buildings in terms of the number of stories and their use of wood [26], **Figure 18**. For buildings up to about six stories, CLT uses substantially more wood to achieve the same function as a light-wood frame building. For buildings over six stories, the use of CLT together with lightwood frame may use less wood than CLT alone. As for buildings taller than 10 stories, the mass timber construction method is employed by using GLT megaframe to support CLT walls, floors, and roofs [26].

The life cycle of a product is defined in the standard ISO 14040 as "consecutive and interlinked stages of a product system, from raw material acquisition or generation from natural resources to final disposal" [27]. This has led to the use of the life cycle assessment (LCA), which is defined as "the compilation and evaluation of the inputs and outputs and the potential environmental impacts of a product system throughout its life cycle" [27]. LCA is a performance-based approach to assessing impacts that building products or systems have on the environment over their lifetime [7], including all activities from raw material extraction/harvesting, materials processing/products manufacturing, transportation, distribution, installation, use, repair and maintenance, and final disposal or recycling [7]. LCA is deemed as the best available tool to compare sustainability of building materials, which includes four main phases, i.e., goal and scope definition, inventory analysis, impact assessment, and interpretation.

**23**

**Figure 18.**

*et al. [26]).*

construction of wood buildings.

**Acknowledgements**

*Lumber-Based Mass Timber Products in Construction DOI: http://dx.doi.org/10.5772/intechopen.85808*

LCA studies on wood buildings are rooted in the assumption of the same life span for wood as other structural materials. Ramage et al. summarized, after conducting a comprehensive review on use of wood in construction, that the buildings are really demolished due to degradation of their main structure, whatever the structural materials [26]. However, some wood components in a building may have a design life shorter than that of the building as a whole, or may require maintenance during the life of the building. There are many factors impacting the lifespan of wood components, including fire and natural degradation. In comparison to other building materials such as steel and concrete, wood is combustible. However, large cross-section wood components, such as those made of GLT and CLT, may perform well in case of catching a fire due to the formation of char layer that can act to insulate the material inside. The burnt wood can still keep large, enough strength to support the integrity of a building. As for small cross-section of wood components, they must be encapsulated in noncombustible material such as gypsum boards or concrete. Steel connectors are widely used in modern wood buildings, thus heat can be quickly conducted through the connectors, degrading the strength and stiffness of the wood connections and materials around them [26]. Caution must be used at time of using steel connectors in

*Use of structural lumber and mass timber products for various structural systems (Adapted from Ramage* 

In a summary, mass timber building systems make it feasible to use wood in construction of mid-rise and tall buildings, industrial structures, and bridges. However, mass timber products and building systems behave in a fundamentally different way in fire than steel or concrete buildings in structural and spatial layout.

This piece of work was financially supported by the New Brunswick Innovation

Research Chair Program, New Brunswick Innovation Foundation, Canada.

More research is required to increase use of wood in construction.

#### **Figure 18.**

*Timber Buildings and Sustainability*

**3. Endnotes**

Brock Commons Tallwood House (**Figure 17**), stands in Vancouver, Canada. This building includes 17 stories of CLT floors supported on GLT columns atop a concrete base with two 18-stroy concrete cores. This 53-m-high building is used as student residence providing 404 bed units. Its unique designed column-to-column metal connector makes a column-panel-column connection, minimizing the accumulation of deformations (i.e., the transverse wood movement) generated from each CLT floor. It was reported that 80% of the work for this tall building was

Environmental awareness coupled with sustainable design and construction practices are increasingly becoming a requirement for many building projects throughout North America and around the world [7]. Sustainable design aspires to use less energy and material resources in conjunction with lowering the environmental impacts on a building from its cradle to grave [7]. The reasons for using wood in construction are attributed to its environmentally friendly attributes, ease of assembly, reduced noise and waste during construction, natural beauty, and cost-effectiveness. Increasing use of renewable and sustainable building materials in construction, such as wood, is a worldwide move. Wood-based materials, such as MTP, consume less energy and emit fewer greenhouse gasses (GHG) and pollutants over their life cycle than traditional energy-intensive construction materials such as steel and concrete [2]. To spur innovation and certify the performance of wood as a construction material, many countries have made a great effort to support the research and development of wood products such as MTP. In Canada, for example, the 2015 Edition of its National Building Code of Canada (NBC) allows to construct wood frame buildings up to six stories. The Canadian have been working hard to the code revisions with an aim at the 2020 Edition of the NBC to permit tall wood buildings up to 12 stories [2]. Their long-term objective is to establish the performance-based codes for the 2025 Edition of the NBC and beyond, which will eliminate the distinction between building materials. This will give architects and developers freedom of choice in their materials. Ramage et al. illustrated the selection of structural systems for multi-story buildings in terms of the number of stories and their use of wood [26], **Figure 18**. For buildings up to about six stories, CLT uses substantially more wood to achieve the same function as a light-wood frame building. For buildings over six stories, the use of CLT together with lightwood frame may use less wood than CLT alone. As for buildings taller than 10 stories, the mass timber construction method is employed by using GLT megaframe

The life cycle of a product is defined in the standard ISO 14040 as "consecutive and interlinked stages of a product system, from raw material acquisition or generation from natural resources to final disposal" [27]. This has led to the use of the life cycle assessment (LCA), which is defined as "the compilation and evaluation of the inputs and outputs and the potential environmental impacts of a product system throughout its life cycle" [27]. LCA is a performance-based approach to assessing impacts that building products or systems have on the environment over their lifetime [7], including all activities from raw material extraction/harvesting, materials processing/products manufacturing, transportation, distribution, installation, use, repair and maintenance, and final disposal or recycling [7]. LCA is deemed as the best available tool to compare sustainability of building materials, which includes four main phases, i.e., goal and scope definition, inventory analysis, impact assessment, and interpretation.

prefabricated and 70% alone was gaining code approval [25].

to support CLT walls, floors, and roofs [26].

**22**

*Use of structural lumber and mass timber products for various structural systems (Adapted from Ramage et al. [26]).*

LCA studies on wood buildings are rooted in the assumption of the same life span for wood as other structural materials. Ramage et al. summarized, after conducting a comprehensive review on use of wood in construction, that the buildings are really demolished due to degradation of their main structure, whatever the structural materials [26]. However, some wood components in a building may have a design life shorter than that of the building as a whole, or may require maintenance during the life of the building. There are many factors impacting the lifespan of wood components, including fire and natural degradation. In comparison to other building materials such as steel and concrete, wood is combustible. However, large cross-section wood components, such as those made of GLT and CLT, may perform well in case of catching a fire due to the formation of char layer that can act to insulate the material inside. The burnt wood can still keep large, enough strength to support the integrity of a building. As for small cross-section of wood components, they must be encapsulated in noncombustible material such as gypsum boards or concrete. Steel connectors are widely used in modern wood buildings, thus heat can be quickly conducted through the connectors, degrading the strength and stiffness of the wood connections and materials around them [26]. Caution must be used at time of using steel connectors in construction of wood buildings.

In a summary, mass timber building systems make it feasible to use wood in construction of mid-rise and tall buildings, industrial structures, and bridges. However, mass timber products and building systems behave in a fundamentally different way in fire than steel or concrete buildings in structural and spatial layout. More research is required to increase use of wood in construction.

#### **Acknowledgements**

This piece of work was financially supported by the New Brunswick Innovation Research Chair Program, New Brunswick Innovation Foundation, Canada.

The author's sincere gratitude goes to Dr. Ian Smith, Emeritus Professor of the University of New Brunswick (UNB), for his kindly reviewing part of the manuscript. The author's thanks also go to Mr. Luji Xiong, Graduate Research Assistant at UNB, for drawing sketches.
