**2.2 Glue-laminated timber (GLT or glulam)**

GLT (also widely known as glulam) is a structural product composed of multiple pieces of finger-joined dimension lumber, or other types of EWP, adhesively face-toface bonded to create a desired form. GLT was first used in Europe in the early 1890s. A 1901 patent from Switzerland signaled the true beginning of GLT construction [9]. A significant development in the GLT industry was the introduction of fully

**Figure 3.** *Two finger joint profiles (left: 29-mm long; right: 13-mm long) used for joining short pieces of lumber.*

water-resistant phenol-resorcinol adhesives in 1942, which allowed GLT to be used in exposed exterior environments without concern of glueline degradation [9]. The manufacturing of GLT is deemed as a one-dimension additive process. The grain of all laminations runs parallel with the lengths of straight members, **Figure 4**. The dimension lumber laminations are not visually graded on the same rules as regular lumber, but follow the grading rules stipulated in Canadian Standard O122 "Structural Glued-Laminated Timber" [7]. Each lamination is visually inspected based on both faces of the piece, and then assigned one of four grades: B-F, B, D, or C [7], in which B-F indicates the highest grade and C the lowest grade. Laminations of higher grades are used in the top and bottom portions of a GLT beam, **Figure 5**, where bending stress is greatest. Specified laminations are also nondestructively graded by machine before assembly to meet both visual and stiffness requirements for particular grades of GLT. Sometimes layers of other materials, such as glass fibers, are incorporated among lumber laminations to add strength or stiffness or to locally reinforce GLT [10]. Moisture content of laminations ranges from 7 to 15% during fabrication. Durable cold-setting waterproof-structural adhesives are used, such as phenol formaldehyde and phenol-resorcinol formaldehyde [7]. Because finger-joined lumber is employed, dimensions of GLT members are in principle only limited by manufacturing and transportation capabilities of a manufacturer. Those capabilities are highly variable, with the most advanced involving fully automated manufacturing processes based on advanced integrated design and manufacturing methods. The automated processes can include robot handling of materials and elements from the arrival of logs at a manufacturing plant to installation of elements at a construction site. A typical GLT member ranges in depth from 114 to 2128 mm or more, in width from 80 to 365 mm, and in length of up to 40 m [7]. GLT is commonly used as beams and columns (**Figure 4**—left and middle), but can be also used as flexural members (**Figure 4**—right). In latter situation, the narrow faces of the laminations are normal to the direction of the load. The Canadian Standard O86 "Engineering Design in Wood" refers to this condition as "vertically glue-laminated" [11]. Usually, GLT is used in dry service conditions or is protected in some way if used under outdoor conditions.

Design stiffness and strength properties of GLT of a given grade are calculated based on engineering properties of the laminations using equivalent linear elastic mechanics theories. A wide range of GLT grades are available with some involving deliberate placements of laminations of different grades to achieve the design properties of GLT elements suited to their particular applications [11]. In general, there are two grade categories for GLT, stress grade and appearance grade [7]. The former defines specified strengths of a GLT member, and the latter the quality of

**13**

**Figure 5.**

**2.3 Nail-laminated timber**

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

finish on the exposed surfaces of the member. For example, some grades suit uses of GLT elements as beams, columns or tension members, **Figures 6–8**. Taking the Canadian Standard O86 "Engineering Design in Wood" as an example, that design standard specifies the grades of GLT bending elements as 20f-E, 20f-EX, 24f-E, and 24f-EX [11], **Figure 5**. Within those designations, numbers 20 and 24 are indicative of the associated specified design strength in bending. E indicates that associated grade properties apply to elements without an inflection in their deformed shapes, with the proviso faces intended to be stressed in tension are correctly oriented. EX indicates that associated grade properties apply to elements with inflections in their deformed shapes. Similar approaches are adopted by other international standards

*GLT members with laminations suiting resistance of bending forces: Upper—Member with a balanced layup intended to maximize material use when the member is loaded in tension on both top and bottom faces (EX grade under Canadian system), and Lower—Member with an unbalanced layup intended to maximize material use when the member is loaded in tension on the bottom face (E grade under Canadian system).*

NLT is manufactured with dimension lumber laminations, stacked on edges, and fastened with nails, to create large-flat structural components, **Figure 9**. Spikes and screws are sometimes used as well. Since the beginning of the nineteenth century, NLT systems were utilized as floor elements in structures known as "mill construction" that originated from cotton mills and sawmills found in the North Eastern United States [7]. The prevalence of the industrial building systems led

which define rules for engineering design of timber structures.

**Figure 4.** *GLT beam (left), column (middle), and panel (right).*

#### **Figure 5.**

*Timber Buildings and Sustainability*

water-resistant phenol-resorcinol adhesives in 1942, which allowed GLT to be used in exposed exterior environments without concern of glueline degradation [9]. The manufacturing of GLT is deemed as a one-dimension additive process. The grain of all laminations runs parallel with the lengths of straight members, **Figure 4**. The dimension lumber laminations are not visually graded on the same rules as regular lumber, but follow the grading rules stipulated in Canadian Standard O122 "Structural Glued-Laminated Timber" [7]. Each lamination is visually inspected based on both faces of the piece, and then assigned one of four grades: B-F, B, D, or C [7], in which B-F indicates the highest grade and C the lowest grade. Laminations of higher grades are used in the top and bottom portions of a GLT beam, **Figure 5**, where bending stress is greatest. Specified laminations are also nondestructively graded by machine before assembly to meet both visual and stiffness requirements for particular grades of GLT. Sometimes layers of other materials, such as glass fibers, are incorporated among lumber laminations to add strength or stiffness or to locally reinforce GLT [10]. Moisture content of laminations ranges from 7 to 15% during fabrication. Durable cold-setting waterproof-structural adhesives are used, such as phenol formaldehyde and phenol-resorcinol formaldehyde [7]. Because finger-joined lumber is employed, dimensions of GLT members are in principle only limited by manufacturing and transportation capabilities of a manufacturer. Those capabilities are highly variable, with the most advanced involving fully automated manufacturing processes based on advanced integrated design and manufacturing methods. The automated processes can include robot handling of materials and elements from the arrival of logs at a manufacturing plant to installation of elements at a construction site. A typical GLT member ranges in depth from 114 to 2128 mm or more, in width from 80 to 365 mm, and in length of up to 40 m [7]. GLT is commonly used as beams and columns (**Figure 4**—left and middle), but can be also used as flexural members

(**Figure 4**—right). In latter situation, the narrow faces of the laminations are normal to the direction of the load. The Canadian Standard O86 "Engineering Design in Wood" refers to this condition as "vertically glue-laminated" [11]. Usually, GLT is used in dry service conditions or is protected in some way if used under outdoor conditions. Design stiffness and strength properties of GLT of a given grade are calculated based on engineering properties of the laminations using equivalent linear elastic mechanics theories. A wide range of GLT grades are available with some involving deliberate placements of laminations of different grades to achieve the design properties of GLT elements suited to their particular applications [11]. In general, there are two grade categories for GLT, stress grade and appearance grade [7]. The former defines specified strengths of a GLT member, and the latter the quality of

**12**

**Figure 4.**

*GLT beam (left), column (middle), and panel (right).*

*GLT members with laminations suiting resistance of bending forces: Upper—Member with a balanced layup intended to maximize material use when the member is loaded in tension on both top and bottom faces (EX grade under Canadian system), and Lower—Member with an unbalanced layup intended to maximize material use when the member is loaded in tension on the bottom face (E grade under Canadian system).*

finish on the exposed surfaces of the member. For example, some grades suit uses of GLT elements as beams, columns or tension members, **Figures 6–8**. Taking the Canadian Standard O86 "Engineering Design in Wood" as an example, that design standard specifies the grades of GLT bending elements as 20f-E, 20f-EX, 24f-E, and 24f-EX [11], **Figure 5**. Within those designations, numbers 20 and 24 are indicative of the associated specified design strength in bending. E indicates that associated grade properties apply to elements without an inflection in their deformed shapes, with the proviso faces intended to be stressed in tension are correctly oriented. EX indicates that associated grade properties apply to elements with inflections in their deformed shapes. Similar approaches are adopted by other international standards which define rules for engineering design of timber structures.

#### **2.3 Nail-laminated timber**

NLT is manufactured with dimension lumber laminations, stacked on edges, and fastened with nails, to create large-flat structural components, **Figure 9**. Spikes and screws are sometimes used as well. Since the beginning of the nineteenth century, NLT systems were utilized as floor elements in structures known as "mill construction" that originated from cotton mills and sawmills found in the North Eastern United States [7]. The prevalence of the industrial building systems led

**Figure 6.** *Conference room built with GLT beams and columns at the University of New Brunswick, Fredericton, Canada.*

the National Lumber Manufacturers Association to publish a guide "Heavy Timber Mill Construction Buildings" in 1916 [7]. In addition, NLT has been used to create deck and diaphragm elements of bridges and buildings for centuries [6]. Like GLT, the manufacturing of NLT is a one-dimension additive process. In North America, individual laminations have a thickness of 38 mm or more and a depth of 64 mm or more, similar to plank decking [7]. The moisture content of laminations is usually 12–16% at time of manufacturing of NLT [14]. The visual or MSR grade of softwood laminations are widely used, such as SS and No. 2 and Better or 1650f-1.5E [7, 14]. Single laminations are commonly employed if the length of prefabricated panels is less than 6 m [14]. The spliced laminations of specific pattern [11, 14] or finger joined lumber laminations [14] are used if longer panels are required. The Canadian Standard O86 "Engineering Design in Wood" [11], for example, specifies connection requirements for fabricating NLT, requiring that nails be long enough to pass through two adjacent laminations and at least halfway through the third, **Figure 9**. For example, 102-mm-long nails should be used to fasten 38-mm-thick laminations, and 152-mm-long nails for 64-mm-thick laminations. Such requirements are based on practical experience and ensure integrity of NLT in various end use situations. NLT shall be spiked together with a staggered single row of nails at intervals of not

**15**

**Figure 7.**

problem due to existence of nails.

*(Source: Photos obtained from CWC [12]).*

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

more than 450 mm [7]. The prefabricated NLT panels typically come in lengths of 3–8 m; however, the panel size is limited by transportation restriction [7]. The drawbacks of using NLT are its slow fabrication process and after-fabrication machining

*A forest of intertwined GLT trees in the Carlo Fidani Peel Regional Cancer Centre, Mississauga, Canada* 

In North America, many timber decks of rural bridges constructed from 1920s through the mid-1960s were made of NLT [15]. Mostly, the NLT was oriented so the lumber laminations were transverse to the bridge span and supported by bridge girders, but for short bridges lumber laminations were sometimes orientated parallel to the span [15], **Figure 10**. Another common traditional use of NLT is in floors of industrial and commercial buildings. The reasons for choosing NLT are as follows: it is well suited to onsite fabrication; it is capable because of the nails of absorbing energy damping vibrations caused by transient or sustained dynamic force (e.g., bridge wheel loads and reciprocating industrial equipment); and it has good fire performance. Disadvantages of NLT include that it is not particularly mechanically efficient if NLT elements are required to have high rigidity when loaded in-plane or as flexural elements, also there have been durability issues associated in particular with bridge applications. The disadvantages stem from the flexibility of nailed interconnections between laminations, and proneness to gaps to form at those intercon-

nections (e.g., due to moisture movements in the laminations).

#### **Figure 7.**

*Timber Buildings and Sustainability*

the National Lumber Manufacturers Association to publish a guide "Heavy Timber Mill Construction Buildings" in 1916 [7]. In addition, NLT has been used to create deck and diaphragm elements of bridges and buildings for centuries [6]. Like GLT, the manufacturing of NLT is a one-dimension additive process. In North America, individual laminations have a thickness of 38 mm or more and a depth of 64 mm or more, similar to plank decking [7]. The moisture content of laminations is usually 12–16% at time of manufacturing of NLT [14]. The visual or MSR grade of softwood laminations are widely used, such as SS and No. 2 and Better or 1650f-1.5E [7, 14]. Single laminations are commonly employed if the length of prefabricated panels is less than 6 m [14]. The spliced laminations of specific pattern [11, 14] or finger joined lumber laminations [14] are used if longer panels are required. The Canadian Standard O86 "Engineering Design in Wood" [11], for example, specifies connection requirements for fabricating NLT, requiring that nails be long enough to pass through two adjacent laminations and at least halfway through the third, **Figure 9**. For example, 102-mm-long nails should be used to fasten 38-mm-thick laminations, and 152-mm-long nails for 64-mm-thick laminations. Such requirements are based on practical experience and ensure integrity of NLT in various end use situations. NLT shall be spiked together with a staggered single row of nails at intervals of not

*Conference room built with GLT beams and columns at the University of New Brunswick, Fredericton, Canada.*

**14**

**Figure 6.**

*A forest of intertwined GLT trees in the Carlo Fidani Peel Regional Cancer Centre, Mississauga, Canada (Source: Photos obtained from CWC [12]).*

more than 450 mm [7]. The prefabricated NLT panels typically come in lengths of 3–8 m; however, the panel size is limited by transportation restriction [7]. The drawbacks of using NLT are its slow fabrication process and after-fabrication machining problem due to existence of nails.

In North America, many timber decks of rural bridges constructed from 1920s through the mid-1960s were made of NLT [15]. Mostly, the NLT was oriented so the lumber laminations were transverse to the bridge span and supported by bridge girders, but for short bridges lumber laminations were sometimes orientated parallel to the span [15], **Figure 10**. Another common traditional use of NLT is in floors of industrial and commercial buildings. The reasons for choosing NLT are as follows: it is well suited to onsite fabrication; it is capable because of the nails of absorbing energy damping vibrations caused by transient or sustained dynamic force (e.g., bridge wheel loads and reciprocating industrial equipment); and it has good fire performance. Disadvantages of NLT include that it is not particularly mechanically efficient if NLT elements are required to have high rigidity when loaded in-plane or as flexural elements, also there have been durability issues associated in particular with bridge applications. The disadvantages stem from the flexibility of nailed interconnections between laminations, and proneness to gaps to form at those interconnections (e.g., due to moisture movements in the laminations).

#### **Figure 8.**

*160-m-long timber bridge with GLT deck structure in Mistissini, Canada. (Source: Photos obtained from Lefebvre and Richard [13]).*

**Figure 9.** *NLT with linear (left) and staggered (right) nailing patterns.*

**Figure 10.** *Transverse (left) and longitudinal (right) oriented NLT bridge decks.*

Recently, use of NLT has undergone resurgence as part of the modern mass timber movement in buildings [3, 4], **Figure 11**. This, in some cases, supports adoption of complicated architectural forms, **Figure 12**, supported by creation of hybrid NLT products which combine lumber laminations with layers of sheathing materials such as plywood and OSB to reinforce the system [14, 17]. Sheathing adequately nailed to NLT can create a diaphragm of the capability to resist lateral forces, and can also help keep the system dry if exposed to moisture [7]. In any such case, it is required to consider the system as an individually designed engineering project.

**17**

**2.4 Dowel-laminated timber**

**Figure 11.**

**Figure 12.**

*obtained from StructureCraft [16]).*

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

*Qingdao Pearl Visitor Centre of exposed NLT roof in China (Source: Photo obtained from StructureCraft [16]).*

*Seven-story T3 Minneapolis building made of NLT floors and GLT beams and columns in USA (Source: Photos* 

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

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

#### **Figure 11.**

*Timber Buildings and Sustainability*

**16**

**Figure 9.**

**Figure 8.**

*Lefebvre and Richard [13]).*

**Figure 10.**

*NLT with linear (left) and staggered (right) nailing patterns.*

*Transverse (left) and longitudinal (right) oriented NLT bridge decks.*

neering project.

Recently, use of NLT has undergone resurgence as part of the modern mass timber movement in buildings [3, 4], **Figure 11**. This, in some cases, supports adoption of complicated architectural forms, **Figure 12**, supported by creation of hybrid NLT products which combine lumber laminations with layers of sheathing materials such as plywood and OSB to reinforce the system [14, 17]. Sheathing adequately nailed to NLT can create a diaphragm of the capability to resist lateral forces, and can also help keep the system dry if exposed to moisture [7]. In any such case, it is required to consider the system as an individually designed engi-

*160-m-long timber bridge with GLT deck structure in Mistissini, Canada. (Source: Photos obtained from* 

*Seven-story T3 Minneapolis building made of NLT floors and GLT beams and columns in USA (Source: Photos obtained from StructureCraft [16]).*

**Figure 12.**

*Qingdao Pearl Visitor Centre of exposed NLT roof in China (Source: Photo obtained from StructureCraft [16]).*
