Veneer-Based Engineered Wood Products in Construction

Abstract

Veneer-based engineered wood products (EWPs) are widely used in construction. Veneer-based EWPs are made of thin veneer sheets or veneer strands with adhesives, mainly including plywood, laminated veneer lumber (LVL), and parallel strand lumber (PSL). This chapter first discusses veneer-based EWPs in terms of their manufacturing, properties, and applications. Secondly, it introduces how veneer sheets or veneer strands intersect with each other in these products, providing additional strength and stable dimensions. Thirdly, this chapter overviews the effects of element dimensions, basic structure, veneer grade, adhesive type, and processing parameters on the properties of these products. Finally, it illustrates the uses of veneer-based EWPs through examples with a focus on their construction applications.

1. Introduction

Due to the significant population growth and the rising housing standards, the need to use structural wood products has been increasing [1]. At the same time, the timber industry must come up with solutions for ensuring the preservation of natural resources because of the growing demand for lumber and decreasing availability of large-diameter old-growth trees [2, 3]. Previously sawn from massive logs, structural lumber is now made from reconstituted wood in various shapes and sizes, which is classified as engineered wood products (EWPs). EWPs can maximize the use of wood and utilize small-diameter logs in comparison with conventional lumber [3, 4]. There are several types of EWPs in terms of the elements used, such as veneer-, strand-, fiber- and lumber-based EWPs, among which the veneer-based group is the oldest but still widely used.

The veneer-based EWPs, or called layered wood composites, are made of veneer sheets or veneer strands bonded with an adhesive [2], mainly including plywood, laminated veneer lumber (LVL), and parallel strand lumber (PSL), Figure 1. These products are largely made from peeled logs and reconstituted wood, which can then be fabricated into large sheets known as engineered panels [7]. The significant advantage of using veneer, as opposed to sawn lumber, is that it can increase the yield of wood materials from logs, particularly from small-diameter logs [8]. Veneer-based EWPs have a more homogeneous structure and uniform mechanical properties than solid lumber, making them a good candidate for building materials in construction.

Figure 1.

Veneer-based EWPs. Top left – plywood (source: photos obtained from Indiamart [5]) top right – thick plywood. Bottom left – LVL, bottom right – PSL (source: photos obtained from think wood [6]).

Veneer-based EWPs differ by wood species, adhesive type, as well as by layup structure. Figure 2 shows the cross-sections (i.e., the width-thickness plane, or x-y plane named here) of four widely used wood products in construction, i.e., solid wood/lumber, plywood, LVL, and PSL. In the y-axis, the dimensional change is similar between solid wood, plywood, and LVL due to limited efficacy of adhesive bonds in this direction, i.e., the radial direction of the wood. However, the dimensional change of PSL in the y-axis is smaller than that of the other three products because of its irregular arrangement of veneer strands in the x-y plane and application of an adhesive. On the x-axis, the dimensional change is largest in solid wood (Note: the x-axis is the tangential direction of the wood.) and smallest in plywood and PSL, with LVL being in between. In other words, solid wood has the largest variability in both x- and y-axes; plywood and LVL have the reduced variability in x-axes, and PSL has the smallest variability in both x- and y-axis.

Figure 2.

Cross-sections of solid wood and veneer-based EWPs.

The first type of veneer-based EWPs invented is plywood [9]. Later, modifications applied to the veneer layups resulted in LVL, and afterward, the long veneer strands were used to make PSL. The veneer-based EWPs have been widely used in construction nowadays [8]. Plywood is usually used as the sheathing material for walls, floors, and roofs, and the web stock for I-joists. LVL is commonly used as beams, columns, and the flange stock of I-joists. PSL is mainly used as columns and beams.

2. Plywood

2.1 Introduction

Plywood is a glued wood panel consisting of several thin layers of veneer with wood fibers in adjacent layers at right angles in most cases. Usually, a plywood sheet consists of an odd number of veneer layers [2, 3, 10]. Each layer is called ply, so the plywood can be deemed as a wood sandwich [7]. The cross lamination of adjacent plies in plywood contributes to improved mechanical properties and dimensional stability in both length and width directions [10].

Plywood is one of the oldest veneer-based EWPs. More than 3500 years ago, a type of plywood was found in ancient Egypt, which is part of the coffin, dating back to the third Egyptian dynasty [8]. Later, around 1500 BC, some images were discovered in which workers cut plywood with an axe-like tool. These images also show that the glue, apparently of animal origin, was prepared in a pot on fire [6]. Furniture constructed from overlapping sheets of wood and inlay had been discovered in Egyptian tombs. Hardwood veneer was preferred due to its attractive texture and shades [11]. The introduction of plywood was linked to the high cost of wood. Due to the shortage of available wood than supply, Egyptians had to import, by sea, ebony and mahogany from East Africa and cedar and pine from Lebanon at a very high price [12]. Later, the ancient Greeks and Romans started producing plywood. Plywood was primarily used for the manufacturing of furniture and household items [11]. Plywood production took off in the 1850s thanks to the Swedish inventor Emmanuel Nobel, who created a model of a rotary lathe [8]. This model made it possible to remove the veneer in a certain and constant thickness from a wooden block. It gave the plywood a uniform thickness and structure [8].

Despite the fact that plywood is now widely used for sheathing in residential and commercial construction, early builders were hesitant to use the newly-born plywood panels because the blood and soybean protein-based glues used were not waterproof, and some panels delaminated when they got wet [13]. In 1934, waterproof synthetic wood adhesives were introduced, which solved the problem and eased builders’ concerns [8, 13]. During World War II, the use of plywood was exploded in many industries such as boats, aircrafts, footlockers, crates, and buildings [13]. It led to the post-war boom in plywood production [8], which was adopted for structural and exterior applications. One notable example of using plywood is the construction of the legendary bomber Mosquito [14]. This aircraft was introduced during the World War II. Spruce wood, birch plywood, and balsa wood were used in the construction of aircraft, which made it possible to achieve the necessary strength with a low weight structure [15]. Plywood and other structural panels have changed the way of constructing light wood-frame houses and buildings [11, 16]. Since the middle of the past century, usage of structural panels has expanded from a few niche applications to a popular commodity such as subflooring, roof and wall sheathing, corner bracing, and concrete forming [16]. Initially concentrated in the Pacific Northwest of the United States, where old-growth, large-diameter Douglas-fir was mostly used the plywood business therefrom expanded into the southeastern regions in the 1970s as the technological barrier of bonding southern yellow pine veneer was removed [13]. As seen from Figure 3, plywood consumption in Canada was rather stable in the last 15 years or so despite the emergence of other new types of building materials. However, Canada also imports plywood from other countries to meet its increasing demand in construction and other industries such as furniture [17].

Figure 3.

Plywood production and consumption in Canada [17].

2.2 Manufacturing

Figure 4 illustrates the key processes of manufacturing three major veneer-based EWPs, i.e., plywood, LVL, and PSL. An example of manufacturing Canadian softwood plywood is given below, which is used for structural applications. Specially chosen peeler logs are transported to a barker, where they are rotated against a steel claw, which removes the bark [18]. Then debarked logs are cut into peeler blocks. A block is placed on a massive lathe, rotating against a sharp knife. When the block turns, a continuous thin layer of wood, i.e., veneer is peeled off, similar to how paper unwinds from a roll.

Figure 4.

Processes of manufacturing veneer-based EWPs.

The whole block is tried to use with an aim to generate a high yield of good quality wood material. The leftover small spindles are used to make other wood products. The long ribbon of the veneer is then cut with clippers into desired widths and sorted. It is also possible to remove defective pieces of veneer. Subsequentially, the veneer is dried to a moisture content of 5% or so in steam- or gas-heated ovens [18]. Depending on its intended use, the veneer may range in thickness from 0.3 mm (0.01 in) to 6.3 mm (0.25 in) [11]. After drying and sorting, the veneer is fed by glue spreaders, which apply an adhesive layer of uniform thickness. Phenol-formaldehyde (PF) adhesives are usually used in the manufacturing of plywood for structural and outdoor applications when exposed to the weather in its service [3]. Veneer sandwiches are sent to the hot press, which is a key step in the production process of curing the adhesive, subjected to a temperature of 150°C (300°F), and a pressure of 1.38 MPa (200 psi). After the press panels are cut to required dimensions, sanded, and graded [18].

In the fabrication of plywood for non-structural uses, such as furniture, cabinets, and indoor decoration, water-resistant urea-formaldehyde (UF) adhesives are used. The UF adhesives can be cured at a temperature of about 120°C (250°F) during hot-pressing, which can also be cured with high-frequency heating system with an aim to reduce the hot-pressing time and increase the production efficiency [3].

Quality control, which includes incoming management of raw materials, such as wood and glue, and manufacturing parameters at all stages of the production, must be applied in order to produce good quality plywood products. Acceptance quality control is the final stage of the manufacturing process. Many plywood manufacturers in western Canada produce structural plywood under the supervision of the British Columbia Council of Forest Industries (COFI), which constantly checks glue bond strength and other properties to guarantee that the products satisfy the Canadian Standards Association (CSA) standard [18].

2.3 Typical species and sizes

Plywood can be made from various types of wood. Softwoods are commonly used to make veneer for plywood in North America, containing Douglas fir, western hemlock, spruces, pines, and firs [14]. These wood species can be divided into various categories based on their strength and use within the plywood structure. Spruce is used to make the majority of construction-grade softwood plywood in Canada [7]. More discussion on softwood plywood is given through the text in following sections.

Of hardwoods, birch, alder, linden, and lauan (“Philippine mahogany”) are most popular for veneer production [7, 10]. These species do not have distinguished earlywood and latewood zones, which are characterized by uniform density and structure, making them easy to be peeled to produce thin and durable veneer.Beautifully grained hardwoods are often combined in several ways to make a unique face pattern [7].

The first standard plywood sheet had a width of 1.22 m (4 ft) and a length of 2.44 m (8 ft), which appeared in 1928 [19]. Such a standard size for plywood sheets has, since then, almost not been changed. The common thickness of plywood varies from 3.2 mm (1/8 in) to 76 mm (3 in) [10]. It depends on the thickness of the veneer and the number of layers. The most common plywood contains 3, 5, or multiple layers. With a three-layer, the plywood is 2–3 mm (0.08–0.12 in) in thickness, which can be used as an underlayment between the subfloor and the tile. Hardwood decorative plywood is often uniformly selected for grain texture, which is widely employed for indoor uses. The universal hardwood plywood has five layers, resulting in a thickness of 4 mm (0.16 in) or so, which can be used for multiple outdoor and indoor applications. Multiple layer plywood with more than seven layers can be classified as thick plywood, which is widely used for structural purposes, requiring acceptable strength and durability under the loading condition [11]. The thick plywood needs a sub-floor or structural sheathing attached to the framing elements of a new canopy.

Hardwood can be peeled or sliced for the production of decorative veneer for making furniture, cabinets, and interior decoration. Slicing results in more loss in raw materials and more intensiveness in labor [16]. Hardwood veneer, such as birch, usually has a thickness of 1.5 mm (0.06 in), whereas softwood veneer is often cut to a thickness of 3 mm (0.12 in) for plywood and LVL production [8].

2.4 Grading

Plywood comes in a range of appearance grades, from flat natural surfaces suitable for finishing to cost-effective unsanded grades suitable for sheathing. More than a dozen typical thicknesses and over twenty different grades of plywood are available [14]. The plywood is usually graded based on the appearance quality of veneer in North America. There are commonly two classes of plywood, each of which has its own set of standards: (a) construction and industrial plywood and (b) hardwood and decorative plywood [3].

In Canada, the two most popular types of softwood plywoods are unsanded sheathing grade Douglas Fir Plywood (DFP), which conforms to CSA O121 “Douglas fir plywood”, and Canadian softwood plywood (CSP), which conforms to CSA O151 “Canadian softwood plywood”. The poplar plywood, which conforms to CSA O153 “Poplar plywood”, is also designated but less uses in construction [14]. The group of DFP can include other species in addition to Douglas fir. For example, the front and back faces are made of Douglas fir, but the inner plies can be made from any of the specified species, including Douglas fir, western hemlock, and the majority of spruce, pine, and fir species in Canada [14]. Plywood that contains other selected Canadian wood species in the face and back plies is labeled CSP. Most species that are only allowed as inner plies for DFP may also be used as the face or back plies for CSP. Three hardwood species, i.e., balsam poplar, trembling aspen, and cottonwood, are restricted to use as inner plies in DFP and CSP [14]. The sizes, grades, specialty panels, manufacturing tolerances, and glue bond quality of plywood are all stipulated in the standards CSA O121, CSA O151, and CSA O153. The structural plywood is put with a legible and durable stamp showing the manufacturer, the bond style (EXTERIOR), the species (DFP or CSP), and the grade [14]. DFP and CSP are both made in a variety of grades based on the appearance and quality of the veneer used for making the outer plies.

Many plywood mills are members of the associations, which are responsible for inspecting, testing, and certifying the products with stamps. These stamps indicate that the stamped products meet the standards accepted by the associations. One of the largest associations in North America is APA – The Engineered Wood Association (formerly American Plywood Association) [20]. There are usually two letters on a stamp, the first indicating the quality of one surface, while the second showing the quality of the opposite surface, Figure 5 [7]. This stamp ensures the customer that this product has followed the association’s stringent quality and efficiency standards [3]. In Canada, the CertiWood™ Technical Center (formerly CANPLY– the Canadian Plywood Association), a non-profit, industry-funded association, represents manufacturers of EWPs [21]. Those mills, being the members of CertiWood™ Technical Center, can put the stamp with the trademark CANPLY on their products [7, 21].

Figure 5.

A sample stamp on plywood: 1- panel grade - panel grades are generally identified in terms of the veneer grade used on the face and back of a panel (e.g., A-B, B-C); 2- bond classification - exposure ratings for APA wood structural panels may be exterior or exposure 1; 3 - decimal thickness declaration; 4 - mill number - manufacturing mill identification number; 5- species group number - classified according to strength and stiffness under manufacturing standard; 6 - product standard [20] (source: photos obtained from APA – the Engineered Wood Association [20]).

The vast majority of construction and industrial plywood is used in applications where structural performance surpasses appearance. Some construction and industrial plywood are manufactured with faces chosen mainly for appearance of either plain natural finishes or lightly pigmented finishes [3]. Structural plywood is available in two exposure durability classes: interior and exterior [13]. INTERIOR plywood is only intended for use in dry indoor applications where the panels should be protected from moisture permanently; which is even glued with a water-resistant interior-use adhesive [13]. EXTERIOR plywood is the only panels suitable for outdoor exposure. They are bonded with a waterproof exterior-use adhesive [13], including EXPOSURE 1 and EXPOSURE 2. EXPOSURE 1 panels are waterproof and designed for applications where long construction delays or exposure to high moisture in service are possible [13]. EXPOSURE 2 panels are water-resistant and designed for protected applications, where only minor construction delays are expected since they are mainly developed for interior use [13]. Sheathing grades that are not listed for appearance usually have the grading stamp on one of the faces, whereas grades such as Good Two Sides are stamped on the edge to avoid affecting the appearance. The strength values stipulated in CSA O86 “Engineering design in wood” [22] are used for Sheathing grade panels based on layups containing only C-grade veneer. Typical DFP and CSP grades include Sanded grades, primarily used in concrete formwork or non-structural applications, and Select and Select Tight Face grades, which are primarily used in floor underlayment applications requiring a smooth and solid surface [14].

Chemical treatments can be applied to plywood to increase its resistance to decay and fire. In Canada, the preservative-treated plywood must be made following CSA O80 “Wood preservation” [23]. To assess the effects of fire retardants or some other potentially strength-reducing compounds, plywood producers shall conduct tests following ASTM D5516 “Standard test method for evaluating the flexural properties of fire-retardant-treated softwood plywood exposed to elevated temperatures” [24] and ASTM D6305 “Standard practice for calculating bending strength design adjustment factors for fire-retardant-treated plywood roof sheathing” [14].

2.5 Properties

The density of plywood depends on the wood species and thickness used, which varies from 400 kg/m³ (25 lb./ft³) to 800 kg/m³ (50 lb./ft³) [3]. This is compared to the density of oven-dry wood, ranging from approximately 320 kg/m³ (20 lb./ft³) to 720 kg/m³ (45 lb./ft³) [3]. Plywood has good machining properties; thus, it is possible to work with it just like with ordinary wood, such as sawing, nailing, and gluing. However, the cross-lamination design of plywood, in contrast to wood that is broken down the grain, prevents it from splitting readily in the grain direction. As a result, screws and nails can be used in structural applications near the edges of plywood panels.

Plywood has exceptional built-in resistance to raking, twisting, or distortion, which is especially crucial when care is taken for transferring large shear stresses generated by powerful winds or earthquakes [11]. Many strength properties are equalized by changing the direction at 90 degrees to the grain with each consecutive wood layer of veneer. This provides plywood with a two-way capacity, i.e., the properties in the width direction are approximately equal to those in the length direction. For example, 6 mm (1/4 in) sheathing plywood on a typical framed construction wall with doors and windows delivers double the rigidity and strength furnished by 19 mm (3/4 in) thick boards laid diagonally. When glued to the framework, the strength values for plywood walls are raised even further [11].

Because structural plywood uses waterproof resins, a weather-resistant panel can be obtained if the edges are properly sealed [10]. Awareness of the allowable design values of a plywood panel is not required except in special engineering applications such as diaphragms and earthquake-resistant shear walls. When properly fastened to framing at the correct spacing, the span ratings alone ensure that the panels can work well under the roof and floor loadings stipulated in the building code. In North America, the design values can be found in CSA O86 “Engineering design in wood” [22], Wood Design Manual [25], and APA – Plywood Design Specification [20] or in its Design Capacities of APA Performance-rated Structural-Use Panels Technical Note N375 [26]. For engineering applications, STRUCTURAL I panels are typically the best choice. The typical values of sheathing grades are listed in Table 1. The properties of plywood vary with the quality of the constituent layers.

Mechanical properties	Metric	Imperial	Remarks
Tensile Strength	27.6–34.5 MPa	4000–5000 psi	Parallel to face; ASTM D3500 [28]
Modulus of Rupture	48.30-68.90 MPa	7000-10000 psi	Parallel to face; ASTM D3043 [29]
Modulus of Elasticity	8.200-10.300 GPa	1190–1490 ksi	Parallel to face; ASTM D3043 [29]
Compressive Strength	31.00–41.40 MPa	4500–6000 psi	Parallel to face; ASTM D3501 [30]
Shear Modulus	0.138–0.207 GPa	20–30 ksi	In-plane (rolling shear) ASTM D2718 [31]
	0.586–0.758 GPa	85-110 ksi	Through thickness (edgewise shear) ASTM D2719 [32]
Shear Strength	1.72–2.07 MPa	250–300 psi	In-plane (rolling shear); ASTM D2718 [31]
	5.52–6.89 MPa	800–1000 psi	Through thickness (edgewise shear); ASTM D2719 [32]

Table 1.

Mechanical properties and testing standards of plywood (source: Wood Engineering Handbook [27]).

2.6 Applications

Plywood is widely employed in structural and non-structural applications [3], which can be an ideal option for use in both wet and dry environments [14]. It was reported that plywood took about 54.8% of the market share in 2017 in the construction sector in North America Figure 6 [21].

Figure 6.

North America plywood market share by segment in 2017 (source: Figure obtained from BCC research, FAO, RAUTE, IMF, National Statistics Offices [33]).

In construction, plywood is mainly used as a load-bearing element in platform-frame structures, including single-family and multi-family housing, such as sheathing and underlayment, since it has good dimensional stability and does not crack, cup, or twist [18]. Plywood panels are used as wall sheathing materials, providing high lateral resistance to shear walls and high racking strength, and assisting in achieving the overall thermal efficiency of walls [16, 18]. Roof sheathing is frequently made of plywood. The stiffness of which constitutes diaphragm action when using prescribed framing and nailing patterns [18]. Also, plywood often finds its uses in the fabrication of I-joists as web stocks, marine applications, pallets, industrial containers, and furniture, Figure 7. Extra thick plywood with special surface treatment can be used for facing concrete formwork in concrete structures [7, 10, 14].

Figure 7.

Plywood applications [20, 34, 35, 36].

3. Laminated veneer lumber (LVL)

3.1 Introduction

Laminated veneer lumber (LVL) is a type of structural composite lumber (SCL) made by gluing several layers of veneer in the longitudinal direction of the wood, which differs from plywood that has the veneer layers cross-laminated. LVL is one of the most important members in the family of veneer-based EWPs [8]. This material was initially used to produce aircraft propellers and other high-strength aircraft components during World War II [18, 37]. The research and development of LVL can be dated back to the 1940s with an aim at making high-strength parts for aircraft structures out of Sitka spruce veneer [3]. LVL was used as a building material since the mid-1970s [18, 37, 38] when the research was focused on examining the effects of manufacturing variables on LVL with a thickness being up to 12.7 mm (1/2 in) [3]. LVL is now widely used as building and packaging materials [18].

3.2 Manufacturing

The veneer manufacturing and drying processes are almost the same as those used in making plywood. Figure 4 illustrates the different processes in the manufacturing of LVL from plywood, largely including veneer orientation during layup, hot press type, and end cutting to produce the length required.

To make veneer sheets, the logs are usually peeled in a lathe. The thickness of veneer sheets in 1.5 mm (0.06 in) up to 6.4 mm (0.25 in) [16, 38], the length is 2640 mm (104 in), and the width is 1320 mm (52 in) or 660 mm (26 in) [18]. The veneer is dried to a moisture content of 6–10%, ideally 6–8% [38]. The veneer is clipped to remove any strength-reducing defects and graded. The veneer sheets are cut to the desired width for billet production [18]. The individual veneer sheets are then joined, with the grain of all veneers running in the direction of a billet’s length direction. End joints between different veneer pieces are staggered along the length of the billet to distribute any defects that could reduce strength. To effectively transmit load, the joints might be scarf joined or overlapped for some distance [18]. Then the veneer sheets are covered with a waterproof phenol-formaldehyde adhesive [18, 37, 38] or phenol-resorcinol-formaldehyde or polyurethane adhesives [39].

The veneer layup of LVL differs from that of plywood. In the production of LVL, the veneer is oriented in the same direction, i.e., the longitudinal grain direction of the wood, providing the super strength in this direction, which is similar to or larger than solid lumber, Figure 8. Thus, LVL is commonly used for beams and columns in the construction of buildings [8, 10]. Veneer sheets in plywood are cross-laminated, making it possess two-way properties, i.e., similar properties in both major and minor directions, as mentioned in Section 2, suitable for sheathing materials.

Figure 8.

Layups of LVL and plywood (source: image obtained from Gong [39]).

The pre-pressing of LVL billets could be carried out in a single-opening cold press or a short continuous cold press [38]. The completed billets are simultaneously exposed to pressure to consolidate the veneer and heat to accelerate the curing of the glue [18]. In general, the press temperature used to produce LVL is rarely higher than 175°C (350°F). For the batch type presses, it is usually 160°C (320°F). For the continuous presses, the temperature might be significantly higher since there is a pre-heating zone. As veneer sheets are relatively low in permeability, it is recommended to avoid using a high press temperature, especially when combined with a long press time [38]. This process is similar to used in manufacturing of plywood, except that instead of being formed into thin flat panels, the veneer sheets for making LVL is formed into long billets up to 25 m (80 in) in length. After curing, the billets are sawn to specific lengths and widths for the target application(s) of a LVL product [18].

During the manufacturing of LVL, the selection of veneer is, in terms of thickness and grade, of great importance. The right veneer thickness can help balance LVL properties and manufacturing costs in the production. The veneer used in the manufacture of LVL must be carefully selected in order to obtain the desired engineering properties. Ultrasonic scanning is often used to sort veneer sheets to ensure that the final product has the desired engineering properties [37]. The individual veneer is typically graded so that the strength characteristics of each LVL can be customized [10]. For esthetic reasons and superior flatwise bending properties, the best veneer sheets are usually used as surface plies, while lower-grade sheets are used for the inner plies [38]. For example, if the final use of LVL is scaffold planks, the higher grade veneer will be put on the plank’s outer sides [18].

With decreasing veneer thickness, the number of veneer sheets required increases for the same density, thickness, and layout technique of an LVL product. As a result, defects in LVL with thinner veneer will disperse more defects than in LVL with thicker veneer. Because of this, as the veneer thickness decreases, the variation diminishes, and the strength values increase. However, as veneer thickness decreases, resin content, press cycle time, and production cost increase [38].

The strength properties of veneer are more critical for LVL than those in plywood in general. As a result, it is highly desirable for LVL producers to avoid using the veneer sheets with deep lathe checks that cause a reduction in veneer strength. Deep lathe checks can decrease LVL’s shear strength and stiffness while having little effect on its MOE. Low shear rigidity can reduce LVL’s MOE rating [38].

3.3 Typical species and sizes

LVL can be made from different softwood and hardwood species; however, in North America, Douglas-fir, larch, southern yellow pine, hemlock, aspen, and yellow poplar are the most widely used wood species for producing LVL [18, 37, 40].

LVL is available in thicknesses ranging from 19 mm (3/4 in) to 89 mm (3–1/2 in) and likely to 178 mm (7 in) [18, 41]. The most typical thickness of LVL used in construction are 38 mm (1–1/2 in) [3] and 45 mm (1–3/4 in) [18], from which broader beams can be conveniently assembled on a job site by fastening several LVL plies [18]. The typical depth is from 140 mm (5–1/2 in) to 508 mm (20 in). Different manufacturers can also provide different widths and depths. At the job site, LVL can easily be cut to a length required [37]. Typical lengths of LVL are 14.6 m (48 in), 17 m (56 in), 18.3 m (60 in), 20.1 m (66 in), and 24.4 m (80 in) [10, 37, 41]. LVL is manufactured in the form of billets with widths of 610 mm (24 in) or 1220 mm (48 in). The required depth of LVL can be cut from these billets [18].

3.4 Grading

LVL is a proprietary product; therefore, its engineering properties and sizes can differ from one manufacturer to another. As a result, there is no general production standard or design values in the LVL industry [37]. However, the Canadian Construction Materials Centre (CCMC) reviews and approves the design values, which are derived from test results following CSA O86 “Engineering design in wood” and ASTM D5456 “Standard specification for evaluation of structural composite lumber products” [37]. Each manufacturer develops the characteristic properties of its LVL products by in-grade testing. The manufacturer is also responsible for checking the properties of its products by constant monitoring and quality management. Each manufacturer publishes its own list of design properties, resulting in a unique grade for a given LVL product [42]. Products that satisfy the CCMC criteria are assigned an Evaluation Number and an Evaluation Report that describes the design strengths. They are then entered into the CCMC’s Registry of Product Evaluations. The manufacturer’s name or product marking, as well as the stress grade, are stamped on the material at different intervals, although this may not be present on every piece due to end cutting [37].

Figure 9 presents a stamp of LVL from APA – The Engineered Wood Association, which shows a qualified LVL grade (e.g., 3100F_b-2.0E), product evaluation reports, the treatment facility, and standard specifications for SCL.

Figure 9.

LVL stamp: 1 – qualified LVL grade (usually represented by design values; 2 – APA mill number; 3 – product evaluation reports; 4 – standard specification for structural composite lumber (source: photo obtained from APA – the Engineered Wood Association).

3.5 Properties

The density of LVL is about 480–510 kg/m³ (30-32 lb./ft³) [43], which is similar to that of the wood made from. Compared to solid wood, LVL has more stable characteristics than solid timber. This is due to the fact that natural defects, such as knots, splits and slope of grain, are dispersed throughout the material or completely removed during the manufacturing, and dried veneer and adhesives are employed [37].

LVL can easily absorb water, resulting in the change in dimensions, in particular in the thickness direction since there are almost no adhesive restrictions. Therefore, LVL should be protected from the weather during job site storage and after installation [3, 37]. Wrapping the LVL materials for shipping to the job site is also critical for minimizing the moisture effect. End and edge sealing are the commonly used approach to avoid moisture penetration and protect LVL products in their services [37].

Both special cutting, notching, or drilling should be performed according to the manufacturer’s instruction. LVL acts similarly to solid sawn timber or glue-laminated beams of equal height, which requires the same fastening and connection requirements as solid lumber [40]. The primary sources of knowledge for design, standard installation descriptions, and performance characteristics are provided in the manufacturer catalogs and inspection reports [37].

3.6 Applications

LVL is mainly used as structural framing in residential and industrial buildings. In the building industry, LVL is widely used for beams or headers over windows and doors on the edge, for hip and valley rafters, scaffold planking, and the flange material of I-joists [3, 10]. LVL may also be used as truck bed decking and road signposts. LVL is chiefly used as a structural component, most commonly in hidden spaces where esthetics is not a concern. Certain manufacturers offer a finished or architectural grade look, but it typically comes at a cost. When using LVL in applications where esthetics is significant, standard wood finishing techniques may be used to accent the grain and preserve the surface layer. The finished wide outer layer of LVL looks like plywood [37]. Figure 10 shows such a complex curved structure constructed Burnaby, British Columbia, Canada, which was designed and built with 53 parallel CNC-cut spruce LVL sections [44]. Each curved structure was panelized into six segments, shipped to the construction site, and assembled into one piece [44].

Figure 10.

Curved LVL structures at Simon Fraser University (Canada) - Ripple Cone Canopy (source: photos obtained from StructureCraft [44]).

Veneer-based EWPs have also been used in the windmill industry, in which wood veneer sheets are used to make windmill blades [45]. Previously, the size of the wooden blade was constrained by the availability of large, consistent-quality tree trunks. Veneering, on the other hand, spreads out defects like knots, resulting in more substantial and more predictable stiffness properties. This makes it possible to make larger wooden blades. When compared to fiberglass, wood laminates provide substantial cost and reduced weight. There are examples of blades made primarily of LVL reinforced with carbon composite spars and coated with a fiberglass composite outer layer [46]. One of the largest windmill blades is 107 meters long (351 ft), which is longer than a football field, produced in Cherbourg, France. It was made from a high-tech sandwich structure consisting of thin layers of glass and carbon fibers and balsa wood veneer [47].

4. Parallel strand lumber (PSL)

4.1 Introduction

Parallel strand lumber (PSL) is known as a composite of veneer strands with wood fibers aligned primarily along the length of the member, i.e., the longitudinal direction of wood [3]. PSL is overall similar to laminated strand lumber (LSL) and oriented strand lumber (OSL) but is made up of veneer strands (sometimes called veneer strips). The length of veneer strands used in PSL is longer than the strands used in LSL and OSL, with a length-to-thickness ratio of around 300.

PSL was invented in 1975 by MacMillan Bloedel Ltd., in Vancouver, Canada, who set out to create a high-strength wood-based material [13]. The first PSL plant was opened in 1982, and its products were first commercially sold for Expo ‘86. MacMillan Bloedel, which is now called Weyerhaeuser, commercialized and patented its PSL products with the brand name Parallam®. The process has been improved over time to produce relatively giant and long beams, and the production and sales have steadily increased [48].

4.2 Manufacturing

The process of manufacturing PSL allows prominent members to be built from small trees, resulting in the more efficient use of forest resources [49]. The first stages in the production of PSL are similar to those used in the production of plywood or LVL. Figure 4 illustrates the unique processes employed in the manufacturing of PSL, differentiating from those in plywood or LVL. To make veneer, logs are turned on a lathe [18]. The thickness of veneer is from 3 mm (1/8 in) to 6.4 mm (1/4 in) [3]. The veneer sheets are then dried to a moisture content of 2–3% before being sliced into long thin veneer strands parallel to one another [9]. After that, the veneer sheets are clipped into long, narrow veneer strands with a length of 2.4 m (8 feet), a width of 13 mm (1/2 in) [18], and a thickness from 2.54 mm (1/10 in) to 3.175 mm (1/8 in) [38].

The production process is designed to use materials from the log roundup and other less than full-width veneer in the veneer cutting stage. As a result, the process uses waste materials from a plywood or LVL operation [3]. The veneer strands are oriented to the length direction of a continuous billet using special equipment (Figure 11) and mixed with a waterproof exterior structural adhesive, such as phenol-formaldehyde, prior to hot-pressing.

Figure 11.

Orientation of veneer strands in PSL (source: photos obtained from WorldPress.com [50]).

The pressing process densifies the veneer strands to some degree, and the adhesive is cured with the aid of microwave technology [18, 49]. A continuous press is employed to produce PSL, which theoretically produces an unlimited length but is constrained only by transportation restrictions [3].

4.3 Typical species and sizes

Douglas fir is used to produce PSL in Canada, and southern yellow pines are employed in the USA. In addition to this, western hemlock and yellow poplar are also used [3, 49, 51]. In general, there are no restrictions on the use of other wood species.

The available stock sizes for PSL have to be compatible with existing wood framing materials and standard dimensions [18]. PSL beams are available in thicknesses from 89 mm (3–1/2 in) to 178 mm (7 in), and in-depth from 235 mm (9–1/4 in) to 457 mm (18 in) [41]. PSL columns come in square and rectangular shapes of a dimension of 89 mm (3–1/2 in), 133 mm (5–1/4 in), or 178 mm (7 in) [41]. Smaller thicknesses can also be used, either individually as single plies or in combination for multi-ply applications [18, 49]. Steel connectors are usually required for larger dimensions [51]. PSL is available in lengths up to 20 meters (66 ft) [41].

The beam-like PSL products can be also ripped into thin boards, Figure 12, which opens a window for non-structural applications [18].

Figure 12.

Boards ripped from PSL (source: photos adopted from Goulddesigninc.com [52]).

4.4 Grading

PSL is a proprietary product, the same as LVL. Therefore, specifications and dimensions are unique to each manufacturer. In North America, both PSL and LVL are treated as the same structural composite lumber [18]. The evaluation procedure and grade determination of PSL are the same as LVL (refer to Section 3.4). Figure 13 presents a stamp of PSL (Parallam® Plus), including a description of the product and uses, the type of treatment, and the treatment facility. The treatment stamp can also reference the treating standards (such as AWPA U1/UC4A by the American Wood Protection Association) and third-party quality program monitor (SPIB - Southern Pine Inspection Bureau) [53].

Figure 13.

Stamp on PSL (Parallam® plus) (source: image obtained from Weyerhaeuser [53]).

4.5 Properties

Since natural defects such as knots, the slope of grain, and splits have been scattered across the material or eliminated during the manufacturing process. The combination of a structural adhesive used with dried wood veneer strands, heat, and pressure employed during pressing makes PSL less warping than solid timber. Therefore, PSL is a type of highly consistent, uniform EWPs [49], which exhibits much less variability and larger load-bearing capabilities than solid lumber [51]. The density of PSL is 720 kg/m³ (45 lb./ft³) [54], which is similar to that of the wood used. Other advantages of PSL are given to its high strength, stiffness, and dimensional flexibility [49]. PSL is less susceptible to shrinkage, warping, and splitting as it has a moisture content of 11% [49].

The texture of PSL is rich due to the grain of wood veneer strands and dark glue lines. PSL is a visually appealing construction material that fits well to the applications that require a high level of finished appearance [49]. The techniques applicable to sawn lumber can be used to machine, stain, and finish PSL. At the end of the manufacturing period, PSL is sanded to ensure exact dimensions and a high-quality appearance. Stain can be used to emphasize the warmth and texture of the wood [49]. It should be pointed out that the special cutting, notching, or drilling of PSL shall be performed in compliance with the manufacturer’s instruction.

As mentioned above, both PSL and LVL are treated as the same SCL in Canadian practices; therefore, their design values are the same. Table 2 lists the key strength properties of sample SCL. Some specific design values can be obtained from manufacturers [41].

Property	Value (MPa)
Modulus of elasticity, E	13,800
Allowable bending stress, fb (300 mm depth)	37.0
Allowable shear stress, fv (perpendicular to glue line or wide face of the strand)	3.7
Allowable bearing stress, fcp (parallel to glue line or wide face of the strand)	9.4

Table 2.

Major strength properties of SCL (source: Canadian Wood Council [41]).

4.6 Applications

PSL is mainly used in residential, commercial, and industrial construction as structural framing components, such as beams and columns in the post-and-beam construction, and headers, pillars, and lintels in the light-frame construction [18]. According to Part 3 of the National Building Code of Canada, CCMC has approved PSL for use as heavy timber construction [51]. Due to the excellent strength characteristics of PSL, it is possible to use it in the design of the roofs with a large span and rooms with open spaces. Figure 14(left) presents PSL beams that are used for an open floor plan at Vancouver Firehall No.15, Vancouver, Canada. Another example is presented in Figure 14(right), showing a portable Concord Pacific Display Pavilion at Vancouver, Canada, which uses a glass-enclosed superstructure made of exposed PSL columns. In addition, PSL is a visually attractive material; thus, it is well suited to applications where the finished look is essential. It can be also appropriate for hidden structural applications where appearance is unimportant [18].

Figure 14.

Floor and columns made of PSL in Vancouver Firehall No.15 (left) and Concord Pacific display pavilion (right), respectively (source: photos obtained from StructureCraft [55]).

5. Endnotes

EWPs are relatively recent structural members that have been widely incorporated in the building industry in North America and beyond. They have been invented and used for making timber buildings and furniture. The family of veneer-based EWPs mainly has three major members, i.e., plywood, LVL, and PSL. Because the majority of veneer-based EWPs are designed to handle relatively large loads, they must be manufactured in accordance with recognized standards or technical guides to ensure that the required engineering design values and applications are met. Veneer-based EWPs have been accepted and acknowledged in the building industry as premium structural materials. It is possible to make these products considerably large from small-diameter logs. The only restriction can be the length of LVL and PSL during transportation [38].

Non-traditional resources (such as under-utilized wood species) can be used to manufacture veneer-based EWPs of better physical and mechanical properties than other traditional structural products (such as solid timber products) [1]. Due to engineering design, removal of defects, drying of wood materials, application of adhesive, and layer-by-layer bonding, the veneer-based EWPs are stronger and more durable than solid wood of the same size. This is an outstanding advantage for constructing a building requiring high strength without a bulky appearance. Typically, LVL and PSL have about three times larger bending strength and 30% larger stiffness than the lumber products of comparable sizes [41].

Interest in veneer-based EWPs products will continue to grow for ecological reasons. For example, restrictions have been introduced in many countries on deforestation of large-diameter old-growth trees. Due to the needs in the construction market, alternative materials must be further developed. The rapid advancement in technology, along with the available raw materials, i.e., small-size fast-growth trees, would inevitably accelerate the development of EWPs [2]. Also, in recent years, the minimization of carbon footprints in construction has reached a consensus. Many architects and engineers have been designing and constructing buildings with 100% solid wood and EWPs. Meanwhile, research on the standardization of the veneer-based EWPs and expansion of their uses is no doubt required. Modernization of existing equipment and improvement of the gluing systems will allow the creation of innovative designs and special shapes that are currently not available for wood products. This will certainly expand the matrix of applications for veneer-based EWPs, as well as making them become more competitive in the market of building materials.

Acknowledgments

This piece of work was financially supported by the New Brunswick Innovation Research Chair Initiative Program by the New Brunswick Innovation Foundation (Canada) and the Collaborative Research and Development Grants by Natural Sciences and Engineering Research Council of Canada (CRDPJ 523922-18).

Conflict of interest

The authors declare no conflict of interest.