Structural Composite Lumber (SCL)
Structural composite lumber (SCL) is a categorization of a group of structurally engineered products that all follow a general application trend. Before specific structural calculations are carried out determining the appropriate material to be used, an architect may highlight that a certain component on a drawing requires the integration of an SCL beam, allowing an engineer to select the correct material. The specific types of SCL follow similar manufacturing techniques as those we’ve already looked at, although the scale of the process is typically much larger. SCL components are not really off the shelf items, and are mostly custom made for specific purposes, though there are some exceptions to this rule in the form of smaller beams. Here we shall look at the different varieties of Structural Composite Lumber.
Wooden I-beams
Wooden I-Beams, also known as timber or wooden I-Joists, are an engineered timber product that are cost effective in their application. I-joists are lightweight, engineered flooring or ceiling joists. They are named as such due to the shape of their through-section, resembling the shape of a capital “I”. These beams are formed with a strip of thin sheet material sandwiched in between 2 routed flanges. There are many variations of these joists in terms of the material combinations used for their construction. Some of these variations include timber batten flanges with Plywood, Laminated Veneer Lumber (LVL) or OSB webs, with other types of manufactured timber materials sometimes being used for the flanges, such as Plywood.
Another name by which Timber I joists can be referred to is TJI Joists, which stands for Trus Joist I-Joist, and is a specific product of the Trus Joist company, though the name of their product has become synonymous with the general concept of I-joists.
Some advantages of I joists over traditional solid timber joists include weight, reactivity, cost, and efficiency. I-joists are generally lighter per length in comparison to a regular joist, thus making the installation of the system much easier. Because of their composite and engineered assembly, they are much less likely to twist, bow, cup, shrink, or expand in comparison to standard timber joists, which allows for a more perfect flooring system. Due to reduced shrinkage and movement in I-joists, squeaks and bounces are very unlikely to develop in finished flooring systems over time. They possess tremendous load bearing capacity in comparison to their size and weight, rendering them more cost effective and labour saving than regular timber joists.
Whilst timber I-joists can be bought at standard sizes and be cut for purpose for smaller jobs, manufacturers are able to produce specific floor layout systems for larger buildings, which include all joists being cut to length, and a floor plan dictating the specific layout to be followed. This is time and labour saving, and can be worth the extra cost of manufacture when weighing up prices against regular timber joists.
Glulam
Glulam timber, or Glued Laminated Timber, refers to an engineered timber product typically used for heavy duty load bearing applications. As the range of engineered wood products go, this variety is one of the among the simplest to produce. The general premise involves taking numerous smaller lengths of raw timber and glueing them together under pressure. With the joints staggered and meshed in a stacked configuration, a longer, more stable and more structurally suitable length of timber is created. This eliminates some of the issues that would occur from creating a beam of a similar size out of one solid piece, namely eliminating potential warping and movement. Due to the specific arrangement of the lengths within a glulam beam and the staggering of the joints, manufacturers can eliminate potential weak points in the beams. Within standard construction, glulam beams can typically be seen to be used as ridge beams in long vaulted ceilings, as well as load bearing support beams for large floor joist systems and large structural openings.
Glulam beams possess a fairly aesthetic appearance and can often form a feature or focal point within a construction, sometimes without serving any structural benefit. Glulam beams can also be shaped or steam bent during their construction, allowing for the creation of very interesting modern timber arches or curves. In these applications, they can often be seen in modern architectural works or sculptures. Another great strength of glulam beams is that they are water and moisture resistant, making them a perfect material for the creation of sports halls and swimming pools.
Laminated Veneer Lumber (LVL)
Laminated Veneer Lumber (LVL) is a type of Structural Composite Lumber (SCL) that is similar in its construction to that of plywood. In the manufacturing process, thin veneers are glued together under heat and pressure to form thick beams. LVL material differs from plywood in that the veneers are generally much longer, and are all oriented in the same direction, whereas the veneers in plywood are rotated 90 degrees with every layer. LVL beams can be used in large structural applications, for example a large ridge beam running through a roof, as well as in smaller structural applications such as structural opening headers in walls. Having reviewed the different catalogues of LVL beam manufacturers and suppliers, existing engineered products range from beams in thickness from around 2” (45mm) all the way up to 5 ¼” (135mm) and can reach as much as 2 feet(610mm) in depth, with some manufacturers offering beams up to a staggering length of 25m. These are just off the shelf products, with any size and length being available to order, calculations and cost depending.
LVL beams share many of the advantages of Glulam beams, in that they are structurally sound and are very resistant to movement, making them a great choice for use in structural applications. Their long length and high load bearing capacity are other benefits of their construction. Due to their many layers, they are rated amongst the most suitable SCL components.
The direction of the grain of the individual veneers always runs parallel with the length of an LVL beam, increasing its strength.
Laminated Strand Lumber (LSL)
Laminated Strand Lumber (LSL) is another type of SCL that closely resembles OSB (Oriented Strand Board) in its appearance and manufacturing process. Unlike OSB, the strands, flakes, and fibres that form a billet of LSL are all oriented in the same direction, parallel to the length of the board/beam. LSL can be used in many of the same applications that the other SCL products are designed for, including engineered studs, TJI Joist cores, as well as aesthetic features. LSL material is one of the cheapest SCL materials. The billets of LSL are much thicker than that of regular OSB, to provide adequate strength in its application.
Cross-laminated timber (CLT)
Cross laminated timber (CLT) is a variety of SCL that is formed from solid timber layers and is often used for modern architectural projects. For the construction of CLT, “veneers” of solid timber are glued together in a perpendicular arrangement relative to the direction of the wood grain. This process is similar in concept to the construction of plywood, with the varying orientation of the layers eliminating the instability that wood possesses across its grain. CLT is made from at least 3 thick veneer layers to achieve the desired structural properties, though beyond that the number of layers per piece is dependent on specification. The layers themselves are typically made from smaller laminations of timber members that are glued together along their width to form timber panels. Creating CLT members from timber panels assembled in this way also eliminates the tendency for the timber to cup or twist.
CLT is also conceptually similar to Glulam construction, though it varies in principle due to the orientation of the laminations. Due to the relatively small size of the raw timber used in this type of construction, CLT members can be assembled in pretty much any shape or size, restricted only by weight. As such, it is often used for complex shaped components within modern architectural works. CLT timber is not often used in regular residential construction.
Parallel Strand Lumber (PSL)
Parallel Strand Lumber (PSL) is a very strong material typically formed into beams that are used in structural applications in modern construction. PSL is formed by compressing very long, thin strands of timber together with resins or glues to produce solid components. The strands are all oriented to run parallel to the length of the beam, and as such gives the beam fantastic load bearing capacity. The strands themselves are often waste products and trimmings from the veneer making process - sustainable quality. PSL is a very stable material due to the high volume of resin and glue within each billet. The natural structural qualities of timber fibres in combination with the specific orientation of the strands within the billet gives each beam great compressive and load bearing resistance, both horizontally and vertically.
Dowel Laminated Timber (DLT) - Brettstapel - Dowellam
Dowel Laminated Timber (DLT) is a variety of SCL construction that utilises dowels in combination with glue to form solid beams and billets out of smaller timber components. The general premise of this method is like that of Glulam construction, though it uses hardwood dowels to align and keep together each individual timber. Early production of this material relied exclusively on the expansion of the dowels in the presence of moisture to keep the single timbers together, though in modern production glue or resin is often used to provide additional strength.
Whilst not necessarily used in extreme structural applications, the benefit of this material comes in the raw material requirement for its production, namely that lower quality timber can be used to form larger more suitable beams.
Originally created in the 1970’s in Germany, this product is also known as Brettstapel, which simply means “Stacked Boards”, or “Board Stack”. Dowellam is another coined phrase - region depending - playing off the term “Glulam”.
Nail Laminated Timber (NLT)
Nail Laminated Timber (NLT) is a material similar to Glulam or DLT, with the main difference being the use of nails as a mechanical fastener during its assembly. In this process, smaller whole timber members are glued and nailed together to create larger sheets or beams that can be used for a wide array of applications. Common applications include lintels/ headers, span dividing beams, wall panels, as well as components in flooring or roofing systems.
In the past NLT was a very common construction method for creating SCL components. This was due to the simple technology involved, in combination with the less common requirement for large structural pieces. As such, the smaller required components in the construction of buildings such as window headers/ lintels, or structural floor beams, were effectively made through the process of nail lamination. Nail lamination is the simplest of the SCL to produce, as it really is just individual timbers nailed together in accordance with structural calculations.
Other engineered timber products
Timber trusses
Timber trusses are shaped timber constructions that form part of a roof assembly. There are many varieties and applications of timber trusses, and they have been used in construction for thousands of years. The intended purpose of each truss is to support and distribute the weight of the roof, bridging the gap between 2 load bearing surfaces. Trusses come in many shapes and can suit a variety of required applications. Trusses are generally made for purpose according to structural calculations, though some modern manufacturers produce off the shelf products for use in smaller constructions.
In traditional post and beam construction, trusses were made by hand from large beams and were held together with classical joinery techniques such as mortise and tenons. These trusses were fashioned in a handful of different patterns, with “King Post Trusses” being a very common choice for roof construction, even as far back as 2000 years ago. These trusses made of larger beams can be set metres apart and support thick purlins on their top side. These purlins would then support much smaller timber rafters - these of which would be set out much closer together to receive a roof covering.
Moving forward in time, mass produced prefab trusses still play a large part in roof construction, though their implementation varies somewhat from tradition. Whilst post and beam construction is still alive in our modern age, cheaper, lighter modern trusses provide a cost effective and time efficient method of assembling roof structures. Engineered timber trusses made of much thinner material than post and beam trusses can be quickly manufactured in factories and delivered in bundles to building sites. These trusses are engineered according to calculations and arrive on site with a specific layout plan. Roof trusses of this type are spaced much closer together and are tied to one another with diagonal and longitudinal braces on the inside, as well as the roof battens that form part of the roof covering system.
Modern trusses can be ordered to almost any size or shape to meet any specification. We will cover the specifics of modern trusses and post and beam trusses in more detail in the roofing section of this book.
Accoya
Accoya is an interesting type of engineered timber, with amazing weather and moisture resistant properties. For the creation of this timber product, a particular species of wood, the softwood radiata pine is cut down and sent to specialised factories. At these facilities, the timber undergoes a process called Acetylation. This process entails bringing the timber down to a very low moisture level in a pressurised tank and introducing acetic acid whilst the timber is under vacuum. This replaces most of the water within the timber with acetic acid, which is resistant to rot and moisture movement. The Accoya company claims, with many sources to substantiate their claim, that the Accoya timber is the most stable timber product in the world.
Thermally stabilised wood/Thermowood
Whilst there are a handful of different patents and manufacturers of thermally stabilised wood, the process remains largely the same throughout and has been used in building applications for hundreds of years. The process involves treating timber intended for construction applications with heat and steam, followed by careful cooling in a specific atmosphere to give the finished product desirable qualities. Manufacturers of this material claim fantastic additional characteristics of the wood, without the use of chemicals in the treatment process. Some of these desirable characteristics include dimensional stability, resistance to rot and pests, increased lifespan of the timber, increased insulative properties, as well as the large environmental benefits through the lack of chemicals used in the process. The modern process of the creation of this timber has existed for around 100 years, though the concept of treating wood with heat or flames dates back hundreds of years further than that.