Emergent Timber Technologies
Table of Contents:
Timber and timber products have attracted much attention in the design and construction of sustainable buildings as replacements for traditional heavy materials, as they are a renewable resource and have a low- unpact nature. Conventional timber and timber products are used for architectural and non-load bearing structures. However, with the advancement of technologies, timber can now also be used structurally in buildings and for high-rise construction. Timber from sustainably managed forests and plantations can be utilised as lumber or manufactured into engineered products. The international collaborative efforts recognise the value of forests for the continued survival and well-being of humankind and have been important drivers for the research and development of timber and timber products that can replace heavy materials in buildings. The benefits of using timber in buildings include low weight, high strength to weight ratio, easy to adjust on site, simple connections and high efficiency in erection, in addition to architectural features and natural characteristics inherent in the product.
The development of engineered wood products (EWPs) is emergent timber technologies that enable more widespread use of timber and timber products in buildings. Popular EWPs include laminated veneer lumber (LVL), glue-laminated timber (ghtlam), cross-laminated timber (CLT) and oriented strand board (OSB). They have been developed over the years in order to enable the construction of prefabricated timber structures that compete with steel and concrete in mid- and high-rise buildings. Mass timber construction (MTC) is an innovative construction method, which mainly utilises EWPs as key structural materials. The adoption of digital design and prefabrication in MTC allow building elements such as beams, columns, floors and walls to be pre-cut, prefabricated and transported to the construction site for immediate installation.
The use of EWPs in construction has been developed since World War II, mainly for non-structural elements, but it has now also been developed for structural applications (Manninen, 2014). In North America, the number of mass timber building per year has increased from under 20 projects in 2014 to more than 200 projects in 2018 (The Beck Group, 2018). Most EWPs are typically made of softwood, such as spruce, pine or fir. The density of softwood is approximately 500 kg/m3 whereas the densities for steel and concrete are 7800 kg/m3 and 2400 kg/m3, respectively. Therefore, the use of EWPs in construction can significantly reduce the weight of the structure, simplify foundation design, and reduce embodied energy and associated CO, emissions.
The light weight and flexibility characteristics of EWPs also mean that building components are simple and safe to construct and the prefabrication method can be applied relatively quickly. Incorporating prefabrication in construction, in turn, can considerably reduce building time as well as labour cost, delays due to adverse weather conditions and environmental impacts. CLT, LVL, glulani and OSB are the most common EWPs used for prefabricated structural applications. In addition, timber can also be combined with RC to form composite structures; timber-concrete composite structure is a typical example.
Cross-laminated timber (CLT)
CLT is the leading innovation among EWPs. It is made of at least three cross-bonded layers (usually three, five or seven) of solid sawn timber. CLT was first developed in Switzerland in the early 1970s and widely used in the 2000s when some European countries, such as Austria, Germany, Switzerland,
Sweden, Norway and the United Kingdom, changed building code to permit multi-storey timber buildings. Approximately 90% of worldwide CLT production volume (around 800,000 m3) is in Central Europe, particularly in the alpine area (Fink et al., 2018). Outside Europe and North America, the CLT market is relatively young and it is estimated that North America consumes 45.000 m3 CLT for buildings each year (Schwarzmann et ah, 2018). In New Zealand, the first commercial manufacturer of CLT, XLam, started production in 2012. Since the market for CLT has increased in terms of both the local and the global demand, the Australian CLT production has reached the capacity of 60,000 m3 per year.
CLT is mainly made from softwood, such as Radiata pine (Finns radiata) or Spruce (genus Picea). However, hardwood is also a potential for CLT production. It is also possible to replace single layers of CLT with other engineered timber products, such as LVL, oriented strand board (OSB), plywood or multilayer solid wood panels (Branduer et ah. 2016). CLT manufacturing offers the possibility of utilising lower-grade dimensional lumber. Hence, low-grade lumber and forest mortality caused by insect, disease and fire could be effectively used (Karacabeyli and Douglas, 2013). According to the United States net annual growth between the harvest removals and mortality from 1952 to 2012, even though the rate of forest mortality slightly goes up, the rate of growing stock is still more than the total amount of timber harvested and forest mortality. This means that the mass timber development demand does not overwhelm the raw material supply (The North American Mass Timber State of the Industry, 2019).
Phenol resorcinol-formaldehyde (PRF), emulsion polymer isocyanate (EPI) and one-component polyurethane (PUR) are the three types of adhesives mainly used for CLT production. PUR, which is a formaldehyde free and light-coloured adhesive, is mostly used in CLT manufacturing. A low required amount of PUR with no additional hardener is used for CLT production (Mohammed and Munoz, 2011). It is important to ensure that adhesives meet specific requirements, such as strength, durability, moisture resistance, heat performance. The overall yield rate of CLT production is around 43%, which means every 1 m3 of the log can produce 0.43 m3 of CLT. However, the production of CLT is a circle process where waste, such as wood chips, sawdust and offcuts, is efficiently reused, mainly to generate the energy for factoiy equipment, kiln drying and local communities (Waugh Thistleton Architects, 2018).
CLT has been used in the construction of housing, multi-storey residential and uon-residential buildings. Using CLT in construction offers several benefits, including short construction time, lightweight (20% the weight of concrete), minimal waste and noise during competitive construction cost, good lateral and seismic load resistance, adequate fire performance, stiffness, and high aesthetic value. CLT can be used for large panel prefabrication to provide floor slabs, roofs, beams, columns, load-bearing walls or shear walls. Typical dimensions of CLT is 20 m length. 50-300 mm thick and up to 4800 nun width. Non-residential and commercial/office buildings need long spans of up to 7 m, mainly for parking spaces at the basement and desired open office layout. CLT has a high strength-to-weight ratio, which allows expanding floor span without increasing weight, and CLT is, therefore, a competing alternative to concrete and steel.
Laminated veneer lumber (LVL)
The manufacturing of LYL is done by bonding multiple rotary-peeled veneers with their grain parallel to the longitudinal axis of the section under heat and pressure. LYL provides a wide range of structural applications, such as beam, column, truss, portal frame post and beam structure, structural decking, I-joist flanges and stressed skin panels. The length can be up to 20 m, the width is from 19 to 200 mm, and the depth is from around 90 to over 2500 mm.
The log will be cut to length, debarked and soaked or sprayed with hot water before peeling in a rotary lathe in order to ensure the quality of veneers. Since LVL is a veneer-based wood product, the quality of veneer is, therefore, one of the driven factors. In the last few decades, technological improvements in wood processing minimise waste and allow smaller diameter logs from young and fast-grown plantation forests (Leggate et al., 2017). Good quality veneers are used for the production of LVL, and offcuts with defects, such as knots, wane, voids or end of log’s veneer sheet, can be used as strands for laminated strand lumber (LSL), oriented strand lumber (OSL) or parallel strand lumber (PSL) manufacturing. This helps to optimise material usage.
However, in order to optimise material usage and increase the mechanical properties of LYL, more research on the potential of using secondary quality wood is necessarily undertaken. Purba et al. (2019) har e recently shown that the knot proportions on the veneer surface do have a negative influence on mechanical properties of LYL produced from secondary quality hardwood. Although thick veneers consume less adhesive and production time, it may reduce the modulus of elasticity and modulus of rapture of LYL. On the other hand, thinner veneer increases LYL strength by better distributing the defects. Using thin veneer leads consumption that is much more adhesive, effort, and time in LYL production. The 3 nun thick veneer optimises mechanical properties of LYL (Purba et al., 2019).
Glued laminated timber (Glulam)
Glulam is an EWP manufactured by gluing several graded timber laminations with then grain parallel to the longitudinal axis of the section. Members can be straight or curved, horizontally or vertically laminated and can be used to create different structural forms. Solid 20 to 50 nun thick laminates are typically finger-jointed into lengths and clamped together by adhesive under pressure. The use of glulam can be in large structural elements, beams, columns, trasses, bridges, portal frames, post and beam structures. Common size of glulam ranges from 60 to 250 mm wide and ISO to 1000 nun deep. There are no limits for length or shape. The dimension is, therefore, determined by transportation capacity (Glued Laminated Timber Association of UK).
Oriented strand board (OSB)
OSB is another engineered structural product, manufactured from thin strands of wood glued together with adhesive under heat and pressure in specific orientations. Wood strands are 100 nun long and 1.2 mm thick and dried to a moisture content of 6-8% before they are bonded together with resin base adhesive (Alldritt et al., 2014). OSB is largely used for wall panels, beams, I-joist webs, floor sheeting and roof panels (Mekoimen et al., 2014). OSB was originated and first produced in the early 1980s in North America. The production of OSB grows originally from 719,000 m3 to significantly 26,632,305 m3 in 2014 in North America (Jin et al.. 2016). OSB is typically made from low- density trees, such as aspen or southern yellow pine, that are relatively fast-growing species. Other types of strand boards such as OSL, LSL and PSL are also manufactured from wood strands in a similar method by bonding with resins under high pressure (Cai and Ross, 2010).
Prefabricated composite structures
Timber can also be combined together with RC to form a composite structure. Timber-concrete composite structure (TCC) is a composite design that combines the best properties of timber and traditional RC in the design of slab structure. It is a composite structure that connects a concrete topping with timber joists or beams (FWPA, 2016). Timber is coimected to overcome the tension stress while the concrete topping is installed in the compression zone using various connection methods such as dowel type fasteners, notches, friction-based connection and adhesives (Yeoh et al.. 2011; Dias et al., 2018). The TCC slab was first developed in Germany in 1922 in response to the shortages of reinforcement in concrete after the first and second world wars (Yeoh et al.. 2011) and it is now widely used in the US for short and medium span bridges, structural application of new buildings and refurbishment of old historical buildings (Yeo et al., 2011; Rodrigues et al.. 2013).
The composite structure overcomes the shortcomings of using only timber or RC in slab design. The TCC design increases the stiffness and load bearing capacity. In addition, the design reduces the volume of concrete and encourages the use of timber to reduce CO, emissions and increase CO, sequestration. The fire performance of TCC structure is competitive with the conventional RC structure, and research results indicate that TCC structure has the performance increased from 60 to 90 minutes (Yeoh et al.,
2011). The TCC slab design can also take advantage of reducing the time for site operation of concrete.
The concrete slab can be prefabricated off-site, integrating connectors for the fixing of timber beams on site (Lukaszewska et al., 2008).
The technique of using pre-tension or post-tension to improve structural performance using unbonded tendon was originally found in a concrete structure in connecting columns and beams in the structure, but now the technology has been applied to EWPs (Iqbal et al., 2010; Negrao, 2012). The teclmique called Press-Lam has been developed to allow EWPs to use high strength unbonded steel cables to create connections between timber beams and columns or columns and walls to the foundations in order to improve the net strength of a timber element (Wanninger, 2015). Research studies indicate that a reduction of the deflections can be achieved with an increase of the bending strength (Negrao,
2012). The prestressed timber structure can be used in multi-storey buildings with large structural timber members made from LVL, CLT or glulam.
I-Joist is another composite wood structure that is designed to replace structural lumber in construction. Composite I-joist is an EWP manufactured from OSB to form the web, and the flanges are made from two LVLs. The I-joist can also be produced by using plywood as web and OSL or sawn structural lumber as flanges. Due to the declining availability of high quality and large dimension lumber, composite I-joist has been developed as a structural element to cany' the load for floor and roof systems (Wilson and Dancer, 2005; Islam et al., 2015). The web and flanges of an I-joist are routing and shaping from OSB and LYL. All components are assembled together with resin, pressed mechanically and heated to accelerate resin cure. The I-joist is less expensive, lighter in weight, stronger and more efficient than a solid sawn lumber beam and has become an important substitution of structural steel and RC beams. I-joist is a common construction element in North America and Europe and can be used for commercial and residential buildings (Islam et al., 2015).