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Production and In-Plane Compression Mechanics of Alternatively Angled Layered Cross-Laminated Timber
Dietrich Buck,* and Olle Hagman
Increasing awareness of sustainable building materials has led to interest in enhancing the structural performance of engineered wood products. This paper reports mechanical properties of cross-laminated timber (CLT) panels constructed with layers angled in an alternative configuration on a modified industrial CLT production line. Timber lamellae were adhesively bonded together in a single-step press procedure to form CLT panels. Transverse layers were laid at an angle of 45°, instead of the conventional 90° angle with respect to the longitudinal layers’ 0° angle. Tests were carried out on 20 five-layered CLT panels divided into two matched groups with either a 45° or a 90° configuration; an in-plane uniaxial compressive loading was applied in the principal orientation of the panels. These tests showed that the 45°-configured panels had a 30% higher compression stiffness and a 15% higher compression strength than the 90° configuration. The results also revealed that the 45°-configured CLT can be industrially produced without using more material than is required for conventional CLT 90° panels. In addition, the design possibility that the 45°-configured CLT can carry a given load while using less material also suggests that it is possible to use CLT in a wider range of structural applications.
Keywords: CLT manufacturing; Crosslam; Cross-ply; Diagonal-laminated lumber; Impact of laminate orientation; In-plane rotation; Grain inclination angle; Mass timber engineering; Solid wood panel; X-lam
Contact information: Wood Science and Engineering, Luleå University of Technology, Forskargatan 1, SE-931 87 Skellefteå, Sweden;*Corresponding author: dietrich.buck@ltu.se
INTRODUCTION
Driven by consumer demand, builders and architects are becoming increasingly interested in the use of ecologically friendly renewable materials and are shifting toward the use of timber products in the construction of larger spans and high-rise buildings. Enhancing the competitiveness and reliability of structural engineered wood products (EWPs) will ensure their future use as sustainable, low-carbon-footprint materials, and as a major contributor to affordable structures (Asdrubali et al. 2016).
Developments in timber engineering resulting from changed industrial conditions have awakened interest in the use of wood as a building material. EWPs, such as cross-laminated timber (CLT), are increasingly used for timber construction and targeted toward a global market. CLT has been gaining acceptance in residential and non-residential applications (Brandner et al. 2016).
CLT is a prefabricated panel of solid wood, conventionally composed of an uneven number of layers placed orthogonally, with adhesively bonded lamellae placed in a side-by-side manner (Fig. 1). CLT panels can be formed as solid straight or curved panel elements. These panels benefit from the homogenized mechanics that result from the interaction of the various layers and lamellae. This layered matrix construction reduces impacts from cracks, grooves, cuts, knots, grain deviations, annual ring width variations, and related heterogeneous features. The cross-layered arrangement forms a product with the advantage of having a more predictive dimensional stability and a greater load-bearing capacity than traditional structural timber (Steiger et al. 2012; Brandner et al. 2016).
Fig. 1. Conventional alternating 0°/90° cross-laminated timber (CLT) panel
Hyperbolic paraboloid shell-shaped timber-roof constructions, with a double-curved hyperbolic shape, appeared in Europe in 1957. The panel used was formed from adhesive-bonded 0°/90° cross-layered timber lamellae as a CLT. Later, this construction material was superseded by the changed availability of industrial steel combined with trends in architectural design (Booth 1997). As a result of advances in industrial automation technology, the concept for CLT as it is known today was motivated in the early 1990s by the need of the Central European sawmill industry to increase the value-yield for sideboards. A further increase in the volume of CLT produced is expected as technology advances and more industrial automation is introduced. Such an increase will attempt to recapture the sizeable market share held by non-renewable mineral-based construction materials, such as steel and concrete, as well as that of traditional wood-based products (Brandner et al. 2016).
Research focusing on the structural performance of buildings made of EWPs has grown and has contributed to an expanded use of wood as a construction material. As reported by Foster et al., as of 2016 the wider use of EWPs for a 300-m-tall building concept was in the exploratory stage. This indicates the potential of EWPs as a future structural material; however, fundamental challenges remain to be addressed with respect to the structural design, due to the increasing impact of lateral and dynamic loads at such heights (Foster and Ramage 2016). A 24-story building in Vienna called “HoHo”, constructed as a structural hybrid based on CLT, glulam, and reinforced concrete, is expected to be completed in 2018. The lateral resistance to wind load in this building is primarily provided by a central vertical concrete core (Xiong et al. 2016).
In 2016, the world’s tallest wooden apartment block was an 18-story, 53-m-tall building in Vancouver, Canada. The structure was constructed mainly of glulam and five-layered CLT, with a concrete elevator shaft that takes the lateral loads from wind (Poirier et al.2016). This raises the question of whether the structural stabilization for wind load provided by the concrete elevator shaft can, in future similar applications, instead be provided by a timber-panel solution. In one exemplification, glulam beams with orientations of ± 45° have been used to provide wind load support (Fig. 2). The principle demonstrated by these beams, that glulam in a ± 45° orientation offers increased wind load resistance, will presumably also apply to CLT panels. The applications of such panels then include replacing ± 45° oriented glulam beams, ± 45° oriented steel rods, or the central vertical concrete core used for wind load stabilization. A CLT panel-based approach in similar cases could involve ± 45° layers acting as a shear wall element. This may provide an alternative design approach that distributes the load more uniformly instead of acting through individual beams.
Fig. 2. A combined glulam and CLT structure. Glulam beams oriented at ± 45° provide structural wind load resistance (Photo used with permission from Alexander Schreyer/UMass)
Compressive stiffness and strength are the principal of concerns when the demand on load-bearing structural applications of CLT is increased. In turn, compression stress as a result of bending in walls can be considerably introduced by wind loads. Increased floor spans contribute to the compression loads on walls due to the weight of the material distributed across a larger area. Design features such as windows and doors increase the concentration of compressive loads on walls. Building elements, such as the upper part of structural subfloors or at the bottom side of overhanging parts can experience compressive loads. The preceding examples illustrate the critical role that compressive resistance plays when CLT is considered in structural design (Dujic et al. 2008; Oh et al. 2015; Schmid et al.2015; Christovasilis et al. 2016).
Wood is an orthotropic material, having different mechanical properties depending on the load orientation. Conventional CLT is therefore fabricated as a 0°/90° laminate, with layers alternating in the longitudinal and transverse directions. Thus, layers oriented transverse to the load orientation are stressed perpendicularly to the grain orientation under in-plane compression. The modulus of elasticity and the shear modulus of Norway spruce in the grain orientation are approximately 25 higher times greater than the values perpendicular to the grain in clear wood (Dinwoodie 2000). Hence, in a conservative design, the contribution of the cross-layers to the global compressive stiffness and strength in 90° layers is generally neglected because of the ratio of mechanical properties of the longitudinal to those of transverse layers (Brandner et al. 2015). If the lamellae are aligned at angles less than 90°, such as 45°, there is a potential to distribute the stresses more suitably along the fiber orientation.
Based on tests of panels containing one 45° layer in the center exposed to a principal in-plane shear proportion, Jakobs (1999) concluded that a CLT with a 45°-layer structure can offer twice the stiffness of conventional CLT. Such an increase in stiffness raises the question of whether CLT can become increasingly relevant if different layers and angle arrangements are combined as industrial automation develops. If so, it would offer a greater freedom in design when using a panel approach, further supporting the re-ascension of CLT in the structural applications market. In response to this, the performance of CLT, including under uniaxial compression, is of interest for CLT with layers at alternative angles.
Erwin Thomas has designed, and his company, Thoma, manufactures Holz100, an industrial product featuring alternating lamellae alignments (ETA 2013). Their product is manufactured using ± 45° transverse layers with the lamellae held together by wooden dowels instead of adhesive (Fig. 3). This approach can partly retain the reinforcing effect in both the major and minor orientations via the ± 45° transverse layers. However, to achieve the desired mechanical performance, adhesive was used in this research to achieve stiffer panels. Gluing load-bearing timber components results in a substantially stiffer panel than the use of purely mechanical connections such as wooden dowels or nails. The stiffer assembly is the result of areal connection bonding, as opposed to multiple pointwise connections with nails or dowels. Bonding between the adhesive and the wood is based on both chemical and mechanical adhesion (Blaß et al. 1995).
Fig. 3. Thoma’s Holz100 is an engineered wood product with lamellae assembled in a ± 45° orientation and connected with wooden dowels
Objective
There were two main objectives of this work. The first was to develop a guideline for industrial production of alternatively angled CLT. The second was to determine the extent to which the load-bearing capacity of such panels can be enhanced. Specifically, to determine the extent to which transverse layers arranged at ± 45° angles can influence the load-bearing capacity of CLT (Fig. 4). This was of particular interest with regard to the in-plane uniaxial compression properties in the principal load-bearing orientation of the panels. The material properties in question are the stiffness, strength, indicated characteristics and the related failure modes. This research strived to create an enhanced CLT product that offers the performance necessary to meet the increase in demand for timber-based building construction materials
Fig. 4. Schematic of the configuration of a five-layered CLT panel with 0° longitudinal and ± 45° transverse layers
EXPERIMENTAL
The experimental data refer firstly to the evaluation of CLT production under industrial conditions and secondly to laboratory testing of CLT panels. Two groups of different types of CLT, configured 0° / 90° and 0° / ± 45°, were the basis for the evaluation of material properties.
Materials
Norway spruce [Picea abies (L.) Karst.] was used to produce CLT panels with transverse layers alternating at either 90° or ± 45°. The panels were produced on Martinsons CLT production line in Bygdsiljum, Sweden using a specially designed process (Fig. 5). The lamellae of the CLT panels were machine-stress-graded by a Dynagrade in accordance with the operating procedure described by the Dynalyze AB patent (Larsson et al. 1998). The timber selected corresponded to C24 grade CEN/EN 338 (2009). The average density of lamellae was 462 kg/m3 at an average moisture content of 8%, in accordance with ISO 3131 (1975) and CEN/EN 13183 (2003) respectively. The surfaces of lamellae were planed along the narrow and wide flat sides through a jointer. The resulting dimensions of a single lamella were a thickness of 19 mm and a width of 94 mm. No finger joints were used in the production of lamellae. The production line cross-cut saw was adjustable, enabling it to cut single full-length lamellae of differing lengths at an angle of 45° for the transverse layers before they were assembled into panels.
A Melamine–Urea–Formaldehyde (MUF) adhesive was used to bond lamellae. Adhesive resin with 29.2% hardener was used and a total of 320 g/m² adhesive was applied. The resin, Cascomin 1247, and hardener 2526, was made by Casco Adhesives AB (AkzoNobel, Amsterdam, Netherlands). This adhesive type and combination corresponds to CEN/EN 301 (2012) adhesive type 1. An industrial separate ribbon spreader 6230, also made by Casco Adhesives AB, was used to apply the adhesive on the wide sides of lamellae during panel fabrication. There was no narrow face bonding of the sides of lamellae.
A high-frequency press SM 6013 HFS from the former Stenlund Maskiner AB, Ursviken, Sweden was used in a single-step procedure to press the lamellae into CLT panels. Vertical and horizontal pressures, respectively 0.37 MPa at 185 bar cylinder pressure and 0.32 MPa at 29 bar cylinder pressure, were applied transversely to the CLT. The duration of the press stage was 290 s and the production temperature of the panels was 78 °C.
After curing in the press, the average dimensions of the CLT panels were 1200 mm in width × 95 mm in thickness × 4136 mm in length. A total of six CLT panels were manufactured, including alternating 90° and ± 45° transverse layers: three panels with alternating layers arranged transversely at 90° (0°, 90°, 0°, 90°, 0°), and three panels with alternating layers arranged at ± 45° (0°, 45°, 0°, – 45°, 0°). During manufacture, every second panel passing through the production line was modified CLT ± 45° followed by conventional CLT 90°, and so forth. Production was performed in an overlapping and simultaneous fashion, resulting in enhanced matching of materials and environmental conditions to ensure panel comparability. All relevant production line procedures and manufacturing parameters were within the nominal range used by CLT company Martinsons when producing conventional CLT.
CLT panels were sawn using computer numerical control (CNC) in systematic sampling to represent the natural material diversity, for a total of 40 samples, consisting of either 90°-configured or ± 45°-configured CLT. Half of these samples were tested in destructive four-point bending (Buck et al. 2016), and the other 20 were examined under in-plane uniaxial compression in this paper. The sample dimensions were measured in accordance with the recommendations of CEN/EN 325 (2012). The final average dimensions of the samples for the compression test were 95 mm in thickness × 180 mm in width × 570 mm in length; all were five-layered.