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3.4 Industrial wood
3.4.4 Glue‐laminated timber (GLT) and the glulam blank
The previous sections have identified the properties and complex behaviours of wood as well as the ingredients and processes involved in EWP production.
If the former is a discussion of wood in its natural state, and the latter is a discussion of the transformations of wood into industrial timber elements and the types of processes involved, then what follows is a survey of the products of these transformations. This research is focused on a particular EWP: glue‐laminated timber (GLT). In the chain of production ‐ from the forest to the building site ‐ glue‐laminated timber is situated between the raw timber outputs of the sawmill and the fabricated architectural timber component.
The physical object that occupies this in‐between space ‐ between sawn lumber and the as‐modelled architectural component with its fixings and finishes ‐ is the central actor of this research. Theglulam blankis the glue‐laminated timber assembly after it leaves the press and before it is planed, surfaced, or otherwise machined to completion. If it is produced
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Fig. 3.17:Joseph Walsh’sEnignumshelves, 2016. Photo: Andrew Bradley
Fig. 3.18:Otto Hetzer’s Patent Nr. 197773 from Müller (2000).
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Fig. 3.19:The material orientation in (left to right) glulam, CLT, CLT with a dominant direction, CLT with non‐perpendicular layers.
for a specifically shaped architectural element, it is thenear net shapeof that element, meaning that it approximates the form of the element, and is subsequently machined and processed to achieve the final form. Using the terms established previously, the glulam blank is the aggregate enabled by structural adhesives that changes the timber paradigm from one of subtraction to one of addition. In this way it offers to link the previously discussed material optimization, spatial variance of material properties, and harnessing of material behaviour in wood and trees with the processes and elemental products of industrial timber.
The design and production of glulams has to take into account the previously described properties and behaviours of wood:grain orientation, thelimits of elastic bending, andend‐grain.
Material orientation
Since glulams are typically slender, axial elements, their material orientation
‐ the direction of the wood fibres ‐ is aligned with the long axis of the glulam element. This is the strongest material orientation for beams and columns as they resist bending and axial forces better than with a perpendicular material orientation.
The material orientation of cross‐laminated timber (CLT) panels provides a counterpoint to glulam material orientation. CLT panels ‐ much like their plywood counterparts ‐ are glue‐laminated panels consisting of layers of lumber that are alternatively oriented in perpendicular directions (Fig. 3.19).
This has the effect of minimizing dimensional distortions due to moisture fluctuations as well as homogenizing the directional strength of the panel, allowing the orientation of the panel to be of less importance.
This leads to a contrast between GLT and CLT: GLT remains anisotropic and unidirectional, while CLT becomes more isotropic and omnidirectional. If
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Fig. 3.20:The Pulpit Rock Mountain Lodge by Helen and Hard uses
non‐standard panel layups for the main structural frames. Image: Helen and Hard (www.helenhard.no)
these two EWPs present opposing ends of this contrast, then there arises a potential to investigate the gradient in‐between, where the component might exhibit a general homogeneity with a bias towards a particular direction ‐ such as in non‐perpendicular CLT layer configurations (Buck et al. 2016) ‐ or start to perform as something in between a beam and a panel. More interestingly ‐ and referring back to the highly localized variations in fibre topology in trees and branches ‐ this modulation of grain orientation could happen on a more localized scale, within the component.
This raises interesting prospects for the functional grading and optimization of glue‐laminated timber assemblies within the constraints of the industrial processes of timber, and invites speculation about what new kinds of timber morphologies could emerge from this way of thinking.
Bending limits
Curved glulam blanks need to confront the elastic bending limits of timber.
Bending introduces stresses within the material which, if exceeded, lead to material failure. In principle, thinner sections of material require less force to impose a particular curvature through bending, meaning that a desired curvature is a function of material thickness. For curved glulams, this relationship is defined in Eurocode 5 as a ratio of 1:200 between the thickness of lamella and minimum radius of curvature (EN 1995‐1‐1 2004).
This has a direct impact on the composition and complexity of manufacturing
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Single-curved
Fig. 3.21:Lamella size as a function of degree and magnitude of curvature (left to right): Single‐curved with low curvature (a single stack of wide boards), double‐curved with low curvature (large square pieces of lumber), single‐curved with high curvature (thin, wide planks), double‐curved with high curvature (very many small sticks).
curved glulam elements: smaller curvatures result in smaller lamella sections, which in turn result in a non‐linear increase in number of lamellae.
Since double‐curved glulams bend around both cross‐sectional axes, they are particularly sensitive to this dimension change, as the lamella count increase both in width and height of the cross‐section. This is a major challenge in current glulam production.
Fibre‐cutting angle and end‐grain
As discussed previously in terms of the effects of anisotropy on the strength of wood, the strength of a timber element decreases sharply as the material orientation changes from being aligned with the longitudinal material axis to being perpendicular to it. Hankinson (1921) illustrates how the ratio between a parallel and perpendicular strength can be up to 1:10 for spruce and similar species (Fig. 3.4). At a threshold of approximately five degrees from parallel, the material strength is greatly reduced.
This threshold is known as thefibre‐cutting angle. This has an important impact on the choice and manufacturing of glulam blanks for free‐form timber components. If the fibre‐cutting angle is exceeded during the machining of the final free‐form piece, the strength of the timber component suffers. This means that the form of the glulam blank and the form of the final timber component must be linked as closely as possible.
This creates a relationship between the designed architectural element and the glulam blank that it is cut from.
This also creates a trade‐off between the manufacturing complexity of the
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Fig. 3.22:The different types of glulam blanks (solid black line) for different curved elements (dashed red line): straight (left), single‐curved (centre), double‐curved (right). Image: Design‐to‐Production GmbH, redrawn by author
glulam blank and its performance demands. Higher curvatures introduce more manufacturing complexity and waste during the production of the glulam blank, however they have a higher strength because of the closer alignment between the free‐form timber component and the material orientation of the glulam blank. Lower curvatures and straight glulam blanks are simpler to produce because they can use larger lamella dimensions, at the cost of a lower strength due to more fibre‐cutting and more waste due to excess glulam blank volume.
Contemporary types of glulam blanks
These considerations of material orientation, bending limits, and the fibre‐cutting angle have led to a classification of glulam blanks according to their curvature. This proceeds from no curvature ‐ thestraight glulam blank
‐ to curvature in a plane ‐ thesingle‐curved glulam blank‐ to out‐of‐plane curvature ‐ thedouble‐curved glulam blank‐ to out‐of‐plane curvature with torsion ‐ thedouble‐curved glulam blank with torsion(Fig. 3.22). As described previously, the degree of curvature greatly impacts the number of constituent lamellae and, as a result, their handling and production (Fig. 3.21).
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Fig. 3.23:A single‐curved glulam press. Photo: Ledinek Polypress (https://www.ledinek.com/polypress)
Straight glulam blank Straight glulams are the simplest blanks to produce.
Since there is no curvature, the timber boards can be of any convenient size that together makes up the desired dimensions of the finished timber component. Straight glulams are typically made by stacking dimensioned lumber vertically to achieve the desired depth.
Single‐curved glulam blank Single‐curved glulams are curved in‐plane.
That is, the curvature is confined to a single plane. This requires the use of a curved or variable press (Fig. 3.23). The timber boards are oriented such that their shortest dimension ‐ the thickness ‐ faces the direction of curvature.
Because of the use of a curved press, the production of single‐curved glulam is more demanding than straight glulams.
Double‐curved glulam blank Double‐curved glulams are curved out‐of‐plane, meaning that their curvature cannot be confined to a single plane. As such, both the width and thickness of the lamellae are affected by the degree of curvature. There are two methods of producing double‐curved glulams, depending on the degree of curvature. For lower curvatures with larger lamellae dimensions, the glulam is assembled and pressed on a multi‐axis glulam press. For higher curvatures that require much finer lamella sizes, the double‐curvature is decomposed into two simpler process steps: the glulam is first formed as a single‐curved glulam, then cut into slices that are parallel to its plane of curvature. It is subsequently bent out of
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plane in another process step.
Double‐curved glulam blank with torsion Double‐curved glulams with torsion are created in a similar manner as without torsion, however in addition to bending out‐of‐plane, the press also introduces twisting around the long axis of the glulam, rotating the cross‐section.
Eurocode
Glulams are graded in Eurocode 5 (BS EN 1995‐1‐1) according to their bending stiffness. Common classes are GL24, GL28, GL32, and GL36. These correspond to bending strength of 24 N/mm², 28 N/mm², 32 N/mm², and 36 N/mm². The strength class of a glulam depends not only on the strength class of its lamellae but also their arrangement within the glulam.
Ahomogenousglulam is made up of lamellae of the same strength class. A combinedglulam contains laminations of differing strength classes: stronger lamellae on the outsides, weaker lamellae on the interior of the glulam (Blass et al. 1995).
In turn, lamellae are graded in two different ways: visual grading and machine strength grading (Blass et al. 1995), both of which are defined in Eurocode 5. The strength class system in EN 338 classifies lumber by its strength in N/mm². These are divided into coniferous (softwood) and deciduous (hardwood) species: C14 in varying increments up to C40 for coniferous, and D30 in varying increments up to D70 for deciduous species.
Visual grading looks at two or all four sides of each plank, assessing the size and quantity of knots, checks, and other perceived defects in the wood (Swedish Wood 2016). The EN 1611‐1 standard then classifies the lumber as G2 (two‐sided grading) or G4 (four‐sided grading) with a value between 0 and 4 to denote the grade quality ‐ with 0 being the highest, with minimal defects and a high visual quality.
Wood species are also classified in Eurocode 5 in terms of durability, in 5 classes. Class 1 is described as ’very durable’ and includes very hardy species such as iroko and greenheart. Class 5 is described as ’not durable’
and includes species such as beech and birch (Structural Timber Association 2014). Spruce ‐ the most common wood species for glulam and CLT ‐ is graded as Class 4, ’slightly durable’.
The relationship between curvature of bending and the thickness of the lamella being bent is addressed in Eurocode 5 (EN 1995‐1‐1 2004), which sets a maximum ratio of 1:200 between the thickness of the lamella ‐ the dimension that is bending ‐ and the smallest radius of curvature of that lamella. This can be increased to a ratio of 1:150 or even 1:100 by the engineering calculations, depending on the anticipated stresses and amount
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Fig. 3.24:Composition of a combined glulam beam, with higher grade lamellae on the top and bottom flanges. Image: Swedish Wood (www.swedishwood.com)
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This directly correlates curvature with the sizing of lamellas, and therefore means that a higher curvature results in more and thinner lamellas, which further results in more wood waste from the extra planing and cutting required. This has repercussions in the glulam production, as more lamellas need to be handled and accurately assembled into the glulam press. This becomes especially troublesome for highly double‐curved glulams: with the exponential increase in lamellas, the production waste, complexity, and cost increase accordingly.
New developments in glulams
Current developments in glulams display efforts to utilize a larger diversity of wood species, new gluing techniques, and reinforcement of glulams with other materials.
Although historically all manner of wood species have been used for glue‐laminated timber, spruce or similar species have been the most common in construction. Glulams using other wood species than spruce, such as beech, are appearing on the market in greater numbers, along with mixtures of beech and spruce (Dill‐Langer and Aicher 2014). Although technically not a wood species, glue‐laminated bamboo, or GluBam, is an attempt to utilize the fast‐growing and abundant quantities of bamboo for large‐scale, structural applications (Xiao et al. 2014) similar to glulam. This effort to diversify the species used for EWPs is driven by a desire to exploit a larger variety of forest stock and capitalize on more forest resources that have so far avoided exploitation.
New gluing techniques such asblock gluingare increasing the scale of possible glulam elements (Aicher and Stapf 2014) by allowing large‐scale glulams to be glued together on‐site, thus avoiding transportation limitations.
Efforts to increase the bending stiffness and strength of glulams include the use of new developments in adhesives (Brunner et al. 2010) as well as through reinforcement of glulams with layers of synthetic materials such as glass‐fibre reinforced polymers (GFRP) and carbon fibre (Fiorelli and Dias 2006; Romani and Blaß 2001).
Double‐curved glulams are still very rare, with only a very few select manufacturers offering them as a product (Hess Timber GmbH). Much of the high technology in glulam production still resides in Central Europe:
Germany, Switzerland, and Austria.
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