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Hybrid wood composites – integration of wood with other engineering materials
Hybrid wood composites – integration of wood with other engineering materials
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16.1 Hybrid wood composites in transport Wood has always been used in association with other engineering materials, particularly in transport and construction. For example, chariot racing was a barbaric sport introduced from the time of the early Roman Empire and became an Olympic sport in 688 BC. Chariots were constructed from a hybrid combination of different wood species and drawn by two horses at high speeds over sometimes uneven terrain resulting in fatigue and impact loading. Oak was used for the axle which was required to be strong, tough and durable. Poplar and other more flexible wood species were used for much of the wheel rim which was clad with metal, usually brass. Other woods used included ilex species (e.g. evergreen oak and holly), ash and elm. The range of densities, mechanical property characteristics and wide availability made wood the effective and only choice of material. Early aircraft made clever use of wood in laminated and solid forms. For example, the Sopwith Pup introduced in 1916, of which there are still examples flying, is comprised of a central plywood module, which protects the pilot in combat, on which the engine is mounted at one end and the spruce space frame fuselage is attached at the other (Figure 16.1a). The propeller is constructed from laminated mahogany. Modern aircraft such as the aerobatic Pitts Specials designed by Curtis Pitts (Figure 16.1b) are aerobatic bi-planes made from up to 75% by weight of wood. Replica versions (e.g. RW2 and RW26) feature airframes constructed completely from wood and covered with aircraft fabric (www.ragwing.net/fleet.htm). The Morgan Motor Company (www.morgan-motor.co.uk) is well known for the hybrid construction of its classic range of sports cars. Engine and axle assemblies are mounted on a galvanised steel chassis and an ash frame, doors and plywood components (floor and wheel arches) are attached to the chassis. Aluminium panels are screwed to the ash frame and the hybrid combination of steel, wood and aluminium components provide the unique handling characteristics of Morgan cars. There are 72 timber components in the Morgan Plus Four frame machined from ash grown in the UK and the plywood is used for the floor panels and wheel arches. These components are combined with wood preservative, adhesives, brackets, hinges and screws to produce the frame (Figure 16.2). Ansell et al. (2004) performed a life cycle assessment of the ash frame and concluded that the lightweight wood frame contributes to superior fuel consumption compared with a steel frame, which represents a tangible environmental benefit. Wood Composites. http://dx.doi.org/10.1016/B978-1-78242-454-3.00016-0 Copyright © 2015 Elsevier Ltd. All rights reserved.
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Figure 16.1 (a) Sopwith Pup (www.ctie.monash.edu.au/hargrave/sopwith2.html). Taken from Wikimedia Commons at: http://commons.wikimedia.org/wiki/File:Sopwith_Pup_-_ RIAT_2008_(2831140422).jpg. (b) Pitts Special. Taken from Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Pitts.s-2a.n74dc.arp.jpg.
Figure 16.2 Outline solid model of the Morgan ash frame created with AutoCAD Mechanical Desk Top (Colin MacPherson, University of Bath).
16.2 Wind turbine and propeller blades Wind turbine blades laminated from thick wood veneer were developed in the USA, based on boat building techniques pioneered by the Gougeon Brothers (1985). The development of wood/epoxy wind turbine blades is described in considerable detail in a NASA report by Spera et al. (1990). In the 1990s, Westinghouse wind turbine blades (Figure 16.3a) were made from softwood veneer bonded with the WEST epoxy system. A gel coat and a woven glass-reinforced plastic (GFRP) were laid down first in the mould to form an exterior weather-proof surface with high torsional stability. Similar blades were developed in the UK by Gifford Technology and in a series of takeovers the design technology was further developed by Composites Technology, the Wind Energy Group (Figure 16.3b), NEC-Micon, Aerolaminates and finally Vestas on the
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Figure 16.3 (a) Westinghouse 600 kW wind turbine, Kahuku Wind Farm, Oahu, Hawaii. (b) Wind Energy Group 400 kW wind turbine, Altamont Pass, California. Taken from Wind Energy Group.
Isle of Wight. Vestas’ multi-megawatt size blades were also hybridised with carbon composites (CFRP). Ansell et al. (1991) describe the structure of wind turbine blades laminated from wood veneer (~4 mm thick). The blade structure is comprised of a D-spar, composite shear web and a foam trailing edge profile to form an aerofoil shape, the whole structure is encased in a GFRP fabric-based composite with a pigmented gel coat outer layer (Figure 16.4). The inner blade surfaces are also protected by a GFRP fabric layer bonded to the wood veneer. The blade is moulded in two halves by hand lay-up followed by vacuum bagging (Figure 16.5). The GFRP serves the dual purpose of maintaining the moisture content of the laminated wood and providing bending and torsional rigidity to the blade. The low density of wood and the thick section of the blade result in a high second moment of area combined with low mass and the 3–4 mm thick veneer allows the blade structure to be built up rapidly in the mould. Initially African mahogany (Khaya ivorensis), available as 4 mm thick veneer, was used followed by more environmentally acceptable Italian poplar. Finally, thin Finnish birch plywood was selected as a substitute for veneer because of its dimensional stability. As multi-megawatt capacity wind turbines were developed, with power output proportional to swept area, laminated wood blades were designed with increased length (Figure 16.6). Marsh (2001) reviewed composites technology for blade design and at Aerolaminates on the Isle of Wight he reported that laminated wood blades matched the lightest synthetic composite blades in terms of weight and offered low density, high specific stiffness, excellent fatigue resistance and high natural damping. Wood-CFRP
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Wood Composites Schematic of wood composite blade Glass/epoxy shear web
Centre of shear Gravity Aerodynamic load
Glass epoxy
4mm Veneers/epoxy Glass/foam sandwhich
Glass epoxy
Foam 36 Steel ‘carrots’ Potted in epoxy grout Blade length 16 m Blade weight 800 kg
Figure 16.4 Schematic of laminated wood wind turbine blade (Ansell et al., 1991).
hybrid blades were subsequently designed with the stiffness required for the manufacture of 50 m long blades. Innovative approaches to the design of large wind turbine blades are considered in a report by Sandia National Laboratories (2003). The aim is to optimise the structural and manufacturing characteristics of 50 m blades. They conclude that wood/carbon/glass hybrids offer advantages in terms of cost of materials, structural benefits and manufacturing costs, assuming that veneer of suitable quality from renewable sources is used. In a materials selection analysis, Brondsted et al. (2005) compare composite materials for wind turbine rotors and conclude that wood has the advantage of high fracture toughness and low density.
16.3 Rotor blades The blades for early propeller-driven aircraft (e.g. aerofoil blades for the Wright biplane) and helicopters (e.g. main rotor and tail rotor for Sikorsky YR-5A) were manufactured from solid wood or laminated wood. During First World War mahogany was the preferred timber for aircraft propellers but walnut, oak, cherry and ash were also employed. Wooden fan blades have also been used extensively in large wind tunnels (Young et al., 1991) comprised of Sitka spruce and compressed birch veneer composite (Hydulignum) at the hub (butt) end of the blade. Laminated Douglas
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Figure 16.5 Lay-up of laminated wood turbine blade at Gifford Technology Ltd. (a) Lay-up of wood veneer and foam into female mould. (b) Release mesh being laid over veneer layers. (c) Blade half is enclosed in a vacuum bag and air is evacuated. (d) Two halves of blade, bonded together, are removed from mould. (e) Studs are bonded directly into the timber at the blade root for attachment to the hub.
Figure 16.6 Laminated wood blade for a 45 m diameter wind turbine manufactured at Lymington, Hampshire.
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fir/epoxy blades were a later development. Young et al. present fatigue data for Sitka spruce in the form of S-N curves and constant life diagrams. The fatigue properties of wood composites for turbine blades are also reviewed by Ansell (2003). In 2010, the wind tunnel at the General Motors Aerodynamics Laboratory in Warren Michigan was thirty years old and during its life it has been used to measure drag in motor vehicles and the aerodynamics of skiing and the performance of yachts. The wind tunnel (Figure 16.7) is the world’s largest with a diameter of 13.1 m and the six blades (Figure 16.8) are laminated from spruce. Hoffmann Propeller (2015) variable pitch propellers are currently manufactured for use in hovercraft (e.g. Griffon Hoverwork, Southampton), light aircraft and airships.
Flow conditioning screen
Contraction section 5.1 Area ratio Vehicle underpass
Laboratory building Control room
Balance platform
Cooling tower
Model holding area Second floor
Freight elevator
Electrical equipment
Direction of airflow
Heat exchanger 175 Vertical airfoil elements using cooling tower water
Turning vanes unique to each corner full height of air path
6 Blade fan, diameter = 13.1 m laminated sitka spruce 4500 HP motor
Figure 16.7 General Motors Aerodynamics Laboratory © General Motors.
Figure 16.8 Fan with six Sitka spruce blades at the General Motors Aerodynamics Laboratory © General Motors.
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In the same manner as some wind tunnel blades, the hub end of the blade is made of densified hardwood and the rest of the blade utilises spruce lightwood. The blade is clad in fibre-reinforced epoxy to provide torsional stiffness. To protect the wood from erosion and the marine environment, the composite is coated with several layers of polyurethane lacquer.
16.4 Hybrid wood composites in sport Many items of sports equipment combine wood with glass and carbon fibre composites, steel, aluminium and polymers. For example, skateboards are laminated from sugar maple to provide a lightweight, compliant, impact tolerant structure. Polo mallets are comprised of an ash or sycamore head with a cane shaft. Archery longbows (www.yewtreearchery.co.uk/woods.htm, www.naturalarcher.com) are still manufactured from laminated wood with a range of species including lemonwood, maple, hickory and yew used for the belly, core, accent and back elements of the bow. Manufacturers of hockey sticks often market wood composite and all composite sticks in their product range. For example, companies such as Grays International produce a range of wooden sticks (www.grays-hockey.co.uk/stick-tech) with mulberry cores and glass and aramid braided fibre composite external reinforcement. The structure of a laminated mulberry wood hockey stick with a GFRP skin (Figure 16.9) illustrates the tight bend at the stick head and the stick shaft spliced onto the blade head. Mulberry can be tightly bent without splitting and the head wood is taken from the centre of the tree. The mulberry wood (Morus spp. including M. alba, M. nigra and M. rubra) is steam bent into as tight a bend as the wood will allow, so that the grain follows the curve. The wood density lies between 560 and 740 kg m−3 and the wood is seasoned for 9–10 months before bending. Ash and hickory have also been used in older designs with smaller curvatures. Fibre composite skins increase axial and torsional stiffness and wear-resistance. Skis evolved from the solid wooden skis of the nineteenth century through to the thick sandwich composites that are used today (Casey, 2001). Wood plays a prominent role in the sport of skiing; the ski core is comprised of wood laminated into a
Figure 16.9 Laminated mulberry wood hockey stick with GFRP skin.
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Figure 16.10 Exploded view of a generic sandwich ski construction with a wood core. From ©Scott Sports SA http://www.scott-sports.com/global/en/technology/wintersports/ SSCL-PSandwich-Sidewall-Construction-Laminate-Paulowina.
sandwich structure as shown in Figure 16.10. The major elements of the ski are the top sheet, the top laminate, the core and side walls, the bottom laminate, base material and steel edges (Lind and Sanders, 2004). The laminate layers may be fibre reinforced composite material (GFRP or CFRP), metal alloy sheets or both (Boehm, 1979). The cross-section seen in Figure 16.11 has a beech wood core and metal alloy and composite laminations. Any of the components can be changed to modify the dynamic characteristics of the ski. Alternatively the ski core is wrapped in GFRP or CFRP to form a torsion box construction (Mastrogiuseppe, 2007). Braided composite materials are used to control the torsional stiffness of the ski. In sandwich and torsion box construction the role of the core is to space the facings to determine flexural rigidity and to act as a damping medium especially in hard, icy snow conditions (Casey, 2003). Lower density softwoods and thick cores optimise damping but the wood should be sufficiently hard to allow bindings to be attached securely to the core. Fischer et al. (2006) comment that the wood core controls the stiffness and damping of the ski. Gross (2013) modelled the structural characteristics of skis using finite element techniques and designed and built an experimental ski with profiled wood core and GFRP skins (Figure 16.12). A combination of unidirectional and biaxially woven glass fabric in an epoxy resin matrix was bonded to the wood core in a sandwich construction. The resin was left clear to allow the integrity of the core-facing interface
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Figure 16.11 Longitudinal section cut through Atomic 777 ski with beech wood core.
Figure 16.12 (a) Bare timber core, profiled, shaped and with sidewalls attached. (b) Finished prototype skis with bindings mounted (Gross, 2013).
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to be observed. The prototype ski was bonded in a ski press at a pressure of 8 bar. The skis, when used in Canada, possessed similar characteristics to mass-produced commercial skis. The design and selection of materials for snowboards is reviewed by Subic and Kovacs (2007). The principles of construction are similar to those for skis. Spruce, fir, ash or maple are sliced into thin strips and vertically laminated to create the wood core in a sandwich or torsion box construction. Increasing the width of the core increases the torsional stiffness of the board. Like skis, surfboards were originally manufactured from solid wood (Gibson and Warren, 2014) to be superseded by much lighter GFRP boards with balsa wood cores and more recently GFRP boards with polyurethane foam cores. Materials selection is now driven by the interest in green products which biodegrade at end of life; this has resulted in a comparison of boards that utilise different reinforcing fibers for laminates (fibreglass, organic hemp and organic cotton) and different resins (polyester, epoxy and bioresin) and polyurethane or balsa cores (Ngo et al. 2010). Ngo and colleagues conclude that the biodegradability of the resin systems used is the controlling factor for chemical breakdown of the board materials, with epoxy resins being the most stable and polyester resins the most easily degraded. Of the low density cores, the balsa wood was more biodegradable than the polyurethane polymer.
16.5 Hybrid wood composite beams Vernacular buildings in the British Isles incorporate timber members including beams, trusses and frames used in conjunction with brick, stone, cob, thatch and lath and plaster. The arrival of the Industrial Revolution relied heavily on timber for the construction of mills, industrial buildings and railways in conjunction with cast and wrought iron. Today, wood composites in the form of panel products, glue-laminated timber (glulam), I-beams, box beams and structural insulated panels (SIPS) are commonly used in construction. Buildings, such as supermarkets and factories, are often constructed from a combination of steel, concrete and glulam in order to produce the most cost-effective solution for a given design. Wood composites are also involved in construction for concrete formwork, slip-proof floors and high security doors in combination with other materials. Some examples of wood composites in hybrid construction are reviewed here. Reinforced wood composite beams include steel-reinforced flitches, FRPreinforced glulam and wood-concrete hybrid floors. Plevris and Trintafillou (1992) review the reinforcement of wood members and present the results of bonding FRP sheets (unstressed and prestressed) to the tension face of wood beams and establish a methodology for optimising the mechanical properties of FRP-wood hybrids. Failure is governed by compressive yield of wood on the top face of the beam followed by fracture of the FRP and finally tensile fracture of the wood. In a similar study on FRP reinforcement for the enhancement of the properties of Irish-grown Sitka spruce, Gilfillan et al. (2003) reinforced laminated wood with GFRP and CFRP in the form of pultruded strip on the compression face alone and on both compression and tension
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faces. Beams, 4 m and 6 m long, were loaded in four-point bending and strength and stiffness were significantly enhanced. Behaviour was predicted successfully with conventional layer theory. A consistent ductile failure mode demonstrated a significant improvement over the more brittle failure mode of unreinforced beams. Composite wood products such as laminated veneer lumber (LVL) behave predictably in bending but properties can be improved by hybridisation. For example, Alam and Ansell (2012) evaluated the flexural modulus and strength of LVL reinforced with steel flitch plates connected together by nails shot-fired through the steel flitch plate into the wood. Increasing the nail density reduced the tendency of the beam to buckle in four point bending but the increased number of splits in the LVL reduced the flexural strength. Alam et al. (2012) assessed the benefits of reinforcing laminated veneer lumber beams with steel reinforcement bonded into LVL beams in the form of a flitch plate or as rods or plates bonded into grooves routed into the top and bottom surfaces of the beam. Flexural yield strength and flexural modulus were measured as a function of the volume fraction of the steel and the structures were predictively modelled with finite element analysis. Yield strength increased linearly as a function of increasing reinforcement volume fraction, while the flexural modulus followed more closely a power law regression fit. Composite concrete-timber structures have attracted considerable interest in recent years as a means of renovating old timber structures and optimising the performance of timber buildings and bridges, in terms of mechanical stability, enhanced damping and acoustic qualities. The concrete is poured onto the timber and, if a true composite action is achieved by mechanically connecting the concrete to the timber, the timber takes tensile loads whilst the concrete resists compression. Ceccotti (2002) reviews the advantages of concrete-timber structures and presents short-term and long-term experimental results. He concludes that the structures are economic to construct, and produce stiff and strong hybrid systems.
16.6 Hybrid wood composites in construction The construction of the Thames Barrier commenced in 1980 and it was operational in 1982. Whilst much of the barrage is comprised of concrete and steel the helmet shells which cover the rotating steel gates are made from glulam laminated from West African iroko (Figure 16.13). The frames are clad in three layers of preservative-treated
Figure 16.13 (a) Thames Barrage. (b) Iroko glulam frame.
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Figure 16.14 Hamar Olympic Hall, Norway © Olasveengen, from Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Hamar_OL-Amfi_indoor.jpg?uselang=en-gb.
cross-laminated spruce with an external ventilated stainless steel skin. The finger joints for the Iroko glulam were tested in tension and shear and the helmet shells were designed according to BS CP 112:Part 2: 1971 (GLTA, 2015). The Hamar Olympic Hall (Viking ship arena) was constructed for the 1994 Norway Winter Olympics and the design is inspired by the shape of a traditional Norwegian fishing boat in upturned form. The structure is 260 m long, and encloses a volume of 40,000 m3. The arched roof comprises 17 double glulam trusses (Figure 16.14) with 10 different spans, the longest of which spans 96 m (TRADA, 2015). The three-pinned spruce glulam arches have triangulated webs and the node connections are based on steel flitch plates which are secured by cylindrical steel dowels. The glulam arches are linked by longitudinal glulam beams which extend the length of the stadium. The arched frames are supported on buttressed reinforced concrete piers which resist horizontal loads. The exterior and interior walls are clad with wood panels protected externally by overhangs of the roof. The Savill Building, constructed in 2006, is the entrance gateway to Windsor Great Park, UK and is renowned for its massive timber gridshell roof. The double curvature roof is connected via steel transfer plates to a steel perimeter tube (Figure 16.15)
Figure 16.15 Detail of Savill Building timber gridshell roof, steel perimeter tube and tubular steel legs (image courtesy of Glenn Howells Architects).
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Figure 16.16 Richmond Olympic Oval roof.
which is connected to tubular steel legs supporting the whole structure. Harris et al. (2008) describe the design and construction of the gridshell, inspired by timber gridshells at the Bundesgartenschau in Mannheim, Germany (1975) and the double-layer timber gridshell at the Weald and Downland Open Air Museum, Sussex, UK (2002). The Savill Garden roof is built from long timber larch laths, spliced together from defect-free pieces, and clad with green oak from the Windsor Estate. The roof is a double layer grid which is laid out as a flat mat and manipulated into a 3D double curvature shape, which is locked into place via timber shear blocks with screw connections. Horizontal shear loads are transferred between the parallel layers of laths, and the deep section enhances flexural rigidity. Steel brackets welded to the steel perimeter tube are connected to Kerto LVL plates, which in turn transfer load from the larch laths, creating an integrated hybrid structure. The Richmond Olympic Oval, Vancouver, Canada, constructed in 2009 for the Vancouver Winter Olympics, is a hybrid structure comprising a composite glulam/steel roof (Figure 16.16) mounted on a concrete and steel substructure (Naturally:wood, 2010). The wood/steel arches span a distance of 100 m with hollow triangular sections through which mechanical, electrical and plumbing services are located. The g lulam is notable for incorporating spruce-pine-fire wood affected by mountain pine beetle infestation and 2400 m3 of 38 × 39 mm2 section SPF timber was used in the roof members together with a similar volume of Douglas fir. WoodWave structural panels located between the arches were made from 19,000 Douglas fir plywood (1.2 × 2.4 m2) sheets, and they enhance the mechanical, acoustic and aesthetic qualities of the roof. Yellow cedar glulam posts are also used externally to support the roof where it extends beyond the walls. The two lower floors of the oval building are cast from reinforced concrete. Not far from the Olympic Oval, the University of British Columbia (UBC) campus showcases some notable hybrid timber structures including lecture theatres, atrium spaces and a biomass fuelled power station. The Earth Sciences Building, completed in 2012 (Equilibrium Canada, 2015), is a five-storey hybrid building c ombining
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Figure 16.17 UBC Earth Sciences Building. (a) Fully cantilevered glulam staircase and (b) fishbone steel connection between glulam members.
c onventional reinforced concrete wings and a connecting atrium space constructed from glulam post and beam and solid wood panel construction. The building features wood-concrete floor panels, a canopy structure comprising five-ply cross-laminated timber panels and CNC machined glulam chevron braces located on a façade face to provide seismic resistance. A key innovation is a fully cantilevered staircase connecting floors constructed from interconnecting glulam plates (Figure 16.17). Parallam structural lumber (PSL) ‘tree’ columns are the major feature of the Macmillan Bloedel Atrium at the UBC Forest Science Centre (Figure 16.18). The Centre was constructed to showcase construction using Canadian forest products. The PSL trees are 13 m tall and arranged in groups of four, supporting truss branches which meet the skylight roof. The column plates and branch plates are connected by steel rods. The walls of the atrium are lined with Douglas fir boards, maple wood veneer and solid panelling.
Figure 16.18 University of British Columbia, Macmillan Bloedel Atrium, Forest Science Centre.
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In the UK, B&K structures are a major player in designing and building sustainable frame structures, such as supermarkets and schools, which include glulam, cross- laminated timber (CLT), timber cassettes, structural insulated panels (SIPS), precast concrete and steel components (B&K Structures, 2015). Each material in the hybrid structure is used to its best advantage with glulam, CLT, cassettes and SIPS fabricated offsite and concrete pouring and steel fabrication carried out onsite.
16.7 Conclusions There are numerous other examples of wood composites in hybrid applications including chemically resistant factory floors and pipelines, the walls of refrigerated vehicles (plywood plus GFRP), baseball bats and chipboard inserts in shoe soles. There are clear advantages in combining a wide range of materials with wood composite products to achieve functional and economic solutions in engineering design.
References Alam, P., Ansell, M.P., 2012. The effects of varying nailing density upon the flexural properties of flitch beams. J. Civil Eng. Res. 2 (1), 7–13. Alam, P., Ansell, M.P., Smedley, D., 2012. Effects of reinforcement geometry on strength and stiffness in adhesively bonded steel-timber flexural beams. Buildings 2 (3), 231–244. Ansell, M.P., 2003. Fatigue of wood and wood panel products. In: Harris, B. (Ed.), Fatigue in Composites – Science and Technology of the Fatigue Response of Fibre-Reinforced Plastics. Woodhead Publishing Ltd, Abington, Cambridge, UK. pp. 339–361 (chapter 12). Ansell, M.P., Hancock, P.W., Bonfield, P.W., 1991. Wood composites – the optimum fatigue resistant materials for commercial wind turbine blades. In: Marcroft, J. (Ed.), Proceedings of the International Timber Engineering Conference, vol. 4. TRADA, Buckingham, pp. 194–202. Ansell, M.P., Murphy, R.J., Hillier, B., 2004. Life-cycle design for engineered timber products. In: Winnett, A. (Ed.), Towards an Environment Research Agenda, a Third Selection of Papers. Palgrave Macmillan, Basingstoke, Hampshire, UK. pp. 211–231 (chapter 9). B&K Structures, 2015. Hybrid structures. www.bkstructures.co.uk/solutions/solutions/hybridstructures (retrieved 19.02.15). Boehm, H., 1979. Influence of composite materials on alpine ski design. SAMPE J (September/ October), 14–20. Brondsted, P., Lilholt, H., Lystrup, A., 2005. Composite materials for wind power turbine blades. Annu. Rev. Mater. Res. 35, 505–538. Gougeon Brothers, 1985. The Gougeon Brothers on Boat Construction – Wood and WEST System Materials. Bay City, Michigan. Casey, H., 2001. Materials in ski design and development. In: Froes, F.H., Haake, S.J. (Eds.), Materials and Science in Sports. TMS (The Minerals, Metals & Materials Society), Warrendale, PA, USA. pp. 11–17, iweb.tms.org/ED/01-5085-11.pdf (retrieved 16.02.15). Casey, H., 2003. Materials in skiing. In: Jenkins, M. (Ed.), Materials in Sports Equipment. Woodhead Publishing Limited, Cambridge, UK, pp. 326–341. Ceccotti, A., 2002. Composite concrete-timber structures. Prog. Struct. Eng. Mater. 4, 264–275. Equilibrium Canada, 2015. UBC’s Earth Sciences Building (ESB), Vancouver – British Columbia. www.eqcanada.com/projects/earth-science-building-esb-at-university-of-british-columbia (accessed 18.02.15).
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Fischer, C., Fauve, M., Combaz, E., Bourban, P.-E., Michaud, V., Plummer, C.J.G., Rhyner, H., Månson, J.-A.E., 2006. Dynamic properties of materials for alpine skis. In: Moritz, E.M., Haake, S. (Eds.), The Engineering of Sport 6. Springer, Paris, France. Gibson, C., Warren, A., 2014. Making surfboards: emergence of a trans-Pacific cultural industry. J. Pacif. History 49 (1), 1–25. Gilfillan, J.R., Gilber, S.G., Patrick, G.R.H., 2003. The use of FRP composites in enhancing the structural behavior of timber beams. J. Reinf. Plast. Compos. 22 (15), 1373–1387. GLTA, 2015. Glulam – performance record. www.glulam.co.uk/performanceHistory.htm (retrieved 19.02.15). Gross, J., 2013. Modelling the structure of alpine skis. Final year engineering project, University of Bath (limited access). Harris, R., Haskins, S., Roynon, J., 2008. The Savill Garden Gridshell: design and construction. Struct. Eng. 86 (17), 27–34. Lind, D., Sanders, S.P., 2004. The Physics of Skiing, second ed. Springer, New York, pp. 201–203. Marsh, G., 2001. Composites enable cheaper wind power. Reinf. Plast. 45 (8), 28–33. Mastrogiuseppe, P., 2007. The effects of core material and thickness on the performance and behaviour of a ski. McGill University, Department of Civil Engineering and Applied Mechanics, Montréal, Québec, www.skibuilders.com/articles/EffectsofCore.pdf (retrieved 17.02.15). Naturally:wood, 2010. Richmond Olympic Oval. www.naturallywood.com/sites/default/files/ Richmond-Olympic-Oval-Case-Study.pdf (retrieved 18.02.15). Ngo, T.T., Hall, J.M., Kohl, J.G., Perry, L.A., 2010. Green surfboards: investigation of product biodegradability at end of life. Sports Technol. 3 (3), 181–191. Plevris, N., Triantafillou, T., 1992. FRP – reinforced wood as structural material. J. Mater. Civil Eng. 4 (3), 300–317. Hoffmann Propeller, 2015. www.hoffmann-prop.com (retrieved 12.02.15). Sandia National Laboratories, 2003. Innovative design approaches for large wind turbine blades. Sandia Report: SAND2003-0723. Spera, D.A., Esgar, J.B., Gougeon, M., Zuteck, M.D., 1990. Structural properties of laminated Douglas fir/epoxy composite material. NASA Reference Publication 1236, DOE/NASA/2032076, accessible at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910000814.pdf. Subic, A., Kovacs, J., 2007. Design and materials in snowboarding. In: Materials in Sports Equipment, Woodhead Publishing Limited, Cambridge, UK, pp. 185–202. Young, C.P., Wingate, R.T., Mort, K.W., Rooker, J.R., Zager, H.E., 1991. Structural integrity of wind tunnel wooden fan blades. NASA Technical Memorandum 104059, accessible at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910013276.pdf.
General Bibliography Blass, H.J. (Ed.), 1995. Timber Engineering: STEP Lectures Volumes 1 and 2. Pub. Centrum Hout, Almere. Forest Science Centre, University of British Columbia: www.forestry.ubc.ca/general-information/ forest-sciences-centre. Glued Laminated Timber Association (GLTA). The association is no longer in existence, but the archive material on this site is excellent. www.glulam.co.uk. Mettem, C.J., 2011. Timber Bridges. Routledge, Abingdon, Oxford, UK. Porteous, J., Kermani, A., 2013. Structural Timber Design to Eurocode 5, second ed. Wiley-Blackwell, Oxford, UK. TRADA, 2015. Wide span sports structures. www.trada.co.uk/downloads/wide_span_wood_ sports.pdf (retrieved 05.06.15).
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16.1 Hybrid wood composites in transport Wood has always been used in association with other engineering materials, particularly in transport and construction. For example, chariot racing was a barbaric sport introduced from the time of the early Roman Empire and became an Olympic sport in 688 BC. Chariots were constructed from a hybrid combination of different wood species and drawn by two horses at high speeds over sometimes uneven terrain resulting in fatigue and impact loading. Oak was used for the axle which was required to be strong, tough and durable. Poplar and other more flexible wood species were used for much of the wheel rim which was clad with metal, usually brass. Other woods used included ilex species (e.g. evergreen oak and holly), ash and elm. The range of densities, mechanical property characteristics and wide availability made wood the effective and only choice of material. Early aircraft made clever use of wood in laminated and solid forms. For example, the Sopwith Pup introduced in 1916, of which there are still examples flying, is comprised of a central plywood module, which protects the pilot in combat, on which the engine is mounted at one end and the spruce space frame fuselage is attached at the other (Figure 16.1a). The propeller is constructed from laminated mahogany. Modern aircraft such as the aerobatic Pitts Specials designed by Curtis Pitts (Figure 16.1b) are aerobatic bi-planes made from up to 75% by weight of wood. Replica versions (e.g. RW2 and RW26) feature airframes constructed completely from wood and covered with aircraft fabric (www.ragwing.net/fleet.htm). The Morgan Motor Company (www.morgan-motor.co.uk) is well known for the hybrid construction of its classic range of sports cars. Engine and axle assemblies are mounted on a galvanised steel chassis and an ash frame, doors and plywood components (floor and wheel arches) are attached to the chassis. Aluminium panels are screwed to the ash frame and the hybrid combination of steel, wood and aluminium components provide the unique handling characteristics of Morgan cars. There are 72 timber components in the Morgan Plus Four frame machined from ash grown in the UK and the plywood is used for the floor panels and wheel arches. These components are combined with wood preservative, adhesives, brackets, hinges and screws to produce the frame (Figure 16.2). Ansell et al. (2004) performed a life cycle assessment of the ash frame and concluded that the lightweight wood frame contributes to superior fuel consumption compared with a steel frame, which represents a tangible environmental benefit. Wood Composites. http://dx.doi.org/10.1016/B978-1-78242-454-3.00016-0 Copyright © 2015 Elsevier Ltd. All rights reserved.
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Figure 16.1 (a) Sopwith Pup (www.ctie.monash.edu.au/hargrave/sopwith2.html). Taken from Wikimedia Commons at: http://commons.wikimedia.org/wiki/File:Sopwith_Pup_-_ RIAT_2008_(2831140422).jpg. (b) Pitts Special. Taken from Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Pitts.s-2a.n74dc.arp.jpg.
Figure 16.2 Outline solid model of the Morgan ash frame created with AutoCAD Mechanical Desk Top (Colin MacPherson, University of Bath).
16.2 Wind turbine and propeller blades Wind turbine blades laminated from thick wood veneer were developed in the USA, based on boat building techniques pioneered by the Gougeon Brothers (1985). The development of wood/epoxy wind turbine blades is described in considerable detail in a NASA report by Spera et al. (1990). In the 1990s, Westinghouse wind turbine blades (Figure 16.3a) were made from softwood veneer bonded with the WEST epoxy system. A gel coat and a woven glass-reinforced plastic (GFRP) were laid down first in the mould to form an exterior weather-proof surface with high torsional stability. Similar blades were developed in the UK by Gifford Technology and in a series of takeovers the design technology was further developed by Composites Technology, the Wind Energy Group (Figure 16.3b), NEC-Micon, Aerolaminates and finally Vestas on the
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Figure 16.3 (a) Westinghouse 600 kW wind turbine, Kahuku Wind Farm, Oahu, Hawaii. (b) Wind Energy Group 400 kW wind turbine, Altamont Pass, California. Taken from Wind Energy Group.
Isle of Wight. Vestas’ multi-megawatt size blades were also hybridised with carbon composites (CFRP). Ansell et al. (1991) describe the structure of wind turbine blades laminated from wood veneer (~4 mm thick). The blade structure is comprised of a D-spar, composite shear web and a foam trailing edge profile to form an aerofoil shape, the whole structure is encased in a GFRP fabric-based composite with a pigmented gel coat outer layer (Figure 16.4). The inner blade surfaces are also protected by a GFRP fabric layer bonded to the wood veneer. The blade is moulded in two halves by hand lay-up followed by vacuum bagging (Figure 16.5). The GFRP serves the dual purpose of maintaining the moisture content of the laminated wood and providing bending and torsional rigidity to the blade. The low density of wood and the thick section of the blade result in a high second moment of area combined with low mass and the 3–4 mm thick veneer allows the blade structure to be built up rapidly in the mould. Initially African mahogany (Khaya ivorensis), available as 4 mm thick veneer, was used followed by more environmentally acceptable Italian poplar. Finally, thin Finnish birch plywood was selected as a substitute for veneer because of its dimensional stability. As multi-megawatt capacity wind turbines were developed, with power output proportional to swept area, laminated wood blades were designed with increased length (Figure 16.6). Marsh (2001) reviewed composites technology for blade design and at Aerolaminates on the Isle of Wight he reported that laminated wood blades matched the lightest synthetic composite blades in terms of weight and offered low density, high specific stiffness, excellent fatigue resistance and high natural damping. Wood-CFRP
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Wood Composites Schematic of wood composite blade Glass/epoxy shear web
Centre of shear Gravity Aerodynamic load
Glass epoxy
4mm Veneers/epoxy Glass/foam sandwhich
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Foam 36 Steel ‘carrots’ Potted in epoxy grout Blade length 16 m Blade weight 800 kg
Figure 16.4 Schematic of laminated wood wind turbine blade (Ansell et al., 1991).
hybrid blades were subsequently designed with the stiffness required for the manufacture of 50 m long blades. Innovative approaches to the design of large wind turbine blades are considered in a report by Sandia National Laboratories (2003). The aim is to optimise the structural and manufacturing characteristics of 50 m blades. They conclude that wood/carbon/glass hybrids offer advantages in terms of cost of materials, structural benefits and manufacturing costs, assuming that veneer of suitable quality from renewable sources is used. In a materials selection analysis, Brondsted et al. (2005) compare composite materials for wind turbine rotors and conclude that wood has the advantage of high fracture toughness and low density.
16.3 Rotor blades The blades for early propeller-driven aircraft (e.g. aerofoil blades for the Wright biplane) and helicopters (e.g. main rotor and tail rotor for Sikorsky YR-5A) were manufactured from solid wood or laminated wood. During First World War mahogany was the preferred timber for aircraft propellers but walnut, oak, cherry and ash were also employed. Wooden fan blades have also been used extensively in large wind tunnels (Young et al., 1991) comprised of Sitka spruce and compressed birch veneer composite (Hydulignum) at the hub (butt) end of the blade. Laminated Douglas
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Figure 16.5 Lay-up of laminated wood turbine blade at Gifford Technology Ltd. (a) Lay-up of wood veneer and foam into female mould. (b) Release mesh being laid over veneer layers. (c) Blade half is enclosed in a vacuum bag and air is evacuated. (d) Two halves of blade, bonded together, are removed from mould. (e) Studs are bonded directly into the timber at the blade root for attachment to the hub.
Figure 16.6 Laminated wood blade for a 45 m diameter wind turbine manufactured at Lymington, Hampshire.
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fir/epoxy blades were a later development. Young et al. present fatigue data for Sitka spruce in the form of S-N curves and constant life diagrams. The fatigue properties of wood composites for turbine blades are also reviewed by Ansell (2003). In 2010, the wind tunnel at the General Motors Aerodynamics Laboratory in Warren Michigan was thirty years old and during its life it has been used to measure drag in motor vehicles and the aerodynamics of skiing and the performance of yachts. The wind tunnel (Figure 16.7) is the world’s largest with a diameter of 13.1 m and the six blades (Figure 16.8) are laminated from spruce. Hoffmann Propeller (2015) variable pitch propellers are currently manufactured for use in hovercraft (e.g. Griffon Hoverwork, Southampton), light aircraft and airships.
Flow conditioning screen
Contraction section 5.1 Area ratio Vehicle underpass
Laboratory building Control room
Balance platform
Cooling tower
Model holding area Second floor
Freight elevator
Electrical equipment
Direction of airflow
Heat exchanger 175 Vertical airfoil elements using cooling tower water
Turning vanes unique to each corner full height of air path
6 Blade fan, diameter = 13.1 m laminated sitka spruce 4500 HP motor
Figure 16.7 General Motors Aerodynamics Laboratory © General Motors.
Figure 16.8 Fan with six Sitka spruce blades at the General Motors Aerodynamics Laboratory © General Motors.
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In the same manner as some wind tunnel blades, the hub end of the blade is made of densified hardwood and the rest of the blade utilises spruce lightwood. The blade is clad in fibre-reinforced epoxy to provide torsional stiffness. To protect the wood from erosion and the marine environment, the composite is coated with several layers of polyurethane lacquer.
16.4 Hybrid wood composites in sport Many items of sports equipment combine wood with glass and carbon fibre composites, steel, aluminium and polymers. For example, skateboards are laminated from sugar maple to provide a lightweight, compliant, impact tolerant structure. Polo mallets are comprised of an ash or sycamore head with a cane shaft. Archery longbows (www.yewtreearchery.co.uk/woods.htm, www.naturalarcher.com) are still manufactured from laminated wood with a range of species including lemonwood, maple, hickory and yew used for the belly, core, accent and back elements of the bow. Manufacturers of hockey sticks often market wood composite and all composite sticks in their product range. For example, companies such as Grays International produce a range of wooden sticks (www.grays-hockey.co.uk/stick-tech) with mulberry cores and glass and aramid braided fibre composite external reinforcement. The structure of a laminated mulberry wood hockey stick with a GFRP skin (Figure 16.9) illustrates the tight bend at the stick head and the stick shaft spliced onto the blade head. Mulberry can be tightly bent without splitting and the head wood is taken from the centre of the tree. The mulberry wood (Morus spp. including M. alba, M. nigra and M. rubra) is steam bent into as tight a bend as the wood will allow, so that the grain follows the curve. The wood density lies between 560 and 740 kg m−3 and the wood is seasoned for 9–10 months before bending. Ash and hickory have also been used in older designs with smaller curvatures. Fibre composite skins increase axial and torsional stiffness and wear-resistance. Skis evolved from the solid wooden skis of the nineteenth century through to the thick sandwich composites that are used today (Casey, 2001). Wood plays a prominent role in the sport of skiing; the ski core is comprised of wood laminated into a
Figure 16.9 Laminated mulberry wood hockey stick with GFRP skin.
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Figure 16.10 Exploded view of a generic sandwich ski construction with a wood core. From ©Scott Sports SA http://www.scott-sports.com/global/en/technology/wintersports/ SSCL-PSandwich-Sidewall-Construction-Laminate-Paulowina.
sandwich structure as shown in Figure 16.10. The major elements of the ski are the top sheet, the top laminate, the core and side walls, the bottom laminate, base material and steel edges (Lind and Sanders, 2004). The laminate layers may be fibre reinforced composite material (GFRP or CFRP), metal alloy sheets or both (Boehm, 1979). The cross-section seen in Figure 16.11 has a beech wood core and metal alloy and composite laminations. Any of the components can be changed to modify the dynamic characteristics of the ski. Alternatively the ski core is wrapped in GFRP or CFRP to form a torsion box construction (Mastrogiuseppe, 2007). Braided composite materials are used to control the torsional stiffness of the ski. In sandwich and torsion box construction the role of the core is to space the facings to determine flexural rigidity and to act as a damping medium especially in hard, icy snow conditions (Casey, 2003). Lower density softwoods and thick cores optimise damping but the wood should be sufficiently hard to allow bindings to be attached securely to the core. Fischer et al. (2006) comment that the wood core controls the stiffness and damping of the ski. Gross (2013) modelled the structural characteristics of skis using finite element techniques and designed and built an experimental ski with profiled wood core and GFRP skins (Figure 16.12). A combination of unidirectional and biaxially woven glass fabric in an epoxy resin matrix was bonded to the wood core in a sandwich construction. The resin was left clear to allow the integrity of the core-facing interface
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Figure 16.11 Longitudinal section cut through Atomic 777 ski with beech wood core.
Figure 16.12 (a) Bare timber core, profiled, shaped and with sidewalls attached. (b) Finished prototype skis with bindings mounted (Gross, 2013).
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to be observed. The prototype ski was bonded in a ski press at a pressure of 8 bar. The skis, when used in Canada, possessed similar characteristics to mass-produced commercial skis. The design and selection of materials for snowboards is reviewed by Subic and Kovacs (2007). The principles of construction are similar to those for skis. Spruce, fir, ash or maple are sliced into thin strips and vertically laminated to create the wood core in a sandwich or torsion box construction. Increasing the width of the core increases the torsional stiffness of the board. Like skis, surfboards were originally manufactured from solid wood (Gibson and Warren, 2014) to be superseded by much lighter GFRP boards with balsa wood cores and more recently GFRP boards with polyurethane foam cores. Materials selection is now driven by the interest in green products which biodegrade at end of life; this has resulted in a comparison of boards that utilise different reinforcing fibers for laminates (fibreglass, organic hemp and organic cotton) and different resins (polyester, epoxy and bioresin) and polyurethane or balsa cores (Ngo et al. 2010). Ngo and colleagues conclude that the biodegradability of the resin systems used is the controlling factor for chemical breakdown of the board materials, with epoxy resins being the most stable and polyester resins the most easily degraded. Of the low density cores, the balsa wood was more biodegradable than the polyurethane polymer.
16.5 Hybrid wood composite beams Vernacular buildings in the British Isles incorporate timber members including beams, trusses and frames used in conjunction with brick, stone, cob, thatch and lath and plaster. The arrival of the Industrial Revolution relied heavily on timber for the construction of mills, industrial buildings and railways in conjunction with cast and wrought iron. Today, wood composites in the form of panel products, glue-laminated timber (glulam), I-beams, box beams and structural insulated panels (SIPS) are commonly used in construction. Buildings, such as supermarkets and factories, are often constructed from a combination of steel, concrete and glulam in order to produce the most cost-effective solution for a given design. Wood composites are also involved in construction for concrete formwork, slip-proof floors and high security doors in combination with other materials. Some examples of wood composites in hybrid construction are reviewed here. Reinforced wood composite beams include steel-reinforced flitches, FRPreinforced glulam and wood-concrete hybrid floors. Plevris and Trintafillou (1992) review the reinforcement of wood members and present the results of bonding FRP sheets (unstressed and prestressed) to the tension face of wood beams and establish a methodology for optimising the mechanical properties of FRP-wood hybrids. Failure is governed by compressive yield of wood on the top face of the beam followed by fracture of the FRP and finally tensile fracture of the wood. In a similar study on FRP reinforcement for the enhancement of the properties of Irish-grown Sitka spruce, Gilfillan et al. (2003) reinforced laminated wood with GFRP and CFRP in the form of pultruded strip on the compression face alone and on both compression and tension
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faces. Beams, 4 m and 6 m long, were loaded in four-point bending and strength and stiffness were significantly enhanced. Behaviour was predicted successfully with conventional layer theory. A consistent ductile failure mode demonstrated a significant improvement over the more brittle failure mode of unreinforced beams. Composite wood products such as laminated veneer lumber (LVL) behave predictably in bending but properties can be improved by hybridisation. For example, Alam and Ansell (2012) evaluated the flexural modulus and strength of LVL reinforced with steel flitch plates connected together by nails shot-fired through the steel flitch plate into the wood. Increasing the nail density reduced the tendency of the beam to buckle in four point bending but the increased number of splits in the LVL reduced the flexural strength. Alam et al. (2012) assessed the benefits of reinforcing laminated veneer lumber beams with steel reinforcement bonded into LVL beams in the form of a flitch plate or as rods or plates bonded into grooves routed into the top and bottom surfaces of the beam. Flexural yield strength and flexural modulus were measured as a function of the volume fraction of the steel and the structures were predictively modelled with finite element analysis. Yield strength increased linearly as a function of increasing reinforcement volume fraction, while the flexural modulus followed more closely a power law regression fit. Composite concrete-timber structures have attracted considerable interest in recent years as a means of renovating old timber structures and optimising the performance of timber buildings and bridges, in terms of mechanical stability, enhanced damping and acoustic qualities. The concrete is poured onto the timber and, if a true composite action is achieved by mechanically connecting the concrete to the timber, the timber takes tensile loads whilst the concrete resists compression. Ceccotti (2002) reviews the advantages of concrete-timber structures and presents short-term and long-term experimental results. He concludes that the structures are economic to construct, and produce stiff and strong hybrid systems.
16.6 Hybrid wood composites in construction The construction of the Thames Barrier commenced in 1980 and it was operational in 1982. Whilst much of the barrage is comprised of concrete and steel the helmet shells which cover the rotating steel gates are made from glulam laminated from West African iroko (Figure 16.13). The frames are clad in three layers of preservative-treated
Figure 16.13 (a) Thames Barrage. (b) Iroko glulam frame.
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Figure 16.14 Hamar Olympic Hall, Norway © Olasveengen, from Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Hamar_OL-Amfi_indoor.jpg?uselang=en-gb.
cross-laminated spruce with an external ventilated stainless steel skin. The finger joints for the Iroko glulam were tested in tension and shear and the helmet shells were designed according to BS CP 112:Part 2: 1971 (GLTA, 2015). The Hamar Olympic Hall (Viking ship arena) was constructed for the 1994 Norway Winter Olympics and the design is inspired by the shape of a traditional Norwegian fishing boat in upturned form. The structure is 260 m long, and encloses a volume of 40,000 m3. The arched roof comprises 17 double glulam trusses (Figure 16.14) with 10 different spans, the longest of which spans 96 m (TRADA, 2015). The three-pinned spruce glulam arches have triangulated webs and the node connections are based on steel flitch plates which are secured by cylindrical steel dowels. The glulam arches are linked by longitudinal glulam beams which extend the length of the stadium. The arched frames are supported on buttressed reinforced concrete piers which resist horizontal loads. The exterior and interior walls are clad with wood panels protected externally by overhangs of the roof. The Savill Building, constructed in 2006, is the entrance gateway to Windsor Great Park, UK and is renowned for its massive timber gridshell roof. The double curvature roof is connected via steel transfer plates to a steel perimeter tube (Figure 16.15)
Figure 16.15 Detail of Savill Building timber gridshell roof, steel perimeter tube and tubular steel legs (image courtesy of Glenn Howells Architects).
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Figure 16.16 Richmond Olympic Oval roof.
which is connected to tubular steel legs supporting the whole structure. Harris et al. (2008) describe the design and construction of the gridshell, inspired by timber gridshells at the Bundesgartenschau in Mannheim, Germany (1975) and the double-layer timber gridshell at the Weald and Downland Open Air Museum, Sussex, UK (2002). The Savill Garden roof is built from long timber larch laths, spliced together from defect-free pieces, and clad with green oak from the Windsor Estate. The roof is a double layer grid which is laid out as a flat mat and manipulated into a 3D double curvature shape, which is locked into place via timber shear blocks with screw connections. Horizontal shear loads are transferred between the parallel layers of laths, and the deep section enhances flexural rigidity. Steel brackets welded to the steel perimeter tube are connected to Kerto LVL plates, which in turn transfer load from the larch laths, creating an integrated hybrid structure. The Richmond Olympic Oval, Vancouver, Canada, constructed in 2009 for the Vancouver Winter Olympics, is a hybrid structure comprising a composite glulam/steel roof (Figure 16.16) mounted on a concrete and steel substructure (Naturally:wood, 2010). The wood/steel arches span a distance of 100 m with hollow triangular sections through which mechanical, electrical and plumbing services are located. The g lulam is notable for incorporating spruce-pine-fire wood affected by mountain pine beetle infestation and 2400 m3 of 38 × 39 mm2 section SPF timber was used in the roof members together with a similar volume of Douglas fir. WoodWave structural panels located between the arches were made from 19,000 Douglas fir plywood (1.2 × 2.4 m2) sheets, and they enhance the mechanical, acoustic and aesthetic qualities of the roof. Yellow cedar glulam posts are also used externally to support the roof where it extends beyond the walls. The two lower floors of the oval building are cast from reinforced concrete. Not far from the Olympic Oval, the University of British Columbia (UBC) campus showcases some notable hybrid timber structures including lecture theatres, atrium spaces and a biomass fuelled power station. The Earth Sciences Building, completed in 2012 (Equilibrium Canada, 2015), is a five-storey hybrid building c ombining
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Figure 16.17 UBC Earth Sciences Building. (a) Fully cantilevered glulam staircase and (b) fishbone steel connection between glulam members.
c onventional reinforced concrete wings and a connecting atrium space constructed from glulam post and beam and solid wood panel construction. The building features wood-concrete floor panels, a canopy structure comprising five-ply cross-laminated timber panels and CNC machined glulam chevron braces located on a façade face to provide seismic resistance. A key innovation is a fully cantilevered staircase connecting floors constructed from interconnecting glulam plates (Figure 16.17). Parallam structural lumber (PSL) ‘tree’ columns are the major feature of the Macmillan Bloedel Atrium at the UBC Forest Science Centre (Figure 16.18). The Centre was constructed to showcase construction using Canadian forest products. The PSL trees are 13 m tall and arranged in groups of four, supporting truss branches which meet the skylight roof. The column plates and branch plates are connected by steel rods. The walls of the atrium are lined with Douglas fir boards, maple wood veneer and solid panelling.
Figure 16.18 University of British Columbia, Macmillan Bloedel Atrium, Forest Science Centre.
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In the UK, B&K structures are a major player in designing and building sustainable frame structures, such as supermarkets and schools, which include glulam, cross- laminated timber (CLT), timber cassettes, structural insulated panels (SIPS), precast concrete and steel components (B&K Structures, 2015). Each material in the hybrid structure is used to its best advantage with glulam, CLT, cassettes and SIPS fabricated offsite and concrete pouring and steel fabrication carried out onsite.
16.7 Conclusions There are numerous other examples of wood composites in hybrid applications including chemically resistant factory floors and pipelines, the walls of refrigerated vehicles (plywood plus GFRP), baseball bats and chipboard inserts in shoe soles. There are clear advantages in combining a wide range of materials with wood composite products to achieve functional and economic solutions in engineering design.
References Alam, P., Ansell, M.P., 2012. The effects of varying nailing density upon the flexural properties of flitch beams. J. Civil Eng. Res. 2 (1), 7–13. Alam, P., Ansell, M.P., Smedley, D., 2012. Effects of reinforcement geometry on strength and stiffness in adhesively bonded steel-timber flexural beams. Buildings 2 (3), 231–244. Ansell, M.P., 2003. Fatigue of wood and wood panel products. In: Harris, B. (Ed.), Fatigue in Composites – Science and Technology of the Fatigue Response of Fibre-Reinforced Plastics. Woodhead Publishing Ltd, Abington, Cambridge, UK. pp. 339–361 (chapter 12). Ansell, M.P., Hancock, P.W., Bonfield, P.W., 1991. Wood composites – the optimum fatigue resistant materials for commercial wind turbine blades. In: Marcroft, J. (Ed.), Proceedings of the International Timber Engineering Conference, vol. 4. TRADA, Buckingham, pp. 194–202. Ansell, M.P., Murphy, R.J., Hillier, B., 2004. Life-cycle design for engineered timber products. In: Winnett, A. (Ed.), Towards an Environment Research Agenda, a Third Selection of Papers. Palgrave Macmillan, Basingstoke, Hampshire, UK. pp. 211–231 (chapter 9). B&K Structures, 2015. Hybrid structures. www.bkstructures.co.uk/solutions/solutions/hybridstructures (retrieved 19.02.15). Boehm, H., 1979. Influence of composite materials on alpine ski design. SAMPE J (September/ October), 14–20. Brondsted, P., Lilholt, H., Lystrup, A., 2005. Composite materials for wind power turbine blades. Annu. Rev. Mater. Res. 35, 505–538. Gougeon Brothers, 1985. The Gougeon Brothers on Boat Construction – Wood and WEST System Materials. Bay City, Michigan. Casey, H., 2001. Materials in ski design and development. In: Froes, F.H., Haake, S.J. (Eds.), Materials and Science in Sports. TMS (The Minerals, Metals & Materials Society), Warrendale, PA, USA. pp. 11–17, iweb.tms.org/ED/01-5085-11.pdf (retrieved 16.02.15). Casey, H., 2003. Materials in skiing. In: Jenkins, M. (Ed.), Materials in Sports Equipment. Woodhead Publishing Limited, Cambridge, UK, pp. 326–341. Ceccotti, A., 2002. Composite concrete-timber structures. Prog. Struct. Eng. Mater. 4, 264–275. Equilibrium Canada, 2015. UBC’s Earth Sciences Building (ESB), Vancouver – British Columbia. www.eqcanada.com/projects/earth-science-building-esb-at-university-of-british-columbia (accessed 18.02.15).
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Fischer, C., Fauve, M., Combaz, E., Bourban, P.-E., Michaud, V., Plummer, C.J.G., Rhyner, H., Månson, J.-A.E., 2006. Dynamic properties of materials for alpine skis. In: Moritz, E.M., Haake, S. (Eds.), The Engineering of Sport 6. Springer, Paris, France. Gibson, C., Warren, A., 2014. Making surfboards: emergence of a trans-Pacific cultural industry. J. Pacif. History 49 (1), 1–25. Gilfillan, J.R., Gilber, S.G., Patrick, G.R.H., 2003. The use of FRP composites in enhancing the structural behavior of timber beams. J. Reinf. Plast. Compos. 22 (15), 1373–1387. GLTA, 2015. Glulam – performance record. www.glulam.co.uk/performanceHistory.htm (retrieved 19.02.15). Gross, J., 2013. Modelling the structure of alpine skis. Final year engineering project, University of Bath (limited access). Harris, R., Haskins, S., Roynon, J., 2008. The Savill Garden Gridshell: design and construction. Struct. Eng. 86 (17), 27–34. Lind, D., Sanders, S.P., 2004. The Physics of Skiing, second ed. Springer, New York, pp. 201–203. Marsh, G., 2001. Composites enable cheaper wind power. Reinf. Plast. 45 (8), 28–33. Mastrogiuseppe, P., 2007. The effects of core material and thickness on the performance and behaviour of a ski. McGill University, Department of Civil Engineering and Applied Mechanics, Montréal, Québec, www.skibuilders.com/articles/EffectsofCore.pdf (retrieved 17.02.15). Naturally:wood, 2010. Richmond Olympic Oval. www.naturallywood.com/sites/default/files/ Richmond-Olympic-Oval-Case-Study.pdf (retrieved 18.02.15). Ngo, T.T., Hall, J.M., Kohl, J.G., Perry, L.A., 2010. Green surfboards: investigation of product biodegradability at end of life. Sports Technol. 3 (3), 181–191. Plevris, N., Triantafillou, T., 1992. FRP – reinforced wood as structural material. J. Mater. Civil Eng. 4 (3), 300–317. Hoffmann Propeller, 2015. www.hoffmann-prop.com (retrieved 12.02.15). Sandia National Laboratories, 2003. Innovative design approaches for large wind turbine blades. Sandia Report: SAND2003-0723. Spera, D.A., Esgar, J.B., Gougeon, M., Zuteck, M.D., 1990. Structural properties of laminated Douglas fir/epoxy composite material. NASA Reference Publication 1236, DOE/NASA/2032076, accessible at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910000814.pdf. Subic, A., Kovacs, J., 2007. Design and materials in snowboarding. In: Materials in Sports Equipment, Woodhead Publishing Limited, Cambridge, UK, pp. 185–202. Young, C.P., Wingate, R.T., Mort, K.W., Rooker, J.R., Zager, H.E., 1991. Structural integrity of wind tunnel wooden fan blades. NASA Technical Memorandum 104059, accessible at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910013276.pdf.
General Bibliography Blass, H.J. (Ed.), 1995. Timber Engineering: STEP Lectures Volumes 1 and 2. Pub. Centrum Hout, Almere. Forest Science Centre, University of British Columbia: www.forestry.ubc.ca/general-information/ forest-sciences-centre. Glued Laminated Timber Association (GLTA). The association is no longer in existence, but the archive material on this site is excellent. www.glulam.co.uk. Mettem, C.J., 2011. Timber Bridges. Routledge, Abingdon, Oxford, UK. Porteous, J., Kermani, A., 2013. Structural Timber Design to Eurocode 5, second ed. Wiley-Blackwell, Oxford, UK. TRADA, 2015. Wide span sports structures. www.trada.co.uk/downloads/wide_span_wood_ sports.pdf (retrieved 05.06.15).