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Cross Laminated Timber

'If the nineteenth century was the century of steel, and the twentieth the century of concrete, then the twenty-first century is about engineered timber'. (Alex de Rijke, dRMM Architects).

When the Stadthaus building in Murray Grove, London was constructed in 2009, it became the largest timber frame residential building in the world. Eight storeys of cross-laminated-timber (CLT) providing the primary structural frame gave the industry a clear demonstration of the capabilities of engineered timber buildings. The sustainability afforded by such a structure combined with increased product quality and design knowledge in recent years suggests that this type of structure will be constructed more frequently in the future. Due to its mechanical properties, engineered timber has limitations in its use for high rise buildings, these being its variation in strength with moisture content, its orthotropic nature, long term creep and the risk of collapse in the event of a fire.

The use of cross laminated timber in the construction industry is still in its infancy with increased popularity for low-medium rise structures due to the significant environmental benefits it offers.

G2 Structural are able to offer a complete structural design service if CLT is within your scope of work with experience in the design of CLT elements on numerous projects. Please contact us for further information.

Source: www.smartlam.com
Source: www.environdeck.com

Structural Properties
CLT can certainly be considered an 'engineered timber' product, with composition much like plywood with layers of softwood timber glued together under high pressure. This typically utilises either 3, 5 or 7 layers that are each positioned perpendicular to each other, although the overall section must be symmetrical. This creates a timber panel with dimensions not limited to the girth of a tree and results in a structural panel that is dimensionally stable, has a high strength to weight ratio and its production is much less energy intensive than that of steel and concrete. The finished product is generally used as floor, roof or wall panels.

Due to the engineered nature of the product, the CLT panels can be manufactured in a large range of sizes, only limited by transportation and manufacturing constraints. The panel thicknesses are typically in the region of 50 - 300mm and up to 4.8 metres wide with panel thickness increments governed by the typical thickness of individual layers and the symmetry requirement for the panels.

CLT structures have proven popular in continental Europe in recent years, however its use in the UK is still quite new, this is largely due to the fact that the main manufacturers are based in Europe, currently there is no UK facility that manufactures CLT panels. One of the main reasons for this is that Spruce trees are by far the most commonly used for the manufacture of CLT panels,such trees are far more widely available in Scandinavia than within the UK.

Typical section of a cross laminated timber panel (Source: TRADA)

The spruce tree is used to make CLT panels as it is fast growing, easy to work with and has good strength due to its long cellulose fibres. However, spruce trees have very little inherent insect or decay resistance which is the main reason CLT should only be used internally if using this type of timber.

The structural properties of CLT panels varies dependant on the manufacturer. The table below has been reproduced from the Metsawood Leno and KLH CLT technical brochures.

Manufacturer KLH Metsawood (Leno)
Material density 470kg/m3 470kg/m3
Moisture content 12-14% 12-14%
Panels loaded perpendicular to span (floors)    
Modulus of elasticity (parallel to direction of grain) E0,mean 12000 10590
Modulus of elasticity (perpendicular to direction of grain) E90,mean 390 410
Shear modulus, Gmean 690 690
Bending strength, fm,k 24 24
Tensile strength, fc,0,k 24 21
Tensile strength, fc,90,k 2.7 2.5
Shear strength parallel to grain, fv,k 2.7 2.5
Normal to grain, fR,v,k 1.5 0.7
     
Panels loaded in plane of the panel (walls/beams)    
Modulus of elasticity (parallel to direction of grain) E0,mean 12000 10590
Modulus of elasticity (perpendicular to direction of grain) E90,mean 10590 410
Shear modulus, Gmean 690 690
Bending strength, fm,k 24 24
Tensile strength, fc,0,k 16.5 14
Tensile strength, fc,90,k 0.12 0.4
Shear strength parallel to grain, fv,k 5.2 2.5
Normal to grain, fR,v,k 1.5 0.7
*All values are in N/mm2 unless noted otherwise

Finite Element Analysis
G2 Structural has experience in the modelling of CLT structures using the finite element method. This complex analysis technique allows for non-linear analysis to be utilised, thus maximising efficiency for our clients. This can be of particular benefit with floor to wall joints under high compression as well as around any openings in a wall panel.

Modelling timber using the finite element method is complex, unlike steel, timber is an orthotropic material so it has differing properties in each of the three main orthogonal directions due to its fibre grain composition. This makes modelling the CLT panels quite complex, in addition to this, changes in moisture content will alter the strength properties so the analysis of timber should be supported by laboratory testing for verification purposes.

The table below shows the elastic stiffness matrix of an orthotropic material. This allows the timber to be modelled with individual layers, each with their own orientation which ensures the material properties are adequate for each layer.

Displacement contours for standard timber floor to wall joint in Abaqus CAE
3.3 Thermal performance
CLT panels can be used to contribute to the thermal performance of a structure, with a low thermal conductivity and specific heat capacity, CLT panels can be utilised to reduce the variation in temperature over the daily cycle. This is useful when CLT panels are exposed internally, maximising its benefit. TRADA (2011a)states that the thermal conductivity of a lightweight aggregate block is similar to CLT but the CLT panel has a greater heat capacity so a 70mm thick CLT panel has a thermal mass equivalent to a 100mm lightweight aggregate block.

It is also worth considering that CLT panels can be used for the fixing of standard insulation materials, ensuring an energy efficient structure. This is quite simple if a cavity is used between the outer skin of cladding or masonry and the inner leaf of CLT panels, much like a standard cavity masonry construction, insulation can be fixed to the outer side of the inner leaf.

3.4 Fire performance
The fired design of CLT structures can be reasonably straight forward if the 'charring rate' method is adopted. Once the fire rating for the structure has been determined and the sizing of structural elements has been carried out, a sacrificial thickness of panel can be added to the structural design requirements in order to give the redundancy required in the event of a fire. The redundancy clearly needs to take into account all faces exposed to a fire.

The design notional charring rate given in BS-EN1995-1-2, for solid timber and glued laminated timber are 0.8mm/min and 0.7mm/min respectively. Specific testing can be used for each product, the charring rates for the KLH CLT panels are given as 0.67mm/min for the outer layer only and 0.76mm/min for all layers including the outer layer, this information has been taken from the KLH UK technical brochure. The degree of redundancy can easily be determined using these values. For example, if a compartmental wall is required to be 140mm thick to satisfy its structural requirements and a 60 minute fire period is to be adopted, the panel would need to be 252mm thick to satisfy its structural requirements and its fire resistance.

This method can add a significant cost increase throughout the structure, in the case outlined above this results in an 80% increase in the thickness of the material, this negates the need for additional protection such as plasterboard lining or intumescent coating, products that are significantly more energy intensive in their production and also adds another trade to the construction of the building. A suitable factor of safety needs to be applied to allow for the fact that tenants may carry out modifications to the property, for example, if a private tenant wishes to install additional electrical services within a property, they may 'chase out' sections of the wall to allow for the installation of cables, this would compromise the performance of the panel in the event of the fire.

The redundancy built into the design can add a significant cost to the structure, this cost increase should not just be considered alongside an alternative fire protection method in isolation, as there are additional benefits to the structure with this built inredundancy. For example, the increase in thickness to the panels also increases the overall thermal performance of the structure, reducing the amount of insulation required, as well as helping with lowering sound transmission between rooms. The redundancy in the structure also improves the robustness of the building which will reduce the cost of expensive detailing between panels.

Environmental performance
Timber is an extremely sustainable product, providing it is responsibly sourced from sustainable forests. As timber absorbs carbon dioxide throughout its growth cycle, the material can be considered carbon negative, this is providing the material is sustainably sourced and the manufacturing process and transportation of the product is kept to a minimum. With regards to engineered timber, the product approaches carbon neutral the more intense the manufacturing process, for example CLT panels are pressure glued together which is energy intensive.

It is clear that the more timber is used within a building the more the overall building approaches carbon neutral. This fact demonstrates the expected increase in the use of structural timber as a construction material.

The Edinburgh Centre for Carbon Management (ECCM) claims that up to 1tonne of carbon dioxide can be saved for every metre cube of timber used on a construction project, instead of more common building materials such as masonry, timber and steel.

On demolition of a CLT structure, the panels can often be re-used, but will eventually suffer from decay. When this occurs, the timber panels can be used as biomass fuel.

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