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Cross-Laminated Timber Construction

BACKGROUND


The use of laminated timber in building construction is not a new concept. Heavy, glue laminated timber frames (commonly referred to as ‘glulam’) have been in use for many years often associated, for instance, with the construction of large sports halls, leisure centres, places of worship, etc....However, recent years have seen significant growth in the development of cross-laminated timber construction, as architects and engineers realise the sustainability and cost-savings potential of this and other timber solutions, as a replacement for conventional masonry, concrete and steel. As a result, it is purported that timber has become the fastest growing construction material in the UK.


Whilst cross-laminated timber can be used on small projects, it is more applicable to larger schemes, where offsite manufacture and speed of construction can significantly reduce costs.

 

CROSS-LAMINATED TIMBER


Cross-laminated timber (CLT) is an engineered timber product with good structural properties and low environmental impact (where sustainably sourced timber is used). It can provide dry, fast on-site construction with good potential for airtightness and excellent thermal and acoustic properties.


Load-bearing wall, floor and roof elements can be factory prefabricated to any dimension and shape, including the provision of openings for doors, windows, stairs and service channels, and delivered to site for rapid assembly. The building envelope can then be insulated and clad with other materials such as timber, brick, render, composite panels or other rainscreen cladding systems.


CLT panels, which are currently imported into the UK from Europe, are normally constructed of softwood spruce. They are formed from multiple timber layers, stacked on one another at right angles to each other, and glued together in a press over their entire surface area. The cross-laminating and bonding to adjacent members, results in improved dimensional stability, strength in two planes and improved structural integrity. Panel thicknesses normally range from 50mm to 300mm, but products as thick as 500mm can be produced. Panels can be up to 20m long and 4.8m wide, but transport constraints are such that a maximum length of 13.5m and width of 3.5m are generally considered practical for the UK.


Although CLT is considered to have a promising future in multi-storey construction, it is anticipated that low-rise, non-residential buildings will be its main application in the UK. Considerable growth in the use of CLT is predicted over the coming years.

 

A CLT Press


Typical CLT Profile


Typical CLT wall detail incorporating external timber cladding and internal plasterboard lining


Typical CLT wall detail with exposed inner face


Typical CLT wall detail with external lime render


Typical CLT floor detail with exposed soffit


Typical CLT floor detail with suspended ceiling


Typical CLT pitched roof detail


CLT floor panels being craned into position


‘Stadthaus’, 24 Murray Grove, Hackney, London

A multi-storey apartment block with floors, external and internal walls, roof, lift and stair cores constructed of CLT panels. There are no columns or beams anywhere in the structure. At the time of completion in 2009, purported to be the tallest timber constructed residential building in the world.


FIRE PERFORMANCE


Unlike lightweight timber frame construction which has no inherent fire resistance, CLT can be designed to provide varying degrees of inherent fire resistance during which it remains structurally stable. Dependent on panel thickness, CLT panels can be produced with fire resistance ratings of 30, 60 and 90 minutes when tested in accordance with ISO 834 – Fire Resistance Tests – Elements of Building Construction.


The fire resistance of CLT panels is achieved through ‘charring’. When the face of the timber panel is exposed to temperatures in excess of 400°C, the surface of the timber ignites and burns at a steady rate. As the timber burns, it loses its strength and a black layer of char on the exposed surface is produced, which acts as a layer of insulation preventing an excessive rise in temperature within the unburnt core of the panel. It is this unaffected core which continues to function for the period of fire resistance. To achieve the designed fire resistance period, there must be sufficient virgin solid timber remaining behind the char layer to sustain the loads applied. Therefore, each CLT panel within the building must be designed for the required fire resistance period and the specific loadings applied to that panel.


Whilst the fire resistance performance of CLT construction may be meritable in satisfying Building Regulations, thereby hopefully ensuring that the structure remains intact and that life safety objectives are accomplished, there are serious concerns that the extent of damage incurred to the building fabric may, in some cases, be such that demolition and rebuilding is the only viable option.


Serious concerns regarding MMC have been expressed by the National Firechiefs Council which have been published (November 2022) in a positioning statement which can be viewed here.  In the context of this Technical Bulletin, this includes the use of engineered mass timber products e.g., Cross-Laminated Timber (CLT); Glue-Laminate Timber (Glulam).


Consultants will be aware of a much-publicised fire which occurred in September 2014 involving a £20m Carbon Neutral Laboratory building belonging to Nottingham University.


The building was constructed of ‘glulam’ timber beams and columns, with walls, floors and roof of CLT panels. Despite close attention having been given to the fire integrity of the timber structural elements at the design stage, the building, which was in the final stages of construction, is reported to have been completely destroyed. This incident which is understood to have caused by an electrical fault, serves as a vivid illustration of the potential risks associated with modern timber buildings, particularly during the period of construction.


Nottingham University Laboratory – architects’ impression


Nottingham University Laboratory – at the height of the fire


EML ASSESSMENT


Consultants must adopt a cautious approach when assessing the EML for a CLT constructed building. Other than in exceptional circumstances, such as when enhanced active and/or passive fire protection measures have been incorporated in the building design, a 100% EML shall be applied.


Any queries concerning a specific CLT case should be directed to the Technical Helpline.

 

FURTHER REFERENCE


This Technical Bulletin has been written as a supplement to TB 18 - Modern Methods of Construction, dealing specifically with cross-laminated timber.


For construction sites, reference should be made to TB 19 – Fire Prevention on Construction Sites and, in particular, The Joint Code of Practice on the Protection from Fire on Construction Sites and Buildings Undergoing Renovation (tenth edition), which is posted in ATLAS.

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