What is Cross Laminated Timber (CLT)? It is a prefabricated panel - PDF document

LCSW CLT Presentation Notes What is Cross Laminated Timber (CLT)? It is a prefabricated panel formed by stacking layers of timber at 90 degrees to the layer below and gluing them together to produce a structural timber panel capable of spanning in two directions. It comes in panels up to 3m wide, lengths can fabricated to suit but most manufacturers suggest a maximum length of 12m to avoid special logistics arrangements. The glue content of CLT is very low- just 0.6% of the panel by volume. It is often used in conjunction with glulam beams and columns where loadbearing walls are not appropriate. CLT is part of a general trend we are seeing where we are moving away from traditional construction techniques and learning from the manufacturing industry- so we are looking at much quicker erection on site because it is all made in the factory, brought to site and simply lifted in to position and screwed together. It requires fewer people on site, it is a much more accurate form of construction with less waste, lower tolerances and better air tightness on site. The most prominent benefit of CLT is that it has the lowest embodied carbon of any widely adopted construction material. Hayesfield Girls’ School in Bath is a CLT and glulam frame clad in Modcell straw bale panels. A carbon analysis was carried out on the structure which found that the embodied carbon is significantly reduced from an equivalent steel or concrete frame even before sequestration is taken in to account. The smaller foundations due to the lighter CLT structure and the reduced time the contractor spent on site due to the prefabricated frame also further reduced the carbon footprint of the building. One of the main advantages of CLT over traditional building materials is speed of erection. The construction of the pop-up workshops on the Holme Lacy campus of the Herefordshire and Ludlow College demonstrates this. The existing pre-fabricated workshops had to be demolished and the new building constructed over the summer holidays so CLT was an obvious choice in this case. The CLT forms the finish, it provides the bracing- no additional sheathing board is required, it is also the only structural timber option to comply with the REI classes. Far fewer add-ons are required than with a traditional steel frame. As well as being speedy the erection is not weather dependant, Kingswood Locality Hub in Bristol was on site during the really cold winter in 2010 - 2011. An adjacent construction site in in-situ concrete was delayed until temperatures increased. Where CLT is exposed the contractor needs to ensure that the panels are protected on site, they cannot be left stacked and sitting in water on site- this is normally dealt with through ‘just in time’ deliveries where the CLT contractor delivers only the panels to be erected each day. Probably the biggest challenge which Integral have encountered with CLT is in ensuring that the initial costings are accurate. Erection time and the time taken to achieve weather tightness is reduced using a CLT frame which results in reduced prelims from the contractor, the QS or contractor needs to look at the project holistically when costing the scheme rather than just comparing the material costs like for like. Once the programme savings are taken in to account the materials are roughly comparable. There are three quality options for CLT- non visual- for structural elements which are hidden, industrial visual grade intended for commercial and industrial buildings and domestic visual for residential buildings or areas where a slightly nicer finish is desirable. The difference between the grades is largely in the amount of finishing to the outer lamella, the maximum allowable width of the glued joints and the maximum size and spacing of knots. The visual finishes can be specified to one side only or two sides if required. Christies Care is an office for a live in care organisation based in Suffolk; industrial visual grade timber was specified in this case which was more than adequate for office use. Glulam timber can have a tendency to yellow with age, at Christies Care the timber frame was lime washed to maintain the pale colour. September 2017

LCSW CLT Presentation Notes CLT is generally sourced from Austria, Switzerland and Germany via third party companies in the UK- some of the more well-known companies are Eurban, KLH and B&K structures. Generally CLT manufacture is standardised with the exception of KLH who supply slightly different thicknesses and layering- this is something to be aware of in initial design; a supplier should be engaged at an early stage to avoid re-working details later on. The consulting engineer will draw the structural intent based on scheme design calculations and then pass these to the CLT supplier once appointed. The CLT contractor will then produce their own fabrication drawings and pass back to the design team for approval. At Hayesfield Girls’ S chool as with most CLT structures the services are all exposed- M&E involvement at an early stage is really important, buy in from the whole design team and early coordination of builders work holes is required with CLT as the holes are all factory cut. This is particularly important where the timber is exposed as there is a limit to the size of hole which can be cut on site. Where timber is not able to provide the required spans or limit deflection to within the required range CLT can be used in conjunction with steel. At the Royal Opera House Bob and Tamar Manoukian Costume Centre heavy roller racking led to tight deflection limits on the floors. In this case a steel frame was used in conjunction with CLT wall and floor panels to provide the most efficient structure. Each contractor has their own preferred method of detailing- another reason why it may be useful to engage a supplier early on. A few things to be aware of when detailing CLT. Fire. CLT is inherently fire resistant and can be used to achieve up to 90 minutes resistance- it does this by charring. Charring forms an insulating layer around the timber which reduces the ability of heat and oxygen to permeate further in to the material and significantly delays further spread of flame. Careful detailing is also key- avoiding steel plates which will then need to be protected. Surface spread of flame treatments can cause more of a problem. The contractor at Steiner Academy, Frome used two different products on the same wall- you can clearly see the difference in colour. In this case the surface spread of flame treatment reacted with a factory applied preservative and discoloured the CLT. Always patch test on a hidden area. Standard details vary between manufacturers- this is another good reason to involve a supplier at an early stage! There are several different ways of achieving the same detail. It is also a good idea to consider construction tolerances- CLT is cut to high precision but the tolerances for concrete upstands are often +/-25mm. Superstructure connections can be made in bearing or shear, as timber is weak in compression perpendicular to the grain bearing connections can mean that large column sections are required to reduce the stress to an acceptable level. However shear connections can drive the size of beam members. Disproportionate collapse also needs to be considered carefully at scheme design stage. There is often a conflict between acoustic requirements and robustness- acousticians will often prefer the floor panels to be split on walls lines to prevent transmission of sound between rooms- this results in larger panel depths as there is no continuity to reduce bending moments. It’s also not a g reat detail for disproportionate collapse. At the Raymond Fenton Community Centre in South Cerney you can see the lintel over the large opening is spanning in the opposite direction to the wall panel. It is possible to make the panel work without changing the span of the outer lamella but if this is desirable it should be highlighted to the CLT supplier at an early stage as it will result in a thicker wall panel (and higher associated cost), if this is not shown on the scheme drawings the contractor will just price the more efficient option. September 2017

LCSW CLT Presentation Notes Similarly the size of the wall panels at the community centre at South Cerney are driven by the very slender columns between openings- this is perfectly possible but a much more efficient and cheaper structure could have been achieved by lining up openings. To conclude. CLT can be very flexible and can be constructed within a significantly reduced programme over traditional building materials. But it needs to be embraced by the whole design team from the outset. September 2017