Wales Forest Business Partnership Unit 6, Dyfi Eco Park, Machynlleth, Powys SY20 8AX Telephone: 0845 456 0342 Fax: 01654 700050 Email:[email protected] woodknowledge www.woodsourcewales.co.uk WALES The Coed y Brenin visitor centre extension - a Case Study
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The Coed y Brenin visitor centre extension - a Case Study Britain’s first Brettstapel building constructed using homegrown softwoods, the extension to the Coed y Brenin visitor centre, was completed in summer 2013. The design process to this achievement started in autumn 2010, when ‘Eco-minimalist’ architects Architype were appointed by Forestry Commission Wales (FCW) to design an exemplar building utilising Welsh grown softwoods.
Figure 1: Architype’s exploded sketch of the new extension (image: courtesy Architype)
The site for the extension is located in difficult terrain, lamellae together to form structural panels. Hardwood cut into a bank and with limited access for heavy dowels ‘super’ dried down to around 6-8% moisture machinery. Mass concrete was specified for the walls of content (MC) can be used with lamellae of considerably the smaller room in the basement and Brettstapel for higher MC so that dowels absorb moisture from lamellae the two side walls of the larger basement room. On the thus expanding and locking them together. Several upper ground floor above the basement, Brettstapel was lamellae are clamped together and drilled right through specified for all walls except the south facing front wall at right angles using an auger 0.5mm smaller than the which is mostly glazed curtain wall supported on a glulam dowel finished size. Dowels can be inserted simply with frame. a heavy hammer. To speed up the process hydraulic or pneumatic equipped assembly lines are used. Lightweight timber frames called ‘Larsen trusses’ (Holladay, 2011) for holding insulation layers within an This type of panel is anisotropic and behaves in a similar outer sheathing panel were specified to be fixed over manner in expansion as would a large panel cut from the OSB sheathing on exterior face of Brettstapel walls. solid timber, it is unlike cross laminated timber (CLT) ‘Warmcel’ cellulose fibre insulation could then be blown panels which are restrained like plywood by the cross- into the void created by the trusses between sheathing tying action of orthogonal layers. However, because of panels. A 215mm deep Brettstapel floor is supported by the parallel alignment of lamellae (and therefore fibres) a whitewood glulam beam running north-south down the Brettstapel panels may perform better structurally centre of the basement. than CLT both as diaphragms and shearwalls; under compression Brettstapel can take up to twice the loading Brettstapel, literal translation ‘stacked planks’, is a type of similar sized CLT panels (Smith, 2013). of solid wood panel pioneered in Germany which uses either nails or wooden dowels to fix parallel softwood
page one Figure 2: A Brettstapel house by Kaufmann; with the render Figure 4: Sohm’s distinctive wavy lamella profile, top surfaces finish it looks like a masonry structure showing machined shadow gaps
At the moment Brettstapel is mostly manufactured Merkle sum up the situation thus; ‘stacked planking’ is in Germany, Austria and Switzerland; Merkle Holzbau an ecological system of construction which although in
Figure 3: A Brettstapel kindergarten by Kaufmann, outer Figure 5: The sides of Sohm panels showing dowel ends timberwork being added page two During winter 2010-2011 Dennis Jones of BRE Wales for Forest Research) was warning of high distortion rates and George Mikurcik of Architype sought Welsh SMEs encountered in drying UK grown Sitka spruce. In order willing to create prototype Brettstapel panels in to minimise risk to the project the decision was made homegrown timber, however by summer 2011 only one between Architype, FCW and WKW to specify Douglas firm had shown interest and no practical tests had been fir lamellae; TRADA rates this species as showing ‘small carried out. Therefore Woodknowledge Wales (WKW) moisture movement’ (TRADA, 1999) making it one of the commissioned Dainis Dauksta to undertake research more stable homegrown softwoods for kiln drying. and prototyping work in partnership with Edinburgh Larch was also considered amongst other species; Napier University’s Centre for Offsite Construction and however with little knowledge of distortion Innovative Structures. characteristics during drying the idea was dismissed.
Figure 7: Distorted larch down centre of picture sitting on much wider, stable Douglas fir board
In spring 2012 Pochin’s PLC were appointed as main contractor and Architype were in discussions with their Figure 6: The first dowelled Brettstapel panel in British preferred Brettstapel sub-contractor. Dainis Dauksta had homegrown timber at ‘Timber Expo’ spent five days with researchers from Strathclyde and Napier Universities visiting Brettstapel manufacturers in One year after the design stage of the Coed y Brenin southern Germany, Austria and Switzerland. This study project commenced the first dowelled Brettstapel panel tour was organised under the auspices of ‘Limesnet’, an using homegrown spruce lamellae was produced in academic working group assembled by Dr Pete Walker of September 2011 and exhibited the same month on Napier Bath University. The study group were advised by Peter University’s stand at Timber Expo (see Figure 6, above). Kaufmann (of Kaufmann GmbH, Oberstadion, Germany) that they used softwood at 15% ± 2% MC for their Tenders to potential main contractors should have gone Brettstapel panels (Kaufmann, 2012). out by autumn 2011 to allow a start on site in spring 2012 but site problems delayed the process. By this time it It was becoming clear that German firms preferred to was known that around 100 square metres of Brettstapel install Brettstapel at a higher MC than UK architects would be required. Specifications were originally drawn and researchers had assumed would be necessary. up by Architype based on those of Austrian Brettstapel By machining shadow gaps into lamellae, subsequent manufacturer Sohm Holzbautechnik which included the lamellae shrinkage could be disguised. Scottish moisture content (MC) of 12% for spruce lamellae. architect Sam Foster was invited to advise on detailing of Brettstapel installation because of his previous This low target MC for spruce was considered to be a experience in project managing construction of Acharacle significant technical challenge considering that spruce Primary School in Scotland where imported Brettstapel is generally kiln dried to around 20% MC in the larger Figure 5: The sides of Sohm panels showing dowel ends panels made by Sohm Holzbautechnik had been used. sawmills and researcher Dr Barry Gardiner (then working
page three A workshop was held at Architype’s office This decision was made because Sohm Holzbautechnik on April 3rd 2012 to discuss the following have a patent application for use of ‘dovetailed’ or angled dowels in Brettstapel panels. Therefore if the topics: patent is granted, any Brettstapel manufactured with dowel angles between 15° and 45° would infringe Sohm’s ● options for sourcing the timber, subcontractor own rights. Because orthogonal dowels were assumed to sourced or FCW donated to project: time and cost impart little racking strength to Brettstapel wall panels, implications 18mm OSB sheathing was specified by the architects to be fixed to the outer non-visual side of these panels. This ● choice of sawmill, their ability to adjust saws and would also act as an airtightness layer. cutting patterns to suit the project specifics Discussions between Pochin and Architype’s preferred ● recommended species for lamellae and dowels sub-contractor were not proving very productive, therefore there was considerable doubt regarding ● permitted number of knots, visual grading procurement of Brettstapel made with homegrown timber; not because of technical challenges, but rather ● moisture content of lamellae and dowels because of the nature of the contract. Under the selected form of contract (JCT intermediate) it was ● sizes of lamellae and dowels clear that the subcontractor would be expected to carry all risk associated with using the innovative structural ● size and shape of grooves in lamellae panels.
● dowelling method (diagonal or perpendicular) Where the risks are high then such problems can be removed by the adoption of a ‘partnering’ approach ● jointing of lamellae to achieve required lengths up to the delivery of innovative projects, where risks are to 6m (finger jointing?) assessed and allocated on the basis of who is best placed to carry them. ● machining options for facing side of lamellae Woodknowledge Wales stated their case: ● finish; compatibility of fire retarding coating Research carried out by Napier University and (HR prof or similar) with OSMO oil finish WoodKnowledge Wales demonstrates that Brettstapel production is now achievable in Britain using homegrown ● transport of timber from sawmill to workshop and (HG) timber. Most technical barriers are understood and transport of panels from workshop to site: prototype panels have been produced in Scotland and protection, handling, size of panels Wales.
WKW presented their research findings and made Various options were discussed during June 2012 including their recommendations regarding species choice and use of imported Brettstapel. This was not an attractive Brettstapel production methodology. Certain key option for the client Forestry Commission Wales who technical decisions were made; 15% MC Douglas fir wanted to showcase use of Welsh softwoods in an lamellae were the preferred option, joined using ribbed innovative building. WKW were asked by FCW to submit beech dowels inserted at right angles through lamellae. other ‘fallback’ options to use HG timber whilst the architects sought new sub-contractors.
Architype offered the work to Williams Homes of Bala who had been one of the original bidders for the scheme and by July 2012 they had agreed to undertake the project. A meeting was held on August 13th 2012 between the architects, FCW, Pochin and representatives of Williams Homes (and their structural engineer Bob Johnson) at Architype’s Hereford office where final specifications were agreed. For the Brettstapel panels Douglas fir lamellae to be 15% ± 2% MC, dowels 6% ± 2%.
Figure 8: Ribbed Dowel page four One difficulty with the public procurement policy was pointed out by Ruth Jenkins of FCW; particular sawmills could not be specified, therefore putting at risk the procurement of the Welsh grown timber required by FCW. Owain Williams raised his concern about weatherproofing of the Brettstapel panels during transport and erection pointing out that wrapping the panels was as problematical as attempting to seal them using suitable oil. It was decided that focussed detailing would be discussed at a further meeting on 23rd August 2012 at Bob Johnson’s office in Shrewsbury. Figure 9: tongue and groove profiles machined into lamellae Nearly two years had passed since Architype were appointed as architects to this project and autumn was Williams Homes started discussions with Pontrilas Group already approaching. There had been concerns about and Woodknowledge Wales regarding kiln drying of timing of construction since the project started and at homegrown softwoods. The kiln manager at Pontrilas this point it was clear that the vital timber elements sawmill was not confident about getting good results with might have to be erected during winter. larch but was happy to dry Douglas fir or spruce to around 15% MC. This was an unexpected turn of events; most sawmillers (including Pontrilas sawmill) had up to this The following decisions were made at point rejected the notion of drying spruce below 20% MC the 23rd August meeting: as being too risky i.e. too high a reject rate.
● Douglas fir and larch were the preferred options for lamellae.
● Four sawmills were shortlisted to supply the softwood.
● Final lamellae thickness to be 50mm with a 2mm shadow gap to be revealed, lamellae to be machined with a tongue and groove profile.
● Panel sizes to be based on 1.2m width to fit with OSB panel size. Figure 10: Mixed Douglas fir and spruce panel showing shadow ● Batches to be manufactured and installed as gaps between lamellae required.
● Panels to be finished with HR Prof fire treatment and Danish oil (Osmo is incompatible with HR Prof).
● Panel to panel joints to be half-lapped with over- sailing OSB and cross-screwed fixings.
● Short sole and head binder plates to be fixed at time of manufacture at same width as wall panels with tongue and groove joints between lamellae ends and plates. The whole panel to be fixed to in situ soleplate at time of erection. Header rails to be fixed along length of wall over binder rails where necessary. Figure 11: A typical internal Brettstapel wall showing binder rails, soleplates and header plates
page five denotes airtight seal throughout all following drawings: floor, walls & roof 140 18 300 15 50 20 20
Vertical board on board timber cladding on horizontal battens
300mm Larsen truss fully filled with Warmcell insulation
15mm Bitroc sheathing board with bitumen sealed joints
140mm Brettstapel massive timber wall, sheathed with 18mm OSB (airtightness layer). OSB to have taped joints and to have overlap over soleplate. Brettstapel panel to be fixed to soleplate from external side using screw fixings to SE specification
2mm Nora Ultra Grip rubber flooring on 18mm OSB on 100 x 50 acoustic battens voids fully filled with semi rigid insulation
Timber soleplate (species to match brettstapel wall), fixed to concrete wall using fixings to SE specification
53.200 FFL OSB sealed to concrete slab using airtightness tape
Protektor 9044 perforated angle
Sto Superlit insulated render system as NBS M21 to 150mm below finished ground level
min Finished
215 ground level 215
Brettstapel dowelled massive timber floor cassettes 75 175 6
175
Sto Silco render system as NBS M21
250mm Sto Therm Classic K EPS thermal insulation with taped joints bonded to RIW waterproofing layer
RIW Structureseal as NBS J40
350mm in situ concrete retaining wall to SE details
350
Architype’s detailing of the junction between Brettstapel 110 DPC detail (first floor level) panels and concrete wall (image: courtesy Architype) 1
CONSTRUCTION ISSUE Project Dwg Title Coed y Brenin DPC details - Sheet 5
Client Dwg No. Revision Forestry Commission Wales D D110 C1 Drawn by Checked by Scales Date GM - 1:5 at A3 18.04.2012
page six Figure 12: First Brettstapel panels at Williams Homes’ factory in Bala, November 2012
By mid-September Tony Hughes of Williams Homes had ordered a mixed batch of spruce and Douglas fir from Sharon Poynton of Pontrilas Group with a specified MC of 15%. This was delivered in October and the first Brettstapel panels were made in early November.
As the Brettstapel panels were being manufactured through November and December 2012, Wales was entering its third wettest winter season ever recorded (Met Office, n.d.). When erection of the glulam frame and the first Brettstapel walls commenced during January 2013 the site was saturated. Figure 13: Brettstapel floor being installed in situ Williams Homes made the decision to install the Brettstapel floor in situ; partly because the site conditions precluded the option of using a crane to install heavy floor panels (which were specified at 215mm thickness) and also because of the difficult shape of the floor plan. Prefabricated panels would have been more problematical (and possibly more costly) to design, manufacture and install. page seven Figure 14: Site conditions at Coed y Brenin in January 2013 When readings were taken through the external layer of OSB in these zones, over 30% MC was encountered. The Woodknowledge Wales held an event at the Coed y decision was taken to remove the Larsen trusses and OSB Brenin visitor centre on January 25th as the Brettstapel to examine the underlying lamellae. floor was being installed onto the glulam frame concrete retaining walls. Attendees were able to witness an Fortunately it was found that the saturated OSB had extreme rainfall episode and the consequent effect on swollen to a thickness whereby the moisture meter the proceeding work; it was necessary to sweep pooling probe pins were measuring MC of the OSB; the lamellae water off the temporary Visqueen weatherproofing and underneath were at safe moisture levels under 20%. even so water was seen to be penetrating it and dripping New OSB was fixed to the wall panels and Larsen trusses between lamellae through the completed Brettstapel replaced. floor. By early summer 2013 all moisture content readings were In mid-February Pochin’s site manager Stuart Gaylard- below 20% and mostly below 16%. The main concern had Rees was expressing concerns over the moisture content been that above 20% MC it is possible for fungal infection (MC) of the Brettstapel panels. Out of 324 moisture to take hold, especially in open structures such as readings, 4 were over 30% MC, 56 were over 21% MC and Brettstapel where lamellae are separated by narrow air there were 28 instances of 21% MC. Moisture content in gaps of generally less than 1mm width. the building now became an issue. Interstices such as these allow water to be transported Woodknowledge Wales advised Williams Homes to deep into panels by capillary action. However, no signs increase air flow throughout the building with large fans of staining or growth of fungal mycelia were observed to avoid pockets of high humidity building up in areas on any Brettstapel panels despite the fact that they where natural airflow was restricted. The building was had been erected through some of the wettest winter not closed off until several weeks later so there was little conditions on record. point in using dehumidifiers. WKW continued to monitor moisture content over the next three months using a hammer probe with long pins to ensure readings were taken within the core of the lamellae.
In the last week of March, there were severe snowstorms and snow cover persisted over the first week of April. Brettstapel panel moisture content in the problem areas did not start to decrease until later in April by which time the building was closed off and solar gain allowed dehumidifiers to function efficiently.
The highest moisture content readings were found in Brettstapel wall panels where water had been swept off the roof in late January. Figure 15: extreme weather conditions in early April 2013 page eight By August 2013 moisture content of Brettstapel panels undoubtedly have been better for all parties to the was averaging 15% and gaps between lamellae had not contract, if risks had been properly evaluated and opened significantly. Homegrown Sitka spruce and Douglas allocated from the outset by adopting a ‘Partnering’ fir has been used to successfully manufacture Brettstapel contract. panels. Untreated Douglas fir has been used in external handrails and decking; all of these applications could It is therefore felt to be essential that all parties work make useful case studies. in collaboration on the pragmatic development of any project and the resolution of its technical challenges. It has become clear that Williams Homes valued highly the experience of working on such an interesting project. The fair allocation of risk-sharing needs to embedded They along with WKW, would however, like to see in all projects, but especially in those of an innovative greater clarity on risk-sharing. In this respect it would nature.
Figure 16: View from the south; homegrown Douglas fir rails, posts and decking stacked on right
Figure 17: Finished interior, looking south through the curtain walling page nine Lessons learnt:
● At the start of this project there was considerable doubt ● We do not have complete, authoritative technical surrounding the technical feasibility of using most of our reports on any other homegrown softwood species; homegrown softwoods for precision solid wood panel giving space for normative thinking and assumptions. manufacture. However, these doubts tended to arise from normative thinking, old assumptions or insufficient ● One of the biggest perceived obstacles at the start scientific data. of this project, kiln drying of Sitka spruce below 20% MC, was achieved relatively easily when ● Initial assumptions regarding (low) moisture content industry overcame their own inertia and took up specifications in Brettstapel lamellae were incorrect; the challenge. Germans firms use softwoods at up to 18% MC (Cheret, et al., 2000). ● Many of the perceived problems in making Brettstapel were resolved by seeing continental practice and ● During 2011, Dr Barry Gardiner of Forest Research was discussing technical issues with researchers or other still warning about the difficulty of drying Sitka spruce. specialists during the ‘Limesnet’ field trip to In the same year Edinburgh Napier University’s Dr John Germany, Austria and Switzerland. Moore’s authoritative research report on Sitka spruce was published; up to that date our technical knowledge ● Public procurement policy in the UK tends to act of the species was at best patchy and limited against using locally grown timber in public projects. (Moore, 2011).
page ten We do not have complete, authoritative technical ● Normal construction industry contractual arrangements reports on any other homegrown softwood species; can be adversarial and act against innovation. Partnering giving space for normative thinking and assumptions. contracts offer a ready-made solution.
One of the biggest perceived obstacles at the start ● We are still learning the limits of what can be done with of this project, kiln drying of Sitka spruce below our homegrown softwoods. 20% MC, was achieved relatively easily when industry overcame their own inertia and took up ● Working through one of the wettest winters on record, the challenge. a successful Brettstapel building utilising home-grown softwoods was constructed in a challenging upland Many of the perceived problems in making Brettstapel location. were resolved by seeing continental practice and discussing technical issues with researchers or other ● From start to completion the project took five years, specialists during the ‘Limesnet’ field trip to demonstrating the difficulty in driving innovation through Germany, Austria and Switzerland. the bureaucratic process. It was however ultimately well worth the effort with technical solutions now available Public procurement policy in the UK tends to act for wider application within the industry. against using locally grown timber in public projects.
Figure 18: The original visitor centre on the left and the new extension on the right of picture page eleven Figure 19: View from south of finished structure
Conclusion
This project demonstrates the challenges to implementation of innovative techniques in construction utilising home-grown timbers. Ironically the construction phase, despite extreme weather conditions, accounted for only 10% of the time taken from inception to completion of the project.
Most importantly, the project demonstrated that Brettstapel construction can be successfully undertaken in the wet climate of western Britain. Primarily however, this project demonstrated that with proper collaboration, innovative projects of high quality can be delivered using home grown timber. This offers great hope for the future of the industry in Wales and the UK.
page twelve Bibliography Cheret, P. et al., 2000. Holzbau handbuch. [Online] [Accessed March 2012]. Holladay, M., 2011. All About Larsen Trusses. [Online] Available at: http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-larsen-trusses [Accessed 15th Setember 2013]. Kaufmann, P., 2012. Moisture content of lamellae for Brettstapel [Interview] (5th March 2012). Merkle GmbH, 2013. Stacked plank construction. [Online] Available at: http://www.merkle-holzbau.de/00engl/home/index_home.htm [Accessed 4th October 2013]. Met. Office, n.d. Statistics for December and 2012 - is the UK getting wetter? [Online] Available at: http://www.metoffice.gov.uk/news/releases/archive/2013/2012-weather-statistics [Accessed 15th September 2013]. Moore, J., 2011. Wood properties and uses of Sitka spruce in Britain, Edinburgh: Forestry Commission.
Schwaner, K., 2012. Brettstapel construction [Interview] (5th March 2012). Smith, S., 2013. Solid Wood Solutions [Interview] (23 February 2013). TRADA, 1999. Section 2/3 sheet 6. In: WOODinformation. High Wycombe: TRADA technology Ltd.
Contact Details: Architype email: [email protected] Dainis Dauksta email: [email protected]
Figure 19: View from south of finished structure Forestry Commission Wales (now Natural Resources Wales) email: [email protected] Pochin’s PLC web: www.pochins.plc.uk Pontrilas Group email: [email protected] Williams Homes and Timber Frames email: [email protected]
Woodknowledge Wales October 2013 page thirteen