BCCB 4 – Structural Frame
Our fourth blog describes how the scheme progressed from starting on site to the completion of the Cross Laminated Timber structural frame…
04 A false start, and then a real one
We started on site in mid April. The first task was to undertake repairs to the adjacent gable wall and chimney, which were in extremely poor condition and needed to be made structurally sound before we began piling. On commencing downtakings to the chimney, it was apparent that the wall was in much worse condition than had been expected and by the time we had established sound existing structure from which we could rebuild and then agreed a method of doing so with our structural engineer the initial two week programme for the works had extended significantly. Unfortunately this impacted even more seriously on the overall programme as it pushed back the earliest possible date for the manufacture of the Cross Laminated Timber (CLT) frame to August, when Egoin, our Spanish supplier, were shut down. So with the CLT unable to be fabricated until September, working back from an install date in October meant that there was no benefit in beginning groundworks and piling until August. So we were now effectively looking at a winter build rather than a summer one. Not a good start!
Fortunately, once works started in earnest we made good progress. The site strip, piling and ground floor slab all ran smoothly and according to programme. A road closure was applied for to facilitate delivery and erection of the CLT frame and the first sections arrived from Spain on 24th October.
A Cross Laminated Timber frame is a relatively new method of construction for the UK. Walls and floors are built up in layers of softwood timber glued together in alternate directions like giant plywood. The pieces are cut to size in Egoin’s factory in the Basque region of Spain with openings for doors and windows already formed. The sections can be up to 12m long by 3m high and are craned in to position on site and screwed together with metal brackets. All of the erection on site has been undertaken by a 3 man team of joiners from Egoin.
A conventional structure for a multi storey residential unit like ours might use a steel frame with concrete precast floor slabs, fireproof cladding to the steel and blockwork walls with plaster finishes. The CLT provides all of those functions in one operation and can be installed very quickly – in our case in under two weeks. It is sustainable (our structure encapsulates 114 tonnes of CO2), requires no wet trades and is accurate, facilitating airtight construction with external joints between sections of timber sealed using airtight tape. The timber sections are oversized so that in the event of a fire, the outer layers will char whilst the inner wood retains the strength to support the building. Our structure is, for now at least, the largest CLT structure in Scotland.
After nearly three years of visualising the project, to see the first pieces of CLT arrive and be installed on site was a very emotional moment. To be able to walk around the interiors of our flats, feeling for the first time the size of the spaces that have been formed, and seeing the amazing sea views we will have out across the Forth has been incredibly exciting. The project, and its unusual method of construction, has generated significant interest and we have had lots of requests for site visits. With the frame complete, we invited our Basque joiners around for a celebratory cross cultural meal of haggis, chorizo stew and tortilla, musically accompanied by Gica on fiddle and Eneko on guitar. On the evening of the American Presidential results, it felt good to make those connections.
With the CLT now complete following two weeks of unseasonably good Autumn weather the challenge is now to get the building wind and weathertight before conditions deteriorate with the onset of winter.