PREFABRICATED TIMBER PANELS ALLOWED THE STRUCTURES TO BE ERECTED QUICKLY
Three new blocks of student accommodation at University of Wales, Swansea, may look much like any other campus residential development - one five-floor and two six-floor functional buildings on a tight site. However, appearances can be deceptive. Excluding an external cladding of non-structural brick work, these lowbudget buildings have been built from structural timber. The architect, Jonathan Hines of Architype, says this one of the first six-storey timber structures built for use, not research, in the UK.
The Building Research Establishment (BRE) explored building tall in timber at its research facility in Cardington. The TF2000 project built the world's first ever six-storey timber-frame building in a venture with the Timber Research and Development Organisation (TRADA). The project was funded by the government and industry and results were published in 2003.
Architype, an innovative practice with a reputation for exploring the limits of timber in construction, consulted with both BRE and TRADA to develop the Swansea blocks to six storeys using a structural timber frame.
'The use of timber was a joint decision from the client and design team, who wanted to procure the job in the brief space of time available, ' explains Hines. The university's PFI contract was awarded to Gwalia Housing Society, a forward-looking Welsh housing association. 'The team looked at the tight site, and the University's brief to get 272 rooms onto it. We saw we needed to create three staggered blocks that would fit onto the tight space while respecting existing surrounding buildings and trees.'
The team had a tight deadline, with a 12-month construction period before the start of the 2004 academic year.
'Timber proved to be a much cheaper option than steel and a more certain construction programme than volumetric, ' said Hines.
'Prefabricated timber panels allowed the structures to be erected quickly, with cladding following on. This allowed work to start inside on lower floors while the upper floors were being erected.' The load-bearing structural panels are made from structural softwood timber, specified to FSC Forestry Stewardship Council standards and from a sustainable source. Each panel is 2.5m high and 3m wide, with thick hardboard Panelvent sheathing fixed to the outside and Paneline fixed to the inside of 140mmdeep by 50mm timber studs. Gaps in the space between the studs are filled with mineral-wool insulation. The external face board is covered in a breathable paper membrane that faces into the ventilation gap between the structural timber panels and the brick cladding. The inner surface is finished with layers of plasterboard to give an acoustic and fire barrier.
The factory-made panels, delivered to site by lorry, were easily bolted together to form cellular room units. Floor joists were then placed over and the whole process started again one floor up.
Building rigidity is provided by the repetitive cellular room layout.
All the floor plans are exactly the same, so vertical loads transfer directly down through the structural timber walls into the reinforced-concrete foundations. Each cellular study bedroom also adds to structural stability, with walls providing racking.
Horizontally, the timber walls are kept in compression by a timber ring beam in the floor void around the outline plan at each level. The ring beam is a second line of defence should any panel fail or the structure be compromised through an accident, such a a car striking the building. The cellular honeycomb nature of the interior will stop a progressive collapse of the floors above in any area where the load-bearing panel is damaged.
'The main problem the team had was anticipating differential movement between the timber structure and the surrounding free-standing stable brickwork cladding, ' says Leah Dempsey of Architype. 'Junctions between the brick cladding and timber stairwell were detailed to allow for differential movement.
The brickwork is tied to the timber structure using conventional cavity ties, with sliding ties to the upper two floors. The brickwork is just cladding but may reduce the wind loading on the timber.' The lift cores are freestanding steel structures of four braced columns clad in timber panels. 'We designed for up to 35mm of differential movement between the timber and fixed elements, this meant adjusting the lifts during the first year to ensure they aligned with the floors, ' Dempsey explains.
'We have designed timber buildings for many years, ' concludes Jonathan Hines, 'but this was the first time that we had built tall using timber. We found it interesting, allowing us to explore new ideas and be innovative and, more importantly, show that there are new and sustainable ways to construct tall buildings.'