The UK’s largest timber building – William Perkin High School in Greenford, West London – has officially opened.
Built to a very tight programme – the school had to open within 12 months of construction work starting – engineers Ramboll chose to use cross-laminated timber for the William Perkin High School.
The four-storey superstructure was built in only four months using 3,800 cubic metres of Austrian prefabricated timber panels, weighing in at 1,700 tonnes and covering a combined floor and wall area of 22,500 square metres.
While the building was originally planned as a concrete frame structure, the team, which consists of architects Feilden Clegg Bradley Studios, contractor Kier Construction, and CLT sub-contractor KLH UK Ltd, switched to a timber superstructure because of the cost and time advantages.
"Even with snow on the ground and delivery restrictions from the surrounding residential area, the majority of the CLT structure was assembled in only 19 weeks. This is because CLT can be put up in any weather or temperature, and with a much smaller and less intrusive delivery schedule than concrete," said Ramboll associate structural engineer Gavin White.
Being less susceptible to winter conditions proved highly useful during the snow storms of Spring 2013. CLT erection continued uninterrupted during the adverse conditions, when more traditional in-situ frame sites would have struggled with the low temperatures.
The school was initially sceptical of the use of timber but soon came to see the benefits of the technology.
Despite the inevitability of knocks and damage, CLT's advantages were clear, said Kier Construction senior site manager Joe Murphy. Whereas concrete has to be protected in certain locations, CLT can be sanded and recoated.
"Impact damage can be repaired and routed for sockets or additions," Murphy said.
For the client, future flexibility and adaptability were also integral to the design.
“The design of the structure enables each of the classroom configurations to be reorganised," White explained. "Solid timber load bearing walls are located where removal is least likely, however the timber enables the formation of further openings to these walls if required. Walls between classrooms are generally not structural and can be reorganised without major revision to the building.”
The choice of CLT and glulam rather than concrete has resulted in fewer, and likely less disruptive, material deliveries, and saved approximately 4400 tonnes of CO2 emissions if timber sequestration is included.
It also offered the ability to commence fit-outs on lower floors while the upper floor CLT frame was still being erected, saving considerable time.
Internally, the timber forms a key design feature for the school. Both the sports hall, which incorporates 11 large Glulam beams, and the building's sunlit central atrium showcase this bright, natural material with timber walls, a floating feature staircase and visible rooflights.
This level of exposed timber required careful consideration to detail. Ramboll worked closely with the design team in fine tuning the timber details to ensure the highest level of finish was achieved.
From a technical specification perspective, the above ground floors are 230-millimetre thick CLT and span 7.5 metres with a 2.5-metre cantilevering walkway. The CLT wall panels range from 95 to 208 millimetres in thickness, decreasing to 95 millimetres at the top floor as a means of saving both cost and weight. Balustrades are 60 millimetres thick.
The panels were supplied in three-, five- and seven-layered panels, with non-visible and living space quality finishes.
"The visible layer has fewer knots and an attractive and smooth finish," said Kay Hartmann, KLH UK's head of design.
The project also made use of building information modelling, winning the Constructing Excellence Awards – BIM Project of the Year
“M&E and CLT structure were used as an integrated BIM model to aid design and perform clash detection,” said White. “This highlighted a number of issues that would have not been obvious otherwise, and definitely helped with on-site co-ordination of the scheme.”
White said early contractor design involvement and detailed fabrication modelling by the subcontractor was integral to fast tracking the design, which was essential to fit with the school’s new term. The project has also proved, despite the building’s massive size, that a fast programme is genuinely achievable.