On course to be the tallest building of its kind, Dalston Lane is a 121 unit residential development in Hackney, designed by Waugh Thistleton Architects. Estimates show that it will use more timber than any other scheme in the world, making it, by volume, the largest CLT project globally.
Ramboll has carried out full structural engineering of the 33m CLT structure, which necessitated the development of bespoke tools, details and techniques, where Ramboll engineers drew on their decade of experience in pushing the boundaries of this dynamic material. Working with specialist timber subcontractor B&K Structures, a series of technical challenges were overcome, to ensure an efficient, buildable and highly coordinated solution.
The CLT superstructure is currently being erected on site and will be completed over several phases. We look forward to providing updates as construction progresses.
The recently completed extension to Mayfield School in Redbridge, north east London, is a showcase of the benefits that extensive use of BIM combined with CLT can bring.
Working alongside David Miller Architects, main contractor Bouygues UK and cross laminated timber (CLT) specialist KLH UK, Ramboll provided a full suite of engineering services for the project including structural, civil, mechanical, electrical and acoustics.
With a structural frame formed from CLT and gluelaminted members, the new facilities provide sports halls, dance studios, teaching and social areas for 800 students and 80 staff. The decision to adopt a timber frame approach was taken at an early stage and was key to achieving the 17 month programme for design and delivery, as off-site manufacturing allowed the frame to be taken off the critical path, reducing time on site, while the lightweight timber frame eliminated the need for piled foundations.
The CLT superstructure was completed in just 12 weeks and fit-out started on the lower floors whilst the upper floor frame was being erected. The CLT was left exposed where possible which reduced the number of follow on trades, as well as creating a high quality finish. In using some 2900m³ of timber, the building’s structure has stored of the order of 2500t of CO2, enough to offset the building’s operational carbon for a period of almost 10 years.
The adoption of a fully integrated BIM approach was also vital to the success of the project. All meetings of the design team were held in the ‘Mission Room’ at David Miller Architects, where a federated model from all disciplines within the design team and key members of the supply chain was used to communicate proposals, check for clashes and to ensure buildability. This rigorous review process allowed the coordinated model to be used to CNC cut the CLT, ensuring that structural and services openings would be ‘right first time’.
The use of an integrated design and construction process at Mayfield School allowed for the procurement and delivery of high quality bespoke solutions, whilst working within the reduced budgets and constrained time frames demanded by the urgent need for new school places.
When completed, Dalston Lane will be the tallest residential CLT building in the world. 9 storeys of CLT structure sit on a first floor concrete podium slab, reaching a height of over 32m. Ramboll are currently working as CLT engineer and form part of a design team which includes B&K Structures, Waugh Thistleton, XCO2, PJCE and Regal Homes.
A building of this size has brought many new challenges, and requires a new approach to design. Complex and ground breaking finite element modelling techniques have been utilised to assess and optimise many aspects of the building, including the building loads, wall panel design and floor deflections and vibration. The upshot of this is that less timber can be used in the structure, reducing weights and improving the sustainable credentials of the construction material.
The first delivery of cross-laminated timber is scheduled to be on site in June 2015, and the entire structure will be complete before the end of the year.
Timber is often regarded as one of the most sustainable materials for mainstream construction projects, while at the same time often criticised as being expensive and architecturally restrictive. Yet new thinking has meant timber can now be approached as an economically viable option for large scale projects that does not mean sacrificing on architectural creativity.
Leading the way in a new timber revolution is cross-laminated timber (CLT). Layers of softwood timber planks, of varying thickness (between approximately 20mm and 50mm), are glued and pressed together at right angles to each other. This creates a solid cross section of timber, which can then be used as walls or floors. The ability to use the panels for both in-plane and out-of-plane loads, as well as the variety of sizes available, creates a key flexibility in design, while timber boasts the lowest embodied carbon of any construction material.
In illustrating just how economical, sustainable and creative timber can be, we believe the recent work completed by Ramboll at William Perkin High School in West London offers a clear example of why timber should now be a serious contender when selecting building materials.
William Perkin High School, Greenford
The £20million school building opened just this year, with a total floor area in excess of 13,000m2 and space for over 1,200 students. The entire superstructure is constructed out of CLT panels, and represents the UK’s largest solid timber panel building.
The construction of William Perkin used some 3,800m3 of timber, which will have generated around 1,300t of embodied carbon emissions. However, this volume of timber is thought to store approximately 3,100t of CO2. Therefore, in taking account of this carbon store, the carbon footprint of William Perkin is negative and can offset the building’s operational carbon for a period of approximately 10 years. The building is therefore ‘carbon neutral’ for 10 years.
CLT is largely produced from timber grown in sustainably managed forests in the Nordic region, Germany, and Austria, and all timber used in William Perkin is PEFC certified – meaning that for every tree felled at least two are planted. As a result, the area of forest in these regions is actually increasing despite the production of significant amounts of timber.
Air tightness is another key factor in the drive to reduce operational carbon because heat (i.e. carbon) escapes through gaps in the building fabric. Without any additional measures to normal fixing details, William Perkin achieved an air-leakage level of 2.8m3/(h.m2), far superior to UK building regulations. Finally, construction resulted in virtually no on site waste from the structural frame erection, which is highly unusual. This is derived from the fact that CLT is pre-fabricated off-site and delivered on-site in a ‘just-in-time’ fashion.
In examining the economic opportunities of CLT, shorter programme length is crucial. By using CLT instead of concrete the superstructure construction period was reduced from an estimated 38 weeks to just 19 weeks. The financial implications of reducing program length are difficult to quantify, but savings are likely to be significant, with wages and all associated costs hugely reduced.
As well as being quick to assemble, the CLT structure enables construction firms to achieve much earlier weather-tightness. The panels themselves form a weather-tight structure and so no further weather-tight measures are required, and the precision cut nature of the panels also allow windows to be pre-ordered. This means that follow on trades, such as fixtures and plaster work etc, can start much earlier in the programme than under traditional structures, saving further cost and time.
By looking at costs as a whole, despite higher upfront costs for the material, with CLT the other efficiency gains should translate into clear cost reductions when compared to traditional steel and concrete frames.
Finally, William Perkin offers clear evidence that timber panel buildings are not restricted to simple rectilinear designs, as many still wrongly believe. It showcases bold yet functional architecture, and highlights its timber frame through employment of extensive timber finishing to the interiors. Features such as the sunlit central atrium, running the 4 full floors, open onto light-filled classrooms and encase multiple open timber staircases. Exterior hightlights such as stainless steel strips within the cladding offer articulation, and the buildings shape fits with both its function and location. Quite simply, the creativity of client and architect need not be restricted in the use of timber, and they may even be inspired by its distinctive character as a fresh and modern material.
With William Perkin High School up and running, the team at Ramboll are excited for the future possibilities for CLT. William Perkin has shown that CLT can be the right solution with its carbon credentials, reduced construction waste and programme length, meaning it should always be considered as a potential structural solution.
Guest Post by Gavin White, Structural Engineer, Ramboll UK
Ramboll collaborated with SDC to provide a new covered seating space for Cambridge charity, Papworth Trust. Ramboll employees donated their time and expertise to carry out the design and construction – resulting in an innovative hinged plywood canopy, inspired by a tortoise shell, fixed to a gabion seating base.
Formed from 15mm thick CNC cut plywood and fixed together with 216 hinges, the canopy spans 5m. Each panel for the canopy was individually decorated on the interior by the clients from the Trust.
Following the annual Chariots of Fire charity relay race in late 2013, where the Papworth Trust were the beneficiaries, the Cambridge office were asked if there were other ways in which they might be able to support the charity. A meeting on site led to the decision to build an interesting external structure, to better make use of the green space adjacent the front entrance.
Office lunchtime scheme design sessions spawned three distinctly different ideas, which were then pitched to the clients. The Tortoise, a seating area with a shell canopy, was chosen to be taken forward.
The base and the canopy were developed by two separate teams within the Cambridge office. The base took on the form of two semi-circular seating areas, 3.5×5.5m on plan, constructed of rock gabion cages with a timber frame for fixing the canopy and seating slats, and for preventing uplift on the canopy.
Meanwhile, the canopy developed from two separate halves to a single shell with two openings. Working with the Computational Design team in London, the planar geometry of the panels was determined, and a SCIA analysis of the shell used to inform the plywood design. Drawing on experience in similar projects, such as the TRADA pavilion and the Fitzrovia chalkboard, detailed fabrication drawings were produced for CNC cutting, including all the holes for 216 bespoke hinge fixings.
Involvement of the centre users was a key driver throughout the process. The design had to fulfill a purpose as a usable, exterior space, and to ensure they were getting what they really wanted, the clients were given the choice of the three potential schemes at an early stage. TO make the structure their own, each panel for the canopy was individually decorated on the interior by the clients from the Trust.
Ramboll worked with local contractor SDC to acquire the materials for the project. Whilst Ramboll worked as quantity surveyors on the design, SDC used their contacts and financial resources to coordinate the orders and delivery of the materials.
Members of the Cambridge office, as well as friends and family, kindly donated their evenings and weekends to carry out the construction of the Tortoise. In addition, SDC provided two labourers for 3 days, and 12 CSR days were also utilised to complete the build. In total, the build took 340 hours of work, spread across four weeks.
The build was formally opened on the 26th June 2014. Papworth Trust is keen to replicate the build at their other facilities, saying “We couldn’t have imagined how fabulous it was going to be – and by consulting with our centre, our service users really felt part of the project”.
Timber Tales is a blog where we share insights about the exciting world of timber engineering, showcasing the latest innovations, beautiful structures and Ramboll’s timber engineering expertise.
We are looking forward to sharing stories about our: recent project wins, completed projects, technical insights, latest research collaborations and much, much more. This year Ramboll is celebrating a decade of design with cross-laminated timber (CLT) and so it is fitting that we whet your appetite for future blog posts by looking back at our largest and smallest timber projects from the last decade.
The smallest: Cavendish Avenue
Cavendish Avenue is our smallest timber project. Using 95mm thick cross-laminated panels of sustainably sourced fir, spruce and pine, the watertight frame was erected in six days. The most striking structural feature of this Modernisst, box-like building is a first floor wall panel designed to support floor and roof loads above. We worked with our client to fit the roof with an architecturally-designed skylight that runs the length of the house. As well as maximising natural light, the skylight provides a visual centrepiece.
The largest: William Perkin High School
William Perkin High School in Greenford, west London, is a four-storey complex and the largest cross-laminated timber building (CLT) in the UK, containing 3,800 cubic metres of timber. The project was initially planned as a concrete frame building but was changed to CLT by the principal contractor, Kier Construction, who preferred the shorter programme CLT construction offers. The school comprises a triangular-shaped building, containing a sunlit full-height central atrium, attached to a rectangular sports hall block. Internally, the CLT is an exposed feature of the triangular central atrium, including a CLT feature staircase.
Head Teacher Keir Smith, is pleased with the completed building saying:
“I initially had reservations to change our proposed school from a concrete frame to a CLT frame which may affect our vision for the student experience. But all the comments we raised were fully discussed and dealt with in the most professional manner. All concerned are now thrilled with the decision we made to convert.”