Sustainable, economical and architecturally inspiring: the rise of timber as a modern building material

William Perkin CLT atrium

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.

CLT Panel
CLT Panel

 

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.

William Perkin Cross-laminated timber staircase
William Perkin staircase

 

Sustainability

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.

Worldwide CLT Production
Worldwide CLT Production

 

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.

Economic advantages

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.

Architectural creativity

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.

William Perkin CLT atrium
William Perkin CLT atrium

Conclusion

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

The Rise of CLT

3D View: Continuous Wall and Notched Floor with Nail Plates

Cross-laminated timber (CLT) is growing in popularity, with worldwide production of CLT expected to reach 1 million m³ by 2015 (Schickerhoffer). The ambition of timber engineers around the world is growing in equal measure as we push the limits of the material, by building ever taller structures.

In 2008 the Stadhaus, in London, became the tallest residential timber building in the world, reaching a lofty 9 storeys. It was a game changer; modern timber structures had never been built this tall before. Five years passed before the Stadthaus lost its title of tallest residential timber structure to the Forte building in Melbourne; a residential timber building one storey higher than its predecessor, standing at 32.2m.

The race continues to build the next world beater, but going higher poses technical challenges, not least ensuring the stability of the building. The uplift forces at the base of a CLT building, due to the wind on the structure increase in proportion to the square of the building height. This rapid increase in loading means that the next generation of timber buildings (10-20 storeys in height) will require the development of new connection details to prevent, the floor plates from crushing between the walls and the building lifting off the ground.

Uplift Force at Ground Floor vs Number of Storeys
Uplift Force at Ground Floor vs Number of Storeys

Recently at Ramboll, we have been involved in the design of, what will become, the tallest residential timber building in the UK. Naturally this has caused us to consider some solutions to overcoming these problems. Here is a selection of ideas that we have been considering:

 

Notched floors and walls

This solution uses castellated CLT floor slabs to provide direct load transfer between walls. Nail plates are positioned at the castellations to transfer uplift forces between the CLT walls.

 

3D View: Continuous CLT Wall and Notched CLT Floor with Nail Plates
3D View: Continuous Wall and Notched Floor with Nail Plates

 

Hardwood Dowels

Hardwood dowels within the CLT floor slabs prevent crushing of the floor by loading timber in the strongest and stiffest grain direction.

3D View: Hardwood Dowels Prevent Crushing of CLT Floor Plates
3D View: Hardwood Dowels Prevent Crushing of CLT Floor Plates

 

Bolt-through Connection

This bolt-though connection uses proprietary high capacity uplift connections bolted through the CLT slab to transfer forces between the walls.

Section: Bolt-Through Connection
Section: Bolt-Through Connection