Renaissance for wood construction
Portcullis House is a stellar example of American hardwood used to good effect, both structurally and aesthetically
Hanham Hall homes are 10 per cent better in energy performance than current UK building regulations standard
Civil engineers and architects need to aim for maximum load, minimum materials
Timber buildings may soon be on course for a comeback, driven by an unprecedented push for sustainable development.
From man's earliest days wood has been used as an important building material. 'The Epic of Gilgamesh', the oldest surviving written story, talks about the magnificent wooden palaces and temples built in Mesopotamia during the early Bronze Age of 3000BC.
When it comes to traditional houses, timber framing dates back to Neolithic times. The shortage of timber and the advent of bricks and mortar curtailed its dominance in the developed world, but the dawn of the modern sustainable culture has given it a new lease of life. Timber and wood-based engineered products are becoming very important as structural materials, especially in the drive towards sustainable technologies and construction.
According to a study last year by the US Forestry Service, greater use should be made of wood in both commercial and residential buildings. "Using wood obtained through sustainable forestry practices in green building applications promotes a healthy environment and a strong economy," Michael A Ritter of the US Forestry Service says. "To take advantage of this win-win opportunity, stakeholders must overcome existing misconceptions about wood as a green building material and help lead the research and development efforts on green building materials.
"Harvesting, transporting, manufacturing and using wood in lumber and panel products in building yields fewer emissions - including greenhouse gases - than the resource extraction, manufacture and use of other common building materials.
"Sustainable forest management can produce stronger, healthier forests that serve as a 'carbon sink' to clean air of greenhouse gases and purify drinking water for wildlife," Ritter continues. "Harvested trees can find value in wood products and systems for green building construction that continue to benefit the environment."
However, the study also found that despite documented advantages, most building professionals and members of the public do not recognise wood as a renewable resource, or the role efficient wood use plays in mitigating climate change and promoting healthy forests.
The wooden Olympics
A prime example of the use of wood in construction came at the 2012 Olympic Games that swept through London in August. The entire London 2012 Olympic Park development achieved dual project certification from the Programme for the Endorsement of Forest Certification (PEFC) and the Forest Stewardship Council (FSC) schemes for its timber usage - the first dual project certification in the world.
This groundbreaking achievement in sustainable timber procurement has played a prominent role in helping the Olympic Delivery Authority (ODA) achieve its commitment of making the London 2012 Olympic Games the greenest games ever with 100 per cent of wood products certified as legal and sustainable.
The Games' east London site comprised seven major venues across the 246-hectare Olympic Park and the thousands of timber products entering the site had to be managed on a daily basis. The ambitious approach was driven forward by the establishment of the Timber Supply Panel that embedded strict policy and procurement procedures and implemented a delivery management system that was forthright in policing and educating stakeholders and their supply chains about sustainable timber and unbroken chain of custody.
The best option
Mainstream UK home builder Barratt has seen the amount of wood it uses grow in recent years. "It is something that we do use quite a lot of as a business and not just from a visual perspective: it is a key element of our standard specification," Stephen Wooldridge, group sustainability manager at Barratt says. "It is not something that we use en mass from a structural perspective - timber-framed buildings or SIPS [structural insulated panel systems] - but the odd project pops up every now and then where it is the best option."
Traditionally, the standard specification for a UK builder would call for either FSC or PEFC certified softwood to be used as roof trusses, floor joists and timber studwork as well as the usual fixtures and fittings.
"There are two aspects to timber-framed buildings - what I sometimes refer to as stick-built timber frame, which is erected on site using a series of timber members that are then clad with material," Wooldridge says. "Then there are the prefabricated structures - often referred to as SIPS. You receive walls and floors as cassette pieces and these are constructed on site.
"One of the benefits of timber-framed buildings is the speed of build. It is a quicker form of construction than brick and block masonry. There are also benefits in terms of carbon content, although the block manufacturers are doing everything they can to increase the environmental friendliness of their material. It can perform more efficiently from an in-use perspective, because there is quite a bit of space within the construction that can be filled with insulation. This allows you to get down to some quite good U-values [heat-loss measurement] on the performance side of things."
One current example of the greater use of wood from Barratt comes at its Hanham Hall development near Bristol, the site of a former psychiatric hospital and the open land and gardens which surround it. The development will become a benchmark for sustainable housing development by using cutting-edge building technology in the form of SIPS panels as well as community heat and power using biomass CHP.
The proposal currently includes 188 homes and 2,400m2 of other uses including a sustainable living centre, office space, a caf' and cr'che.
The homes are 100 per cent better in energy performance than the current UK building regulations standard. The historic building will be refurbished to the highest UK sustainable office standard for reuse as a community and employment facility.
"It is a SIPS construction," Wooldridge says of Hanham Hall. "It is going to be built to the new zero carbon definition, which is going to be the building regulations by 2016. In order to get to that standard, they have gone down the SIPS panel route with its very high insulation properties."
There is, however, a dark cloud on the horizon for timber-framed buildings and that is its cost, which offsets the technology's prime benefit, speed of construction. "At present the benefits of building quickly have rather been negated by the state of the economy - we can't sell houses quickly so there is no need to build them quickly, therefore it comes down to cost and that is where brick and block do just nip it. Potentially we could be looking towards more and more wood buildings in the future but that will be guided by cost and the uncertain regulatory landscape with Part L of the building regulations [Conservation of fuel and power]."
Arrival of hardwoods
Traditionally wood used in buildings has been the fast-growing softwood varieties, but there is a growing consensus that hardwoods have their part to play as well. The visual benefit of using hardwood in construction has long been acknowledged, as the colours and grain patterns in oak and chestnut are aesthetically far superior to the softwood of fir and pine traditionally incorporated into European structural design.
Timber is the most sustainable constructionmaterial, because as well as it being renewable, it also absorbs carbon dioxide as it grows. Coupled with the increasing volume of sustainable North American hardwood being produced, it would seem American hardwoods would be a timber high in demand.
Until recently, European integration of hardwood into weight-bearing applications was avoided due to the lack of sound research into the load-bearing qualities of the material. To address this sparse area of research, the American Hardwood Export Council (AHEC) undertook a high-profile British project in 2001 to prove the structural benefits of the wood, using an Arup-designed hardwood latticework roof as the crowning feature of the new parliamentary building in London, Portcullis House.
For many years, regional strength classes and species properties have been a safety sticking point for architects wanting to use American hardwoods in structural design. As with most building materials, there are various standards that address the yield, durability, density, knot grading, strength and stiffness of the wood, but due to hardwood's varying nature it often does not dovetail into a specific category across every regional standard.
Bringing America to London
In Europe, softwoods and hardwoods are separated into two profiles, C14-C50 for softwoods and D30-D70 for hardwoods. While the existing class system is an acceptable method of representing strength and density for the softwoods, the grading does not provide accurate profiling of the four American hardwoods at the heart of the AHEC's research.
For example, while American white oak fits the strength and density profile of the D50 legislation, American red oak can only achieve level D40, despite being stronger during bending than its white cousin.
The Portcullis House latticework project set out to increase the popularity of sustainably produced American Hardwoods, quickly becoming one of the most complex timber roof structures in Europe. Constructed from American white oak, the arch-shaped roof supports a glazed roof in the building's 50m x 25m courtyard.
The project's architect, Michael Hopkins & Partners, proposed a very light structure with a preference for American white oak due to its grain pattern and texture. As research on the strength qualities of American white oak was so sparse, Arup commissioned a testing project from the Building Research Establishment (BRE) to determine whether the design in this material was feasible.
Although no single visual grading standard is accepted throughout Europe, BRE tested up to 1,000 of each species to the UK grading rule BS 5756, as it complied with European measurable standards. Rigorous testing included moisture content, elasticity, density and bending content of the wood samples.
The testing revealed that the American hardwood would be of a good enough quality to build the latticework, with the strength needed to support a glazed ceiling. The strength and density of the wood allowed the lattice's basic frame modules to be made as fine as 200mm by 100mm at cross-section, with each one bolted to stainless-steel blades which lock into a steel-sphere at the centre. Cigar-shaped columns provide structural support for the roof, which features glass panels on a grid of tensioned metal rods, creating a stunning yet solid structure.
The Timber Wave
AHEC has continued its positive contribution to American hardwood's reputation by commissioning the American oak 'Timber Wave' again with Arup, in collaboration with Amanda Levete Architects and Cowley Timberwork.
Benefiting from the original American hardwood research carried out by BRE for Portcullis House, the Timber Wave is a spiralling wooden sculpture erupting from the doorway of the V&A. It is a steel-jointed wooden structure, featuring American red oak glulam chords, which are created by gluing thin slices of curved timber together.
CAD was used to model the sculpture and addressed the trade-off between the thickness of the wood and the tightness of the curve. The tighter the curve, the thinner the laminate. Traditionally in glulam the laminates are 35-40mm thick, but here they are only 7mm. The wood has been treated with a biocide stabilising oil to ensure it can withstand external use after its removal.
"We wanted to demonstrate the technical and conceptual properties of the material," Amanda Levete, founder of Amanda Levete Architects, says. "Normally I feel uncomfortable if a design doesn't have a function at its core, but the purpose here is very clear, to create something that is self-supporting, that is made from timber with minimal steel connectors."
Stress test: Ultra-lightweight construction
Researchers from the University of Stuttgart together with Bosch Rexroth have invested their efforts in realising the structural engineering ideal of maximum load capacity with minimal consumption of materials.
They have constructed a wooden shell that is much thinner than anything previously deemed possible, measuring 4cm in thickness, and spanning a surface of over 100m2. This extreme slimness is made possible through the use of an adaptive structure.
Structures are traditionally designed for an exact maximum stress. However, researchers concluded that this type of stress generally occurs only very rarely and for a short period. Therefore a good deal of existing materials design serves these extremely seldom peak loads. The ultra-lightweight structures are aimed at achieving a drastic saving of materials and a better reaction to dynamic loads through an active manipulation of the structure. In the case of the Stuttgart wooden shell this manipulation is achieved through hydraulic drives, which rest on the points of support of the shell and generate movements that compensate in a specific way for deformations and material stresses caused by wind, snow and other loads.
The wooden shell is supported at four points. Three of these can be moved by hydraulic cylinders and freely positioned in space. Sensors record the load status at numerous points on the structure. Targeted movements of the points of support counteract variable loads (for example, through the loads derived from snow or wind) and thus reduce deformations and material stresses. Compared to conventional, passive structures this considerably reduces the use of materials for the shell.
The load balancing takes place through a control system which was especially developed for hydraulic drives. The core task of the control system is to implement the complex hydraulic control tasks of the shell structure. In this way the supporting structure can react to a change in the load status within milliseconds.
An active vibration dampening and the adaptation to changing loads can be applied in many areas of construction such as in stadium roofs, in high-rise buildings or in bridges.
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