Bamboo is “stronger than steel”, fast growing and very sustainable – but is it a realistic alternative structural material?
Mankind has been harvesting bamboo for millennia. Its new shoots are eaten, a fermented bamboo paste is reputed to cure impotence and sterility, and the mature culms (stems) have been used to fabricate a vast range of artefacts, from medieval weapons to modern bicycles. Bamboo has also had many structural applications, most commonly low rise vernacular housing but also more demanding structures such as short span simple suspension bridges.
Bamboo scaffolding is still much used in the Far East, particularly in Hong Kong, and can rise up many storeys. Its actual construction, like most bamboo structures, is intuitive and craft based. As a structural material, bamboo is in much the same position as timber construction was until the 1940s, with little reliable guidance on how to design and construct bamboo structures to modern standards.
Recently, however, the growing acknowledgment that sustainability is a key parameter in selecting construction materials has spurred research. One of the leading research teams can be found in London’s Fitzrovia, at the headquarters of consultant Arup.
“It all started about five years ago, with a brief to develop affordable seismic resistant housing in El Salvador,” says Arup senior engineer Sebastian Kaminski. “Although there was no tradition of bamboo construction and no suitable local bamboo, we were able to come up with an improved version of the local “bahareque” technique – basically a form of wattle and daub – that could be used in many other areas of the world.”
The original prototypes used a timber structural frame with infill panels made up of matrices of local cane – a smaller relative of bamboo – clad externally with galvanised chicken wire and plastered with a cement based mortar. Foundations were a thin reinforced concrete slab with reinforced masonry upstands.
Shake table testing at Cambridge University showed the design had excellent seismic performance, a key factor in El Salvador, which is one of the world’s most active seismic zones. It can also be adapted to take advantage of local material availability, so an “all-bamboo” version is a realistic option. Arup then realised the basic principles could be adapted to take advantage of local material availability anywhere, so an “all-bamboo” version was a realistic option.
We’ve found it possible to change perceptions through proper design
Arup Advanced Technology & Research associate director Andrew Lawrence
What Arup has dubbed “engineered bahareque” houses can now be found in developing countries around the globe. Arup has recently produced a design guide on behalf of the International Network for Bamboo and Rattan (INBAR). Arup Advanced Technology & Research associate director Andrew Lawrence, however, says there is often initial resistance to the concept of bamboo housing, even where bamboo construction is traditional.
“It has a bad reputation for durability,” he explains. “Unlike some timbers, bamboo doesn’t contain any natural toxins to protect it against fungus and insects.
“If it’s constantly exposed to the elements it can last less than two years. However, we’ve found it possible to change perceptions through proper design.”
Bamboo’s thin walls, usually only around 10% of the outside diameter, exacerbate the problem. Paints and varnishes are only a partial solution, Lawrence explains.
“The outer skin is smooth and silica-rich, so coatings don’t adhere well, and tend to crack and allow moisture in. Keeping it dry is the best solution.”
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Well-dried bamboo is also less susceptible to insect attack. Powderpost beetles are attracted to the sugars and starch in the sap, so lowering the sap content is an important part of bamboo harvesting and preparation (see box below). Termites can be even more dangerous, as they can digest cellulose as well.
Boron-based preservatives are a safe, cheap, effective and readily available option for protecting bamboo against fungal rot and insect attack – unfortunately their water-soluble nature means they are only viable for interior use. More effective copper-based treatments exist, but these add significantly to the costs.
There are those who believe that bamboo is “stronger than steel”. This is a popular misconception, Kaminski says. “A good bamboo will have similar properties to high-grade hardwood, parallel to the fibres at least.
“Depending on species, allowable stresses in bending, tension and compression will range from 10N/mm2 to 20N/mm2. Allowable shear stresses are lower, due to bamboo’s tendency to split.”
Arup advises that bamboo beams should never be heavily loaded, otherwise there is a risk of crushing or shear failure at the supports. Bamboo works best in axial tension or compression, although connecting members in tension is still a challenge, and more research is urgently needed, says Lawrence.
“Traditionally bamboos have been lashed together.
“This works, but strengths are both low and unpredictable. What we need are better bolted joints, similar to those used for timber.”
We’re only recommending bamboo construction for one or two storey housing, where escape times are very short
Arup associate structural engineer Sebastian Kaminski
A “low-tech” solution is generally to be preferred. Filling the open end of the culm down to the next nodal diaphragm with cementitious grout helps transfer stresses: simple pipe clamps can restrain any splitting tendencies.
Performance in fire is another issue that has to be addressed. Although bamboo will char at much the same rate as timber, being hollow it loses proportionally much more of its cross sectional area in a given time.
“In practice this isn’t a problem,” Kaminski asserts. “We’re only recommending bamboo construction for one or two storey housing, where escape times are very short”.
Engineered bahareque housing uses cement renders to protect bamboo from water and fire. Its excellent performance in earthquakes, however, has little to do with any unique properties of bamboo. Historically, bamboo structures have survived seismic events primarily because of their lightweight nature and energy absorbing connections, nailed joints in particular performing well.
There are rumours that a 100m high bamboo structure is now on the drawing board
Modern bolted and grouted joints are much less accommodating, but effective nailed joints are difficult to achieve due to the risk of splitting. More resilient bolted joint designs are needed.
Architects are beginning to design non-traditional bamboo structures, Lawrence reports. “These are usually very lightweight structures with more rigid bolted connections.”
There are rumours that a 100m high bamboo structure is now on the drawing board, somewhere in the world. Surprisingly, perhaps, the real centre of research into whole bamboo construction is Colombia. In the Far East the focus is mainly on processed bamboo, in many forms, from furniture to fabric.
Wherever it occurs, the increasing research effort should eventually lead to bamboo becoming a mainstream structural option in many parts of the world. Its rapid growth and outstanding sustainability should see to that.
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More than 1,400 species of bamboo are known to exist, growing naturally in countries as far apart as Australia and Argentina, the Himalayas and the south eastern United States. Around 100 of these are suitable for construction.
Bamboos are members of the grass family, and include some of the fastest growing plants on Earth. Growth rates of more than 900mm in 24 hours have been recorded, peaking at 1mm every 90 seconds. Stems are columnar rather than tapering, with leaves and branches only emerging at full mature height.
Some species can be as tall as 30m or more, with stem diameters up to 300mm. The stems – more properly culms – have a limited life span, and reach their maximum dimensions in the first year.
Harvesting takes place when the culms are fully hardened. This can take between three and five years, depending on the species. To reduce sap levels and minimise the risk of insect infestation and fungal attack (see main story), the culms are harvested during the dry season, traditionally at dawn or dusk.
Subjecting the harvested culms to traditional leaching with water reduces sap levels even further. For the highest levels of pest resistance the leached culms must be dried slowly and evenly to avoid cracking the outer skin. Typically, structural grade bamboo will be available in 6m to 10m lengths and diameters up to 100mm.
Engineering a better world | Sustainable materials