Advancing the structural applications of bamboo

Atelier One’s ongoing journey with bamboo started 10 years ago. The practice was invited to visit the Green School in Bali. The school’s bamboo structures demonstrated the potential of a material that many had previously dismissed. Critically, a new treatment process had been developed, sustainably eliminating the risk of insect attack and thus increasing the lifespan. With the strength of bamboo significantly exceeding that of timber, and with only three years required for a culm (stem) to reach full strength, Atelier One was convinced that bamboo can make a significant contribution to the necessary transition to zero carbon construction.

The meeting could not have come at a more opportune moment. IBUKU, the architect for the Green School, was hitting engineering obstacles in its pursuit of bamboo structures. It had reached the limit of what its engineer could justify with analytical methods. Thus started Atelier One’s journey to fully understand this remarkable material.

Over the next few years, Atelier One began to examine the material in detail, talking to experts around the world and studying papers to build up the necessary knowledge, before attempting design. After this initial research phase, the engineers began carrying out engineering design for simple post and beam structures. However, it became clear that these structures were not exploiting the true potential of the material. Due to the low shear strength of bamboo, connections were dominating design.

Atelier One progressed to exploring gridshell structures. The natural flexibility and axial strength of bamboo make it ideal for such surfaces. The practice developed a number of these structures for the Green Village alongside IBUKU (the architect) and PT Bamboo Pure (the builder).

In 2019, Atelier One met with one of the bamboo craftsmen who had been advising it, Jorg Stamm, and Elora Hardy of IBUKU. The Green School was looking for a replacement for its existing gymnasium which had reached the end of its life. The new structure had to utilise the existing foundations. Together they devised the Arc, a 14 m high by 19 m span roof, consisting of a series of bamboo bundle arches connected by bamboo lattice anticlastic saddles, topped with a bamboo mat surface. The structure’s strength would come from its form, with the arches and saddle surfaces acting compositely.

The composite behaviour of such a structural system needed to be fully understood. Due to its unprecedented form, the lack of available test results and the material variability of bamboo grown under different con- ditions, a progression of load tests was completed. First, single culms were tested, then bundles lashed in a variety of different configurations, to check the level of composite action that could be considered. Next came testing different patterns of ‘reb-reb’, a process of notching the bamboo to allow increased curvature. Finally came full-scale load testing of a sample arch and saddle, to check the composite action, allowing calibration of Atelier One’s analytical models.

The Arc has received attention from around the world. It stands as testament to the amazing potential of bamboo.

Bamboo grows to full height in 12 months and reaches its mature strength in three to four years, compared to 20 to 30 years for timber. As a natural material, bamboo has the potential to sequester carbon whilst oxygenating the air. Once a culm is cut, a new culm grows in its place, and this can be repeated five or six times. It means that a single bamboo clump can produce 1000 m of bamboo. These factors combined mean that a properly managed bamboo plantation can sequester phenomenal amounts of carbon, dwarfing that possible, per hectare per year, for a forest.

For the Arc, whole culms harvested from near the site were used. This means that the emissions due to processing and transportation are minimal. The quantity of bamboo required is drastically reduced, due to the structurally driven form, with the arches and saddle surfaces all contributing to the strength.

Combined with the use of existing foundations, the resulting carbon footprint of the building is negligible. This is demonstrated by the A++ rating.

Bamboo can be grown on previously degraded land. It binds the soil together, regenerating it and allowing other plant species and wildlife to prosper. Clumping bamboo, rather than running bamboo, should be chosen, and a multiculture must always be considered.

Bamboo has the potential to make a huge contribution to a more sustainable construction sector, both in its natural form and laminated. Atelier One believes that the time is right for the widespread adoption of bamboo, based on its strength and speed of growth. Work carried out on the Arc has helped demonstrate the possibilities of the material to a wider audience.

When it began exploring bamboo, Atelier One developed relationships with local craftspeople in Bali, learning from the bamboo expertise that had been handed down through generations. Now, with a better understanding of the material, Atelier One has been able to introduce new typologies and structural principles to them. This two way information exchange is a hugely satisfying experience for all involved.

The Arc is the gymnasium for the Green School – the right environment for teaching symbiosis with the environment and cultivating future sustainability leaders.

The Green School has close links with BambooU, a series of residential courses designed to train people to use bamboo. The Arc is an inspiration to the participants.

Once it was decided to pursue the structural use of bamboo, Atelier One realised that it was necessary to fully understand the material. The practice dedicated significant time to developing its own technical guide to bamboo and an index of key players in bamboo. Many of in order to learn from their insight and experience.

As a result of 10 years of study, Atelier One feels it is now in a position to help make sure bamboo is properly utilised. Bamboo is sometimes referred to as ‘vegetable steel’. However, it must not be considered as a straight substitute for steel, or even timber. It is closer to ‘vegetable carbon fibre’, and it must be detailed as such.

The material strength of bamboo far exceeds that of timber, as indicated in the tabulation.

As an example, Atelier One has been able to develop forms that work in tension and compression rather than bending. The forms utilise the rather than fighting it. Atelier One has helped move away from the use of dowelled connections that are limited by the low relative shear strength of the material. It has also helped to introduce the use of geometric stiffness, with forms drawing on nature rather than traditional post and beam construction.

Name Dendrocalamus bamboo Timber (C24)

The Arc is Atelier One’s most successful implementation of these principles to-date. The form is extremely material-efficient, with the entire surface contributing.

The novel form and infancy of bamboo codes meant that extensive testing was required to justify the structure of the Arc. Working closely with the builder, PT Bamboo, Atelier One carried out a series of load tests. These started off with checking the mechanical properties of the individual culms to account for the specific growing environment and progressed in scale. Bundles of culms and notched beams (reb-reb) were then tested.

Finally, a full scale prototype of a single bay enabled the checking of the performance of the composite arch saddle structure under dead and wind loading, with recorded deflections used to confirm and calibrate the analytical models.

It has taken significant research and development, over a period of 10 years, to reach this point. The Arc provides a valuable precedent and

PROJECT DATA

Length

41 m

Width

23.5 m

Height

14 m

Floor Area

760 m2

Construction Time

8 months

PROJECT CREDITS

Project

The Arc at Green School Bali

Location Bali, Indonesia

Client Green School Bali

Design IBUKU

Conceptalisation

Jörg Stamm

Structural Engineer

Atelier One

Builder

PT Bamboo Pure Website for further information www.greenschool.org/bali ibuku.com www.atelierone.com is an important milestone in the aim to champion bamboo as a mainstream construction material.

All images by Tommaso Riva, unless otherwise stated.

The Arc has already won a number of design awards and has received considerable media attention.

The combination of arches and saddle surfaces working compositely in this way is novel. Also, due to structural bamboo being in its infancy, design guidance is limited. The load testing that was carried out has facilitated both the justification of the design and a better understanding of the composite behaviour of bamboo bundles.