From July 24th – 28th I attended the International Conference of Non-Conventional Materials and Technologies (IC-NOCMAT). The first day of the conference was a workshop held by an engineer and bamboo specialist from ARUP, a structural design firm that has an international development arm. I found the workshop to be very useful because it helped me understand the strengths and weaknesses of bamboo as a material and the importance of appropriate design for its use. Here are some of the biggest points I took away from it:
- Bamboo can last forever if it can be kept free of beetles, termites and rot
- The best and safest form of treatment is the use of Boron (and it’s also readily available as it is used as a fertilizer for agriculture), but it’s biggest weakness is that it can wash out if exposed to moisture
- Therefore all exposed bamboo structures need to be protected entirely from rain and splashing and for rain a 45 degree angle must be considered when designing roof overhangs because rain can always come at an angle.
- There are three ways the bamboo can be treated with Boron. The most traditional way is piercing the inner nodes with rebar and using a bath (7-14 days in cold water, 7 hours in hot water). There is also a “boucherie” method where the end is clamped and the liquid is forced through the longitudinal cells of the bamboo. The last way is called VSD where the bamboo are stood up on end in a scaffold, all the nodes are pierced except the last one, and the bamboo is filled up for 7-14 days.
- In some cases fire protection needs to be a consideration. Bamboo walls can be protected using mud plaster, cement/lime plaster or gypsum plasterboard. The plaster would need to be applied to a matrix that helps it adhere to the wall system. 25mm of mortar or 12mm of gypsum plaster board provides a 30 minute fire rating
- Bamboo is strongest in compression. I always thought it was strong in tension too but because of the connections, bamboo is much weaker in tension. One design consideration is to consider using bamboo in compression where it is strong, and use steel rods in tension.
- Another thing to consider is that bamboo is weak in the cross-sectional direction. Because of this a design should try to create direct load paths. For example, it is better for columns to move past beams so that there isn’t a heavily loaded column bearing on a beam
- Connections are always the weakest point and a few things to keep in mind when designing them are to minimize holes, pre-drill all nails and screws, use dry bamboo (and keep it dry), reinforce against splitting, consider corrosion protection to steel, fill nodes with cement mortar (that will not shrink or expand), and design out areas where water can collect.
- There are few codes and standards available but the most well developed one is the Colombian code NSR-10G developed specifically for the Guadua variety. ISO has developed codes 22156 and 22157 but they currently have errors. The good news is there is an updated version in development. In the meantime a good guideline is the IStructE Note series.
That is a summary of the most valuable lessons I took from the workshop. Of course this summary does not replace the need to consult an experienced structural engineer when working on a specific project. I hope that I will get an opportunity to work with bamboo in Congo! The reason why it has become established as a building material in Colombia is because the government supported it, standards were developed, and architects such as Simón Vélez have made some beautiful projects out of it that are showing the possibilities of what can be done with good design and craftsmanship. So perhaps the same is possible in Congo and we can start using a building material that is available, affordable, and highly renewable!
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