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Bamboo Structural Connection

Bamboo Structures: Composite Beams, SEB & Design Principles

Composite Beams

When using bamboo poles in their natural round form, it is possible to combine them for increased strength.

Composite beams can be made from two or more members fixed together using bamboo, timber or metal pins and galvanized wire.

The combination of culms prevents deflection (sagging) and provides an overall greater strength.

The assembly system is hands on, and uses the bamboo culms in their natural round form. The method is simple and no special tools are required, only a simple jig. The method allows for easy remote construction.

When whole culm bamboo structures fail, their round profile may flatten but the structural fibres which run along the culm length stay intact, which allows for replacements of members before structures collapse entirely. Timber products in contrast, fracture then break at the point
of stress.

Reference Documents:

THIN BAMBOO CULMS FOR TRUSSES. Use of Two and Three Culms in Composite Beams, October 2012, S. Nienhuys

Lamination of Splits

Another main usage for bamboo is to process it into splits (strips). Due to the linear growth pattern and formation of bamboo fibres, splits retain the fundamental strength and structure of bamboo.

When splits are laminated, a more homogeneous and rigid material called plybamboo is created. This is a very similar idea to plywood, where the material grain direction is alternated with every layer, thus creating a more uniform and stable material overall.

It is possible (link to 2.8mb INBAR PDF) to laminate straight strips to produce panels, beams and flooring boards.

More recently, engineers have been working on laminating splits or crushed bamboo to create structural beams known as Structural Engineered Bamboo (SEB) structures, which are very similar to Engineered Wood. 

“Engineered bamboo is made from raw bamboo culms, which through pressure and heat form a laminated composite that is then glued together to form structural parts… the main applications of SEB are structural systems (columns and beams), structural glazing systems for buildings (for entrances, roofs, façade systems), as well as curtain walls and floor-to-ceiling frames.”

Source: ArchDaily

“The only obstacle…lies in the access to knowledge about solutions and examples of buildings and product designs already completed with this material. Spreading awareness of the key performance and sustainability advantages of bamboo over other structural building materials will be an important part of ensuring its future use”.

Source: ArchDaily

3 tips for bamboo design

Keep it dry

Bamboo is like a giant straw. Any contact with moisture, either from touching the ground or being exposed to rain, will result in the bamboo fibres absorbing via capillary action some of this water (mostly from the cut ends), which could eventually lead to fungal attack or rot, which ultimately leads to failure. A simple and practical solution for ground water is to keep bamboo off the ground. Use masonry or other water-resistant materials as the base foundation / connection for any bamboo columns. In terms of water from above, a simple roof eaves overhang that projects at least 70cm should offer sufficient protection.

Keep fibres intact

As much as possible, keep your holes and cross cuts neat, and avoid stressing the material so that it does not forms cracks along its length. A great way to prevent bamboo from cracking or splitting, is to pre-drill “pilot holes” for any screws, bolts or anchors.

Use appropriate connections

Traditionally, bamboo is joined by forming what is known as a “fish-mouth” connection, and the pieces lashed or pinned together to form a structure. In modern times, it is common to use steel threaded bar to connect posts and beams. In some cases, these connection points are reinforced by pumping cement mortar into the connecting internode (the hollow part of the bamboo). Drill “pilot-holes” for screw connections to avoid bamboo splitting. Do not use nails as they split the bamboo.

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