Bamboo is a material that embodies values of sustainability and durability while being cost-effective. A fast-growing grass, bamboo has been used as a building material since time immemorial. With over twelve hundred species across 110 genera, this primitive grass is a versatile material. A Bamboo consists of a rhizome which is its anchor underground, a culm that is its stem, nodes, and internodes.

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Parts of a bamboo: source: ©https://gardenerdy.com

 While we widely use bamboo in various small-scale cottage industries, its use in the construction industry is because of its structural and economic advantages. A Bamboo culm has twice the compressive strength of concrete and equivalent tensile strength of steel, thus making it one of the strongest construction materials. We cannot directly use bamboo for construction post-harvesting. An untreated bamboo culm has a brief life span and is prone to rot and insect attack. Prophylactic treatments such as water immersion, curing, smoking, chemical application, and whitewashing with slaked lime remove all the starch, protecting the bamboo from deterioration. 

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Bamboo culms being left to dry in the sun: source: ©https://www.bamboo-earth-architecture-construction.com
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Bamboo culms being treated with borax powder: source: ©https://www.bamboo-earth-architecture-construction.com
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Bamboo culms being immersed in a solution of boric acid : source: https://www.bamboo-earth-architecture-construction.com

Although bamboo has several advantages, construction with bamboo has its challenges. A bamboo culm has a round profile which makes the connections complicated. As the fibers in bamboo only grow in the vertical direction, it is not suitable for cross loads. The outer surface of the bamboo lacks grip and is very slippery, making connections difficult. Primarily there are two types of connections that are used to address the aforementioned problems while constructing with bamboo.

1) Traditional connections

  1. A) Friction tight rope connection
  2. B) Wedge connection 
  3. C) Plug-in bolt connection
  4. D) Positive Fitting 

2) Modern connections

  1. A) Bamboo-Tec
  2. B) Transportation Armature
  3. C) Induo-Anchor Technique
  4. D) Pan knot space truss

Friction tight rope connection 

The ‘friction tight rope’ connection is widely used in traditional construction. These connections use natural materials like rattan, coconut fiber to join the bamboo culms together. Lashings, wraps, fraps, and clove-hitch are some knots used in these connections. To get tighter connections, we use green bamboo strips. These strips are watered before use and shrink while drying, resulting in a stronger connection.

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Friction tight rope joinery using rattan lashing: source: ©https://bambus.rwth-aachen.de
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Friction tightrope joinery using bamboo strips: source: ©https://bambus.rwth-aachen.de

Wedge connection 

In a wedge connection, a wedge-shaped wooden piece is driven at the joint of two bamboo members.

However, this connection requires additional reinforcement through the use of lashing or bolts. 

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Diagram showing the Wedge connection: source: ©https://bambus.rwth-aachen.de

Plug-in bolt connection 

The Plug-in bolt connection is not widely used and works on the similar principle of the tenon and mortise joint in wooden joineries. We must take care to avoid using the plug-in bolt connection close to the edge of the culm to prevent the splitting of the bamboo.

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Diagram showing the plug-in bolt connection: source: ©https://bambus.rwth-aachen.de

Positive Fitting 

Positive fitting connections are widely used in traditional bamboo construction. These connections involve carving a hole in a bamboo culm and inserting a bamboo of a slightly smaller diameter. The joint is further strengthened using bolts or dowels. The disadvantage of this jointing technique is the reduction of the strength of the bamboo because of the cutting of the hole.

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Positive fit connection: source: ©https://www.google.com

 Bamboo-Tec

Bamboo is a natural material and is thus subject to the irregularities of diameter within a single culm. The ‘Bamboo-Tec’ connection is a modern technique developed by Bruno Huber to counteract these irregularities. This connection uses bamboo culms that are cut to the desired length and are capped with artificial resin. The end caps are made of steel or aluminum and have circular grooves on the inner side. These grooves facilitate a stronger connection between the cap and the bamboo.

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Diagram of the bamboo tec connection from the patent: source: ©https://bambus.rwth-aachen.de

Legend:

1 single strunt

2 free end

3 connection element

4 bore

5 strunt axis

6 adhesive

8 notches

9 notches

10 thread bore

11 cross bore

12 joint element

13 sphere

14 rotation axis

 Transportation Armature  

This method involves the threading of hollow bamboo culms with a metal armature. We fill the residual cavity with concrete. This connection increases the speed of construction by eliminating the time required for the fabrication of dowels.

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Diagram showing the threaded armature: source: ©https://bambus.rwth-aachen.de

 Induo-Anchor Technique

The Induo-Anchor technique is used for bamboos with larger diameters. The joint comprises an anchor, which is a spherical node usually made of cast-iron. The node is drilled and tapped at varying angles to create desired connections. We thread the bamboo culms into the nodes using conical end bolts.

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Image showing the central node: source: ©https://www.google.com

 Pan knot space truss

A pan knot space truss comprises two components: a ball knot and thread rods which form the intermediate connection between the central knot and the bamboo culm. The advantage of this connection is the provision for dismantling. We use this connection for canes with a smaller diameter.

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Image showing the pan knot space truss: source: ©https://bambus.rwth-aachen.de
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Image showing the dismantled pan knot space truss: source: ©https://bambus.rwth-aachen.de

The conventional applications of bamboo in the building industry range from scaffoldings to structural and non-structural elements of a dwelling. The construction of a bamboo house is characterized by a frame-system similar to timber construction. The wall, floor, and roof elements are interconnected and depend on each other for stability. While we join whole bamboo culms to form the structural frame of the house, halved bamboos are used to form floor decks, walls, and roof tiles. We weave together, flattened bamboo strips to create mats that are then used as screens or walls. Door, windows, rain gutters, pipes, roof coverings are some non-structural applications of bamboo.

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Traditional bamboo house in Tripura: source: ©https://www.flickr.com
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Woven bamboo strips wall: source: ©https://arkitrek.com

An unconventional application of bamboo is its use as concrete reinforcement. The tensile strength and strength to weight ratio of bamboo is comparable with steel making it replaceable. With the availability of steel steadily declining, bamboo is a viable and eco-friendly alternative. 

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Casting of bamboo reinforced plinth: source: ©https://arkitrek.com

 Bamboo with its diverse applications and fast-growing characteristics is a sustainable alternative for steel and timber. This material has been an integral part of the vernacular construction industry for ages and with the development of new joineries and construction techniques, bamboo steadily becoming a part of the contemporary construction industry.

Author

Anushri Kulkarni is a 24-year-old, Mumbai based architect with a passion for green architecture. She is a voracious reader and consumes all genres with equal gusto. Apart from being an architect, she is also a budding architecture and interior photographer.

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