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Construction with Bamboo
Diplom-Ingenieur Christoph Tönges
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joints

Already several architects and engineers occupied themselves with the development of a useful connection of bamboo tubes and presented many different solutions, reaching from simple lashing and glueing techniques over combined solutions to complex connections, for example, injecting concrete and transferring forces through screws and steel tabs.

Joints of Arce-Villalobos
 (78) Oscar Antonio Arce-Villalobos, 1993
 
Diagonal section through Guadua

To design a connection, that can take advantage particularly of the enormous tensile strength of Guadua it is inevitable to take a look at its material properties at first. The fibers of the outer third of a section of the bamboo culm wall are the most efficient. So the goal is to introduce the forces into this part of the culm wall as distributed as possible to avoid axial splitting of the bamboo material.

In an investigation of the tensile strength, the connections of Simón Vélez, failed because the outside wall of the Guadua culms split. The linear order of the bolts was also a reason. The tests were executed at the FMPA-Stuttgart in the course of the construction of the ZERI Pavillion.

 (79) Diagonal section through Guadua
 

After a thorough examination of existing connections and the qualities of Guadua, I also wanted to execute some experiments.
I was interested especially in two different ideas of power transmission.
1. transfer of forces through a large number of screws and
2. transfer of forces through a cone:

Idea of I.C. David Trujillo Cheattle Idea of J.J.A. Janssen
 (80) I.C. David Trujillo Cheattle, 2001  (81) J.J.A. Janssen, 1981
 
Prototype 1 Prototype 1

ad 1: A sheet metal with a certain order of holes serves as pattern to drill holes for the screws and as a 'washer' for the screws at the same time. The large amount of screws and their regular distribution should provide an good power transmission to the bamboo cross section ie. to its hard outside skin.
A connecting means (bolts, sheet metal) is put into the end of the tube and then is filled up with mortar.

 (82) Prototype 1, C. Tönges, 2002  (83) Prototype 1 (without mortar)
 

ad 2: The other idea is to cut the bamboo end axially crosswise to generate a cone by tightening a steel clamp. This cone is filled up with mortar as well. This cone should transfer the force to the outer high-tensile fibers.

Slitting with a chainsaw Prototype 2
 (84) Slitting with a chainsaw  (85) Prototype 2, C. Tönges, 2002
 
Tests in Manizales

Tests

In order to find out how the connections behave under influence of forces I tested the prototypes at the laboratory for material examinations at the Universidad Nacional de Colombia in Manizales.
The specimens were clamped into a testing machine and were exposed to a continuously increasing tensile force. The behavior of the materials at different forces was observed.

 (86) Tests in Manizales
 

ad 1: The mortar cylinder transferred the forces to the screws. With increasing tensile load the screws were pressed into the bamboo (approximately 0,5 cm) and twisted in direction of the force at the same time. The end of the bamboo tube splitted. With further increase of traction, the mortar broke.
The capability of the connection could easily be increased by using a mortar of better quality, however, this test revealed two arguments against this type of connection: The manufacturing expenditure is very high and the connection itself is not stiff enough, ie. it shows too much initial slippage.

Failure, prototype 1
 (87) Failure, prototype 1
 
Failure, prototype 2

ad 2: The mortar failed here, too. A cylindrical piece of mortar with the diameter of the initial opening broke out of the connection. No measurable distortion of the steel clamp nor splitting of the Guadua culms were observed.
The capability of this connection type can be increased with a better mortar, too. Consequently the measurements are not meaningful. Finally, I would like to conclude that the construction principle of the prototype 2 appears interesting for further testing.

 (88) Failure, prototype 2
 

Further development

In summer 2002 I did further experiments at the University Aachen (RWTH Aachen) to develop a high-perfomance connection to build bamboo space frames.

Since winter term 2002/2003 I'am working at the Institute for Structural Design for the course 'Construction with Bamboo'. Now we are working together at the development of the connection and at the design of structures made with the connection.

> Read more about the State of the art.

Tetraeder
 (169) Tetrahedron by 'Construction with Bamboo'
 
Bibliography
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Author and Copyright: CONBAM
Translation: Victor Löffler --- 06.08.2002 --- last update: 23.07.2005