Bamboo Connections

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Bamboo Connections

Student: Chris Davies

Supervisor: Pete Walker

Department of Architecture and Civil Engineering

The University of Bath

2008

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Department of Architecture &

Civil Engineering

Assessment Cover Sheet

Name: Christopher John Davies

Student Number 040924472

Programme: Civil and Architectural Engineering

Year: 4th

Unit Code and Title: AR40223 MEng Dissertation for Civil and Arch Eng

Tutor/Lecturer:

Title of work: Bamboo Connections

Date submitted Monday 20th April 2009

(Late submission may be penalised)

Cheating and Plagiarism Declaration

I certify that I have read and understood the entry in the programme handbook1 on Cheating and Plagiarism and that all material in this assignment is my own work, except where I have indicated otherwise with appropriate references. I agree that, in line with Regulation 15.3(e), if requested I will submit an electronic copy of this work where relevant for submission to a Plagiarism Detection Service for quality

assurance purposes.

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Abstract

The intent of this study is to review a range of existing bamboo connections used across the world and publish reliable data on the strength of these joints. The author also hopes to use the knowledge learnt from this study to develop a new connection that would be suitable for a specific purpose in bamboo construction.

Four connections were originally tested using a range of materials and techniques. These include a traditional bamboo tenon and dowel joint, a plywood gusset plate joint, a concrete filled joint and a steel tube joint. The tested samples were all made into orthogonal joints that connected together two pieces of Moso bamboo culms. The culms all ranged within a 70mm-80mm diameter.

The four connections tested were adapted from other author’s research to be compatible with the available resources in the workshop. The maximum bending moment capacity, shear capacity and tensile strength of each joint were found through a series of tests. Tests were carried out by holding the bottom culm in place and then pushing or pulling the top culm through a loading jack. The test rig was especially cut and welded to fit the size of the connections.

The study into the construction of these joints and extensive testing enabled the strengths and weaknesses of each joint to be found. The failure patterns were closely monitored and reported for each connection. Testing was continued after the initial signs of failure to observe the post-failure behaviour of the joint.

The traditional bamboo joint failed in a brittle manner through the weaknesses of the bamboo tenon. The tenon failure either occurred at the base or around the hole where the dowel passed through. The strength of the connection was much higher than expected and sole use of bamboo in creating the connection is an important benefit. However the connection was time consuming to construct and it was hard to obtain a secure fit between both culms as there is no part of the joint to tighten.

The plywood gusset plate connection was of sufficient strength so that the bamboo culms were always the first to break. The gusset plate would be able to be produced in factory conditions on a large scale making the construction of the joints very quick. The individual results were very similar in strength and reliable showing a possibility of this connection being able to be part of a bamboo building code.

The concrete filled joint failed around the steel eye bolt that ran through both the top and bottom culms and failed in a ductile manner. The concrete helped to stop the bamboo culms cracking around the steel dowels. This allowed for the connection to carry a large bending moment. The negative aspects of the joint were that it was hard to construct and the concrete mortar ruined the natural beauty of the bamboo culms.

The steel tube connection had a sufficient moment capacity to cause the bamboo culms to fail before the steel yielded. The joint would be easy to produce on a large scale and could be easily assembled. However it did not provide any tensile strength and left the ends of the culms exposed. The steel tubing was only applicable to a small range of bamboo culm diameters as using too much material to fit a larger culm would weaken the joint.

Studies into these adapted joints enabled a new connection to be created and tested for a specific type of bamboo construction. The new joint is made from canvas with a steel “T” strip reinforcement and hook and loop straps. The joint is able to be produced in large quantities and transported to the area of need. The initial version of this joint was modified after testing to respond to the first version’s weaknesses. The final results showed that the joint didn’t provide any tensile strength but the steel bar gave the connection an amount of bending and shear resistance. The canvas joint is simple and quick to assemble making it ideal for its purpose. This connection is most suitable for temporary housing and post disaster relief where bamboo is abundant in supply but there is a lack of tools and skilled labour.

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List of Figures

Figure 1 – Simon Velez Cathedral, Columbia 2

Figure 2 – Methods to prevent crushing 6

Figure 3 – Traditional Lashing Techniques 7

Figure 4 - Elastic Band Lashing 7

Figure 5 – Bamboo Scaffolding 8

Figure 6 – Nienhuys Improved Tied Butt 9

Figure 7 – Tongue Joint 9

Figure 8 – ‘Horn’ Connection 9

Figure 9 - Full Lapped (Left) and Half Lapped Joints 10

Figure 10- Butt Joint with Side Plates 11

Figure 11 – Sleeves and Inserts 11

Figure 12 – Gusset Plate Design 12

Figure 13 – ITCR Internal Gusset Plate 13

Figure 14 – Timber Insert 13

Figure 15 – Nienhuys Expandable Connection 15

Figure 16 - Duff Steel Connection 15

Figure 17 – Improved Steel Connection 16

Figure 18 – Modern Variations on steel connections 16

Figure 19 – Steel Tube Connection 17

Figure 20 – Tensile Test Using Bamboo Fibre Board and Drift Pin 18

Figure 21 – Bending and Shear Test Apparatus 20

Figure 22 - Set up for Bending Moment Test 20

Figure 23 – Shear testing from the side and top 21

Figure 24 – Bending testing close up and from the top 21

Figure 25 – Set up for tension test 22

Figure 26 – Tension Rig in action 23

Figure 27– Bamboo Tenon and Dowel Connection 24

Figure 28 – Bamboo Tenon and Dowel Components 24

Figure 29 - Components of Plywood Gusset Plate Connection 25

Figure 30 – Gusset Plate Dimensions 26

Figure 31 – Concrete Connection Components before Mortar Injection 27

Figure 32 – Stages of assembly of steel tube 28

Figure 33 - Completed Steel Tube Connection 28

Figure 34 - Bamboo Tenon and Dowel Bending Test 1 30 Figure 35 – Bamboo Tenon and Dowel Bending Test 2 31 Figure 36 – Bamboo Tenon and Dowel Bending Test 3 31 Figure 37 – Bamboo Tenon and Dowel Bending Results 31

Figure 38 – Bamboo Tenon and Dowel Shear Test 1 32

Figure 41 - Bamboo Tenon and Dowel Shear Results 33

Figure 42 – Bamboo Tenon and Dowel Tension Test 1 34 Figure 43 – Bamboo Tenon and Dowel Tension Tests 2 and 3 34 Figure 44 – Bamboo Tenon and Dowel Tension Results 35

Figure 45 – Plywood Gusset Plate Bending Test 2 36

Figure 46 – Plywood Gusset Plate Bending Test 3 36

Figure 47 - Plywood Gusset Plate Bending Results 37

Figure 48 – Plywood Gusset Plate Shear Test 1 37

Figure 49 – Plywood Gusset Plate Shear Test 3 37

Figure 50 – Plywood Gusset Plate Shear Results 38

Figure 51 – Plywood Gusset Plate Typical Tensile Failure Pattern 39

Figure 52 – Plywood Gusset Plate Tension Results 39

Figure 53 – Concrete Filled Bending Test 1 40

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Figure 55 – Concrete Filled Bending Test 3 41

Figure 56 - Concrete Filled Bending Results 41

Figure 57 – Concrete Filled Shear Test 1 42

Figure 58- Concrete Filled Shear Results 42

Figure 59 – Concrete Filled Connection after Tension Failure 43 Figure 60 – Bamboo Tenon and Dowel Tension Results 43

Figure 11 – Steel Tube Bending Failure 44

Figure 62 - Steel Tube Bending Results 44

Figure 63 – Steel Tube Shear Test 45

Figure 64- Steel Tube Shear Results 45

Figure 65 – Canvas Mark 1 Front 50

Figure 66 – Canvas Mark 1 Back 50

Figure 67 – Canvas Mark 1 Bending Results 51

Figure 68 - Canvas Joint Mark 2 Back 52

Figure 69 -Canvas Joint Mark 2 Front 53

Figure 70 - Canvas Joint Mark 2 53

Figure 71 – Canvas Joint Mark 2 Failure 54

Figure 72 – Canvas Mark 2 Bending Results 54

Figure 73 – Canvas Joint Mark 3 55

List of Tables

Table 1 – Bamboo Tenon and Dowel Connection 30

Table 2 – Plywood Gusset Plate Connection 36

Table 3 – Concrete Filled Connection 40

Table 4 – Steel Tube Connection 44

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Acknowledgements

The author would like to thank the continued support of the lab technicians who helped in the testing of the connections. In particular the assistance of Will Bazeley, Neil Price and Brian Purnell have helped the tests run smoothly and enabled reliable data to be published.

The support and funding from Engineers Without Borders has made it possible to perform all the desired tests and order enough lengths of bamboo culms. Additional ideas from EngINdia and Developing Technologies have also helped in the development of the project. The author would also like to thank David Trujillo, Troy Whyte, Michael Oxlon and Ron Dennis for their advice and support.

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Figure 1 – Simon Velez Cathedral

1. Introduction

The use of bamboo in construction is long established and still used extensively as a structural material in many countries. There are over one thousand species of bamboo each with their own individual properties (Gratani et al., 2008). Bamboo is the fastest growing plant on the planet and it has been measured to grow as fast as 1.2 metres in a single day

-(Farrelly, 2003). Bamboo is not limited to structural applications and can also be utilised in culinary, medical and decorative areas. Bamboo is strong but lightweight and is therefore an extremely versatile building material.

The use of bamboo in modern architecture can produce some amazing structures such as the church designed by Simon Velez seen in Figure 1.

In some countries bamboo is seen as a poor man’s material and modern alternatives such as glass, steel and concrete are now used in the place of bamboo. However the increasing need for sustainable building construction and the use of natural local materials have made bamboo a popular material once again. Along

with this increase in use has come an increase in research into bamboo structures. Modern studies find bamboo can be an excellent structural option when compared to modern materials.

Bamboo is often mistakenly considered to be a type of timber. In fact bamboo belongs to the true grass family Poaceae. Unlike timber bamboo has no knots or rays making the culms evenly stressed across their length. The culms are hollow but split into chambers through diaphragms at the nodes. Its qualities as a strong, light and elastic material make it ideal for structural use. It is easy to grow and can be cultivated by individuals without the need for complex tools or machinery. Culms can be cut with a machete from a bamboo area without destroying the entire area which makes bamboo a very renewable material. Bamboo does not need to be cut to shape before it is used and this lack of sawing or logging results in no waste material from the crop.

However bamboo is limited in its application through its durability and it needs to be

processed and treated to ensure its long term usage. Untreated bamboo has a lifetime of one to three years if in contact with the atmosphere and soil, four to six year if under cover and ten to fifteen years if under cover and not in a very humid climate (Janssen, 1995).

Good detailed design such as keeping the culms away from the ground and under cover will help to increase its life span by ten years (Janssen, 1995). Stone foundations separating the bamboo from the soil and large overhangs on the roofs are common ideas found in good

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bamboo with slaked lime) the culms are used to increase durability of the bamboo. These methods are inexpensive although the long term durability is still not ensured.

Modern chemical treatment methods can increase the lifetime of bamboo to twenty-five years (Janssen, 1995) although this may result in environmental damage. Another problem is that, unlike timber, it is hard for chemical treatments to penetrate into bamboo culms and

preservation tends to only really occur at the ends of the culms. The additional cost of using chemical preservatives can be justified if the additional lifespan provided by their use is considered necessary.

The natural tapering of bamboo and the varying structural properties make it a very hard material to categorise and development of standards is hard because of the way bamboo’s properties vary with geographical location and age (Arce, 1993).Bamboo culms are used in construction in the form that they are harvested and the natural shape of the culms has to be brought into designs. Irregular node spacing on the bamboo culms and varying thicknesses can cause considerable problems in design.

There is a lack of structural data for bamboo and there are no complete bamboo building codes (Jayanetti and Follet, 1998). This makes the development of bamboo structures very difficult and can result in unsafe large bamboo constructions.

The other major problem with bamboo is in the jointing techniques used. This is because of the culms irregular shape and also because bamboo is hollow. There are many types of traditional joints but their structural efficiency is low (Herbert and Evans, 1979).

This study has two aims towards the development of bamboo connections. Firstly it aims to publish reliable data on the strength and failure characteristics for a range of joints. These connections are built using a range of materials and techniques and connect Moso bamboo culms. The strength of more modern designs will be compared to the strength of traditional techniques. The objectives are to find the advantages and disadvantages of each joint tested and compare which types of construction they are most suited to.

The second aim of the project is to develop an entirely new joint from the understanding of existing connections. By looking at the failure characteristics and compatibility of existing joints the author hopes to develop a joint suitable for a specific type of construction. The aim is to create a joint that can be easily constructed, removing the need for skilled labour on the site. This should be a precast connection that can be made off site and easily transported to be assembled. This is especially important in post disaster shelter construction where quick and easy to build structures are often needed and there is likely to be a lack of knowledge of detailed construction techniques.

The project will focus on orthogonal joints to allow for a range of joints to be compared. The initial joints tested will be modified from the previous work of others so they are able to be tested in a similar manner and are compatible with the available resources.

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2. Literature review

2.1 Introduction

The historic weakness in bamboo structures is its poor durability, the lack of design codes and weak connections. This project focuses on the design of bamboo connections. In any structure the connections are vital for determining a successful design. Connections can provide an attractive focal point for a building but can cause aesthetical and structural problems if not designed to a high standard. A well constructed joint can add to the architectural features especially with a material as naturally beautiful as bamboo.

This report looks at jointing techniques found world wide. Some of these are commonly used and have been for years, others are in experimental stages and still being developed.

Much of the information found shows the design and development of connections but it is hard to find published results on any individual structural performance. The use of bamboo in construction is widely based on experience of generations of workers and full design

standards are non existent.

The jointing of bamboo is very different to any other material and requires an understanding of how the material acts under loading. Traditional wood jointing techniques are generally not applicable because of the hollow and cylindrical nature of bamboo. Techniques used in steel or concrete might not be suitable because of the lack of standardisation in bamboo culms. The tapered nature of bamboo is very important to consider as the jointing techniques need to accommodate a range of diameters. Lawson (1968) reports that, when comparing both ends, a three meter length of bamboo could easily vary in diameter by over twenty millimetres. Different types of bamboo will vary considerably even within bamboo culms of the same species. Any joints designed should therefore cater for this variation even if the culms used in testing do not have wide variations in diameter.

Standardised joints will not fit in every situation unless a certain amount of flexibility is introduced. This is why the traditional method of bamboo connections being cut and constructed on site to fit is still popular. This brings flexibility into the structure but also needs a high level of skilled labour which is not always available.

2.2 Basic Jointing Rules

Bamboo construction techniques do not have the standardised design and construction codes found in the UK but there are some basic rules to be followed. Hidalgo (1998) published basic guidelines to be followed in bamboo joint construction. These are applicable to all jointing techniques and are followed by many builders in South America. This study will be used to construct a traditional joint and the basic rules will be followed to maximise the structural capacity. The study does not publish results on the performance of each joint so further testing will be needed. The Hidalgo rules are based on the experience of many generations of

bamboo workers so it is assumed to be the most efficient way of construction.

2.2.1 Rule 1 - Construct joints near nodes

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culm splitting. Any holes or openings that need to be cut should be done close to the nodes. However, having a node near each joint is not always practical in construction.

2.2.2 Rule 2 - Avoid openings in culms

Preferably there should be no openings cut into the bamboo near the joints. If a hole is required it should be as small as possible and of a circular shape rather than rectangular. This will reduce the chance of a high stress concentration on one part of the culm. Nailing into bamboo to connect it to other culms is generally avoided as this can induce high local stresses in construction and cracking of the culms.

2.2.3 Rule 3 - Treat the bamboo culms

The strength of bamboo connections can be increased through the use of seasoned culms as opposed to using green culms. Bamboo shrinks when it is dried and cracks can form as a result. Using seasoned culms allows any culms with cracks to be noticed before construction and used in a suitable place. This stops cracking in any key structural element. Using green bamboo in construction may also result in weaker joints over time as the culms shrink when they dry, this can make certain types of connections looser in fit. This will reduce the strength of the joint, the durability and the life span.

2.2.4 Rule 4 - Securely fit joints

It is important for all the joints to be strongly secured and fit well together. It is common for the end of bamboo culms to be cut to facilitate jointing. The usual way to achieve this is to make a fish mouth cut into one end of the culm. This can be done with either a chisel, saw or a circular power drill. A secure fit will stop the ends of the bamboo culms being accessible and so reducing the risk of water penetration or insect attack. Careful attention to detail and a secure fit of culms will also ensure a reduced risk of the culms splitting or cracking. A secure fit also allows for stresses to be evenly spread across the entire connection.

2.2.5 Rule 5 - Make durable connections

The joints should be just as durable as the bamboo structure. A deteriorating joint can be very hard to repair or replace especially if it connects many culms together. The durability of the joints is dependent on three main factors. These are the choice of materials, the detailed design and the quality of craftsmanship. An example of improving durability through the choice of material is the use of galvanised wire in lashing instead of natural materials. Improving durability through design considers both the type of joints used and the clever detailing of the joints such as keeping the joints hidden away from the natural elements.

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2.2.6 Rule 6 - Reinforce culms under high point loads

Joints should be designed so that there is no high point load transferred onto another culm tangentially. The hollow cylindrical nature of bamboo culms results in a poor tangential loading capability. If this cannot be avoided, the area likely to crush should be reinforced. This should be done either with a small cut of bamboo filled around the node or a wooden cylinder. This can be seen in Figure 2.

2.3 Tools needed in bamboo jointing

Janssen (1999) writes that many traditional forms of bamboo construction can be carried out with just a machete. A drill is a useful tool in more complex joints and some joints require the use of a hammer. The tools used depend if the bamboo being used is green or dry.

If the bamboo culms are to be dried then they need to be seasoned. The most basic method of seasoning requires a trough, heavy stones, preservative and a plastic cover.

2.4 Traditional jointing techniques

Most of the available traditional jointing techniques have been developed over time with knowledge being passed on from generation to generation. The development of the best practice for these connections has not been based on laboratory testing so published results are generally not available.

2.4.1 Lashing

Lashed joints use the friction in the rope to secure the joint and Farrelly (2003) notes that this is still the most common method of jointing bamboo. Lashing is found in construction across the world and can be traced back over thousands of years. Lashing bamboo with steel wire is used in scaffolding to create structures up to forty stories high (Building Department, 2006) in China. Other materials such as Velcro straps or plastic bands can be used in place of wire or rope.

Many culms can be joined together in one place with the use of lashed joints and the size of each culm can vary. The jointing technique can prove to be very effective but is not

standardised and the joint’s strength depends of the skill of the worker. There is very little pullout strength in lashed joints and they are susceptible to loosen over time. The tension capacity of the joint can be improved by drilling holes through the culms and tying the lashing through the holes. This weakens the strength of the culm but increases the strength of the joint. A common technique for lashing bamboo is shown in Figure 3.

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Figure 3 – Traditional Lashing Techniques

Arce-Villalobos (1993) looks into the materials are that commonly used in the lashing which includes cocos/sago palm fibre, bast, strips of bamboo and rattan. Bamboo ropes can be created by twisting together bamboo fibres creating a more resistant connection.

Arce-Villalobos found that a 60mm diameter bamboo rope created in this way has the tensile ability to support up to 14 tons. Using saturated green bamboo as a lashing material is preferred as when the bamboo strips dry they shrink and form a stronger, tighter connection. The bamboo strips are normally plaited together so that when they shrink the whole joint is tightened. This technique requires green bamboo to be readily available which is not always possible. The joint takes time to reach its full strength while

the green bamboo is drying. A structure created using this form of lashing may take several weeks to be able to achieve its full load carrying capacity.

Traditional lashing materials are now being replaced with more durable materials such as zinc coated iron wire and plastic ties. In Thailand the predominant technique used to tie bamboo structures together are plastic bands as seen in Figure 4 (Bambus, 2002). A piece of wood or metal is tied in with part of the lashing. This piece is then twisted and secured to tighten the lashing holding the bamboo culms.

Velez (2003) designs and photographs a collection of lashed joints and showed that lashing techniques can be made aesthetically

pleasing when skilfully made. The project will consider the aesthetics of the joints built and Velez’s designs provide a useful comparison. Janssen (1981) described the use of many lashed joints in his Ph.D thesis that are used worldwide. His testing of the joints was based on their use in a king-post truss and the lashed joints were not individually tested. Janssen’s study of common types of lashed connections is useful but the results are based on their performance as a whole truss. The individual strength of the joints is hard to calculate from these studies.

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Figure 5 – Bamboo Scaffolding

Ramanathan (2008) studied the use of lashed joints in scaffolding and was able to publish data of the strength of the connections. In Hong Kong in 1970 nylon strips replaced the use of bamboo strips for lashing in scaffolding construction. These nylon lashed joints still showed gradual weakening over time because of weathering. Safety measures called for scaffolding design codes to be published which included values for the strength of the nylon lashings. Chung (2002) published the resistance of the nylon lashing to be 1.1 kN and noted that a factor of safety of 1.1-1.25 should be applied. These figures are based on a set type of nylon lashed joint, the

specification for this joint are given in the Hong Kong Building Department (2002). The culms are specified as being free from defects, to be 3 to 5 years old and air dried in vertical

positions indoors for three months. Two common types of bamboo are used and the minimum external diameter and wall thicknesses are set for both types. As well as this, the nylon has to have a set strength of 0.5kN and be of set dimensions. The lashing technique is also stated with five rounds of nylon strips to be completed. The rest of the Building Department (2002) code specifies every aspect of scaffolding construction including maintenance and bracing required.

If bamboo is to be used as a serious building material then it is encouraging that work is being done to turn this specification into an ISO code (Janssen 1999). Builders can now be taught how to construct lashed joints through the design guides and designers will have faith in the perceived strength of the connections. This is the only way safe construction practice can occur in multi storey bamboo scaffolding.

The factor of safety is still relatively low when considering the large amount of manual work that is required in the construction of bamboo joints and the possible flaws in the materials used. The nylon strips can be mechanically produced to a suitable standard but bamboo culms may have flaws in them that are not visible. When considering the amount of joints that are being constructed and the high possibility of one connections being constructed, a safety factor of 1.1 (Chung 2002) may not be appropriate.

The problem with lashed joints, even if the design and construction is carefully considered, is with their stiffness. Many types of construction call for a joint which has the ability to carry a suitable amount of moment and not just fix the culms in place. The main hindrance for designers using lashing in connections is the safety problem as a uniform quality is very hard to achieve and is dependable on the labour hired. It is also very hard to stop the culms

slipping out of place and they have a poor tensile capacity. Over tightening of the lashing to prevent this can cause local crushing at the ends of the culms.

Another major problem with lashed joints is poor fire resistance. Bamboo culms are also prone to failure under fire because of their hollow form and the cost of preventing potential

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2.4.2 Butt Joints

Jayanetti (1998) looks into other common joining techniques that only use natural materials. In bamboo structures the most common joint needed is when two bamboo culms run

perpendicular to each other. These are commonly joined through a butt joint and many techniques have been developed, these have been collected together by Jayanetti.

The studies by Jayanetti do not publish results on how the joints acted under loading and can only be used as a best practice guide. His work is simply a collection of designs found across the world. Replication of these joints and testing is necessary if the joints are to be compared to modern and man made joints. Testing is also necessary if these joints are to be included in bamboo building standards.

A butt joint is formed by sawing or chiselling the end of one of the culms so it sits comfortably on the other culm. A secure fit will ensure a good load transfer between members. Nienhuys (1976) reported on saddle butt joints where the joint was tied secure through notches in one of the culms. This gives the connection some tensile resistance and keeps it secure, improving its structural performance.

Stulz/Hidalgo (1981) looked into the possibility of cutting the bamboo to leave a long tongue as seen in Figure 7. This tongue is then lapped over the joint and tied for additional tensile strength. The joint requires the bamboo to be green and is not applicable to seasoned dry culms.

Figure 7 – Tongue Joint

The traditional saddle butt joint can be adapted by cutting one of the culms to leave two ‘horns’, this can be seen in Figure 8. The bamboo culm with the prongs cut then attaches through the other bamboo culm. A large amount of craftsmanship is needed to line up the horns on each side of the culm, especially if the culms are not at right angles to each other. This also requires drilling holes in the bamboo which can drastically reduce the strength of the joint.

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Figure 8 – ‘Horn’ connection

Janssen (1984) reported the strength of this joint as being 8kN and has stiffness in shear along the upper member of 0.98kN/mm. These results are from when the joint is designed to be part of a truss and the results are based upon the horned joint being placed at 45 degrees to the horizontal. The test results also include a large standard deviation which allows for the variable properties of the bamboo culms and variable quality of joint construction. The report published by Janssen does not specify the exact dimensions of the prongs for the connection. This makes the connection hard to replicate and the results difficult to compare with other connections. The construction of this joint requires a considerable amount of time and skilled labour. This was agreed by Siopongco et al. (1987) who said that the effort required to shape this joint is not reflected in its structural performance.

2.4.3 Splice Joints

Splice Joints are necessary in bamboo construction either to extend a culm to span between two points or to create a stronger beam by tying culms together. The following variations of the splice joint were collected by Jayanetti (1998). The joints are subject to the previously discussed limitation of having a lack of reliable data published on their strengths.

2.4.3.1 Lapped Splice Joints

Full lapped spliced joints are a very traditional method of jointing bamboo. The two bamboo sections are overlapped in the same line by at least one node. The overlapped area is then tied together to form one member and this can be reinforced with wooden dowels. The resulting joint is very secure and simple to construct but can result in very bulky joints. This technique is used in long load carrying beams where many bamboo culms need to be attached together to form one strong beam.

A half-lapped splice joint can be used if the design requires the joint to be no thicker than one bamboo culm. The bamboo culms are cut in half, overlapped and tied with the addition of dowels if required. The overlapped area should be the length of one node as can be seen in Figure 9.

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Figure 9 - Full Lapped (Left) and Half Lapped Joints (Right)

2.4.2.2 Side Plate Splice Joint

This joint is formed by placing two bamboo culms end to end. To hold the culms together a bamboo culm of larger diameter is split into quarters and placed over the joint as shown in Figure 10. The two side plates are then tied together and can be dowelled for additional strength. It is important to cut the bamboo culms near the node to provide maximum strength to the joint and to use culms of similar diameter.

2.4.2.3 Sleeves and Insert Splice Joint

Weaker joints can be produced using sleeves and inserts in a very simple method of construction. Short lengths of the culm are used to completely surround the butt joint or can be inserted in the middle of the joint. This requires the short length of bamboo to be exactly the right

diameter to create a solid joint. The simplest of butt joints is when one culm of bamboo of smaller diameter is inserted inside a culm of larger diameter as seen in Figure 11.

Figure 11 – Sleeves and Inserts Figure 10 –Side Plate Splice Joint

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2.5 Modern Connections

Modern bamboo connections tend to utilise the strength of other materials to strengthen any areas of bamboo’s natural weakness. Some of the connections discussed show results on the strengths of the joints while others are simply design ideas.

2.5.1 Gusset Plate

Gusset plates can be attached to the outside of the culms for support as is shown in Figure 12. This connection can be used to bind many culms together. The gusset plate is commonly

constructed from a timber such as plywood and fixed to the culms with bolts or dowels.

Duff (1941) found that the addition of the gusset plate improved the rigidity of the joint by making use of the in plane

moment of inertia of the plywood. Attaching gusset plates to the side of the bamboo is a flexible technique and a mass produced gusset plate can be used for bamboo culms of varying diameter. The technique does require drilling into the bamboo which can considerably reduce the strength of the culm. However bamboo members can be drilled to fit off site and then assembled together in place.

Sonti (1959) investigated the practicality of lashing the bamboo culms to a steel plate. As part of his study a large dome was created made entirely from this type of joint. Results are not published on how the individual joints performed just of the structure as a whole. Sonti’s study stated that the rigidity of the joint was taken by the steel plate avoiding any damage to the bamboo. The lashing used was shown to be the weak point in the structure which was to be expected. If steel is to be used as gusset plate it would be make sense to use other man-made materials for the lashing and not traditional fibres.

A plywood gusset plate joint was one of the joints tested by Janssen (1984) when exploring bamboo trusses. The strength of this joint measured by Janssen was greater than 16.5kN and the stiffness of the joint in shear along the upper member was 1.17kN. Janssen used a

plywood gusset plate with steel bolts but none of the dimensions are given in the paper. This makes the results difficult to compare with any new connections.

The timber plate can also be placed inside the bamboo culms and glued in place. This requires slots to be sawn into the bamboo culms. This technique was developed by the Instituto

Tecologico in Costa Rica (ITCR) as is shown in Figure 13. The disadvantages of this joint are that the ends of the culms are exposed and also it can be hard to make the glue stick to the bamboo. Cutting a void through the bamboo culm wide enough for the gusset plate to fit causes a considerable loss of structural capability of the culm. An advantage of this joint is its visual appearance as the gusset plate is hidden within the culms. Results are not shown on the performance of this connection but it would be expected to be much weaker than the normal external gusset plate connection.

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Figure 13 – ITCR Internal Gusset Plate

2.5.2 Wooden insert

A connection developed by Dr Arce at the ITCR (1993) is a wooden insert as seen in Figure 14. The insert is fitted inside the bamboo culm and glued in place. This adds additional strength to the bamboo when loaded in the

tangential direction. The wooden insert should be of sufficient depth to transfer any stresses over a large area. The result is a larger value of second moment of area and the shear stresses are distributed more evenly. The increased internal area allows for the bending moment capacity at

the joints to be increased. When two culms with a wooden insert are joined the connection is no different to traditional wood joints. This results in the bamboo being able to be treated as a timber joist and nails and screws can be used with greater effect.

One of the main problems with this connection is that bamboo culms are rarely perfectly spherical and vary in diameter. To solve this problem the culm is cut with two slots to allow for some flexibility in the joint. This must be done while the bamboo is still green and the culm is flexible. The inner surface of the culm must first be cleaned before the insert is placed inside. Sand paper can be attached onto a power drill which can remove up to 5mm from the internal diameter (Arce, 1993). This allows for a fixed size of wooden insert to be used for bamboo culms varying in up to 10mm in diameter but weakens the culms. This is not a perfect solution to the problem of variable internal diameters but does allow for large scale production of the timber inserts in a workshop. The inserts could then be produced locally on a large scale and once these are produced the rest of construction requires no skilled labour. All that is required is the cutting of the bamboo and sanding of the inside of the culm.

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Another advantage of the timber insert is its adaptability. The connection is forgiving if the length of bamboo cut is slightly short. The connection can also be applicable to many different types of joint because of the range of cuts that can be made to the end not inserted into the culm. This eliminates the need for complex angled cutting of the culms and makes construction simpler, essentially making bamboo a material that can be used by an unskilled workforce. Only small amounts of glue should be needed and the cost of the wood should be low when compared to other modern jointing techniques. The insert also prevents any insects or moisture entering inside the bamboo culms which could damage the culm.

Development of the connection by the ITCR has been predominantly in the practicality of the joint. The strength of the connection still needs testing to be further developed. The study by Dr Arce is directly relevant to the project as it suits the needs of a precast and adaptable connection.

2.5.3 Concrete Infill

Concrete can be filled inside the bamboo culm to increase the strength of the joints. Morisco et al (1995) looked at this technique using a combination of steel bolts and concrete. Bolts are placed into the bamboo and then the concrete is poured to fill around the bolts and up to the diaphragms of the culms. A wire brush is used to clean the inside of the culm and help the concrete bond to the inside. Morisco also looked at the addition of gusset plates to a concrete filled joint for additional strength. Morisco’s work was designed for spliced joints but could easily be adapted for orthogonal joints. This work was unpublished and no reliable data can be used. The paper does offer ideas on design of mortar filled bamboo joints but these would have to be independently tested. Morisco’s work recommends using green bamboo as the culm will shrink with the concrete. This is not possible to test with the resources available in the UK and a new version of the connection would have to be developed.

2.5.4 Pin Connection

A jointing technique developed by the UK Building Research Establishment (Herbert and Evans, 1979) is named the Herbert shear pin connector. Small steel sleeves are attached to the culms and these sleeves are all bolted together through pins. This allows for multiple culms to be connected at one point although the connection works best where all the culms are in the same directional plane. The culms can overlap at any point which brings a larger amount of adaptability to the connection. The overlapping of the culms can result in very bulky connections especially if more than two culms are connected. There is a lot of steel used in the Herbert shear pin connector but is very strong in certain directions. Its disadvantages are that the pin joint used makes the connection carry little moment and both ends of the culm need to be secured.

2.5.6 Expandable Joints

Expandable joints are considered by many to be the future of bamboo connections. Expansion joints are generally used to fit internally into the culms. These have the advantage of being able to fit into a range of diameters of bamboo. Janssen (1998) reported on Nienhuys’s (1976) expandable connection shown in Figure 15 (the report by Nienhuys is in Portuguese and so results are not shown in this paper). Development of an effective simple expandable joint to fit inside a culm could be developed to suit multiple culms and different angled connections.

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use with bamboo structures. The problem with expandable joints is that over tightening the connection could split the bamboo culm. The connection designed by Nienuys would not provide any tensile strength unless it was used with adhesive.

Figure 15 - Nienhuys Expandable Connection

2.5.7 Steel Inserts

The use of steel in joining bamboo culms was first suggested by Duff (1941). The initial design suggestion can be seen in Figure 16. This uses a specially designed steel fitting to connect multiple culms in one place. The steel also ensures the bottom of the bamboo culms is not exposed.

Figure 16 – Duff Steel Connection

Further work to these steel joints was carried out by Spoer (1982). This design can be seen in Figure 17. Mortar is used to fill the void in the culm between the bamboo and the steel tube and an adhesive is used to bond the steel tube to the mortar. The nodal sphere means that lots of bamboo culms can be connected into the same place and the angles of the culms can be varied. This makes this design ideal for being placed on top of a column head where other connections could prove to be too complicated.

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The technique is still being used today and is favoured by architects for its modern look and ease of construction once the steel connection is placed inside the culm. Stainless steel is used for durability and aesthetical reasons. Figure 18 shows the use of nodal steel connections in modern construction.

Figure 18 – Modern Variations on steel connections

The use of steel in bamboo connections has been developed further through Gutierrez (1998) and the Costa Rican Bamboo National Project. In their research the steel was used not just for the connections but to run down the centre of the culm. The steel bar is connected at each end with welded plates which keeps the steel in place.

Bamboo acts well under compression but does not react well under tension. The steel bar in the centre of the culms carries these tensile forces instead of the bamboo. Connecting the steel bar at each end to plates ensures the bamboo is not subject to high forces. The connection is similar to the nodal connection and the end of the steel rod is threaded to allow for the culms to connect together through a receptacle. Further studies in the project added concrete mortar into the nodes at either end of the bamboo culms. This helped to keep the steel bar in place and prevented any insects entering into the internal nodes of the culm but added a

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2.5.8 Preformed Tubing

Preformed tubing made from a range of materials can create a basis for a joint. The bamboo culms are then inserted onto this tubing internally or externally. If the tubing is fitted internally the end of the bamboo culm should be cut in the direction of the grain. This allows the culm to increase in size to fit onto the steel tubing and considers the likely variation in diameter of the culms. A jubilee clip is used to hold the culms firmly to the tubing once they are able to fit on. The steel tube joint developed by Nienhuys (1978) is seen in Figure 19.

The cutting of the bamboo to allow for it to fit is an obvious flaw in this connection because the cut is likely to split further along the culm. A steel tube should be able to carry

considerable force and the bamboo culm is always going to be the first to break. The bamboo culm is likely to split in the direction of the fibres and cutting along this direction will

introduce a high stress concentration for the forces to act along.

The addition of the jubilee clip will stop the culm splitting under a very low loading but cracking is likely to occur over time. The advantage of cutting the green culms and providing a jubilee clip is that the connection will have some tensile strength.

2.6 Bamboo in Timber Connections

Bamboo has been developed as a connector for timber structures. This has mainly been

developed in Japan after the government introduced laws about recycling in construction. The use of natural materials instead of steel bolts in connections allows for much simpler

dismantlement of a building. Wood and bamboo connections also make buildings more earthquake resistant due to their much lighter weight than the steel alternative.

2.6.1 Bamboo Fibre Drift-pins

The use of bamboo fibre as a drift-pin and plate has been developed by Mori et al. (2008). Bamboo is useful in this situation because of its high-strength fibre and availability.

To construct the drift pins and boards the four year Phyllostachys bambusoides bamboo was cut into lengths to avoid the nodes. These pieces were then split into eight pieces before being treated with an alkali solution. This allowed the bamboo to be separated into fibres before being pressed into either drift pins or boards.

The testing of the pins and board was done with 12mm diameter pins or 12mm square pins. The pins were initially tested for bending and achieved a bending strength of 350-400MPa and a Young’s Modulus of approximately 50GPa (Mori et al., 2008). A shear test was carried out using bamboo fibre board and drift pins connecting two glulam beams. The average bearing strength for this connection was 16kN with average yield strength of 4.5kN. The displacement of the connection reached 30mm before it lost all load carrying capacity.

A tensile test was carried out with a sample made up that can be seen in Figure 20. The results from this test showed each drift-pin to have the strength of 17.2kN.

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Figure 20 – Tensile Test Using Bamboo Fibre Board and Drift-pin

Further work on bamboo drift-pin joints was carried out by Tanaka et al (2008). A technique was developed to locally reinforce connections where drift pins are close to the edge of a connection. A glued-in-rod system was used to reinforce the surrounding area to the pin. This involved drilling holes into the wood around the bolt hole, filling the holes with adhesive and then inserting the bamboo rod. The addition of these reinforcing rods brought the shear strength of the connection from 25kN when unreinforced to 50kN. There was also an increase in ductility of the connection when the bamboo reinforcing rods were inserted.

These studies by Tanaka and Mori show that timber structures can benefit from using bamboo in the connections. Bamboo has all the benefits of the natural quality of timber but has a high strength fibre structure which makes it more suited for use in certain timber connections. These reports publish results from the connection which makes it possible to compare similar connection designs and there are several tests so the results can be relied upon. Bamboo fibres are used in these connections which require a large amount of treating and processing to form back into structural components. Temperatures of 120oC and a pressure of 19.6MPa are needed to form the bamboo drift-pins used by Mori. The extra work needed removes one of the qualities of bamboo connections in their ability to be used without any considerable treating or reworking.

These studies are not directly relevant to the current project as bamboo is used in conjunction with timber structures. Ideas from these studies could be incorporated into new bamboo connections that use processed bamboo fibre pins and boards to connect bamboo culms. This would probably result in an overdesigned joint where the bamboo culm was always the first to fail. It would seem more sensible to follow Tanaka’s work and look into methods of

reinforcing the areas around the connection.

2.6.2 Laminated Bamboo Lumber

Bamboo can be laminated to connect timber beams. Steel bolts are used to connect this laminated board to the timber beams or columns. Under testing from Zhang et al. (2008) the laminated bamboo lumber joint showed a high strength, rigidity and excellent deformability.

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2.7 Findings

A review of the literature on bamboo connections shows the large range of techniques that already exist around the world and the continued work that is being carried out on modern joints. There are many designs being developed for bamboo connections but little data published on the strength of these joints.

This is especially the case for the traditional joint designs. These have been designed over a number of years through expertise passed down through generations and not through strength testing and modifications. Considerable amounts of testing are needed on these traditional joints if they are to be used in safe modern construction.

Modern methods tend to use other materials to create composite connections. Results have been published on some of these connections that show a development on the production of bamboo building standards. Different composite joints used steel, concrete and timber with bamboo culms showing the adaptability of the plant. The review also showed that bamboo has a use in connections for timber structures mainly when broken down into bamboo fibres. This report will show that all connections have their ideal application. Every connection has advantages and disadvantages and the strength of the joint is just one of many properties that must be considered. Failure modes must be noted and the ability to manufacture the

connections to a similar strength. Economic reasons often govern the choice of connection and this is the predominant reason why lashed joints remain very popular along with their simplicity.

The quality of labour available is another primary concern when choosing the type of

connection. Many modern joints are precast and allow easy assembly which removes the need for skilled labour. Most traditional joints require a skilled work force for the joints to reach their full capacity.

Overall it was shown that choice of connection used is dependent of the size and use of the structure. If any sort of bamboo building code is to be developed a large amount of research and testing needs to be conducted into all sorts of connections designed.

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3. Main chapters

3.1 Laboratory Testing

One of the problems with bamboo connections is the lack of design standards and reliable strength characteristics. This project looks at the bending strength, tensile strength and shear capacity of four connections with the aim of publishing reliable data. A computer was used to record the variations in the displacement transducers and the loading applied in all tests.

3.1.1 The Test Rig

3.1.1.1 Bending and Shear Tests

Figure 21 – Bending and Shear Test Apparatus

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moment capacity and Figure 23 shows the new positioning of the equipment for shear capacity testing. The load cell acts at a height of 180mm in the bending tests and a height of 40mm in the shear tests.

For the bending tests a small rubber strip was placed on the point of contact between the top culm and the load cell. This was to help ensure no local crushing of the top culm occurred and can be seen in Figure 24.

A steel section was placed between the load cell and the top culm in the shear tests. The load was only able to be applied 40mm above the bottom culm because of load cell resting on the circular steel section. The steel section was used to increase the amount of pure shear the base of the connection was subject to. This steel section can be seen in Figure 23.

Figure 23 – Shear testing from the side and top

The bamboo joints are held in place through specially welded steel tubes which are clamped to the floor through 30mm bolts. These tubes have an internal diameter of 80mm to ensure every culm could fit in the apparatus. Small pieces of wood were used to ensure a secure fit with smaller culms inside the steel tubing. A steel angle bolted down on one side stopped the bottom culm slipping out.

Displacement transducers are needed to measure the rotation and deflection of each bamboo joint. These were placed at heights of 180mm and 40mm above the top of the bottom culm. They were used to record the displacement and rotation of the top culm.

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3.1.1.2 Tension Test

Figure 25 – Set up for tension test

The test rig for the tensile testing used the same set up to hold the bottom bamboo culm in place. The loading on the joints was applied using a tensile jack with the top culm placed parallel to the floor. The tensile force was applied to the top culm using a welded square steel section with a part cut out to insert the joint. This set up can be seen in Figures 25 and 26. This rig allows for much greater forces to be loaded onto the connection that if the bamboo culm was clamped at its top end and pulled. The necessary force needed to tighten the clamp would have caused the bamboo culm to crush locally. Another option would have been to create a strong tensile connection on the other end of the top culm and apply the loading through that. This option was not used because any failure of the joint away from the bottom culm would affect the results and make them unreliable. Using an adhesive to hold the top culm would not work because glue does not bond well to bamboo.

In some cases it was necessary to modify the connection to allow for the square steel section to fit between the top and bottom bamboo culms. This negated the need to create a tight fitting between the top and bottom culms

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Figure 26 – Tension Rig in action

3.1.2 Methodology

Before each test the bolts were loosened to allow for the connection to be placed inside the apparatus. The parts were then reassembled and all the bolts tightened to ensure the apparatus did not move when the connection was loaded.

The transducers and loading jack were placed against the top culm and zeroed at the start of each test. Each joint was then slowly incrementally loaded using the jack while the displacement transducers recorded the change in deflection at the bottom and top of the top culm. These results were all fed into a computer.

The joints were observed for any signs of visual or audible signs of cracking. These cracks were carefully recorded as the failure mode was considered just as important as the overall strength capacity of the joint when considering the use of the joint.

After the initial failure the sample was photographed and the visible cracks noted. Each connection was either tested to destruction, until the displacement of the top culm inferred with the top displacement transducer or until the load cell ran out of jack. The joint was then removed from the rig to examine the crack patterns.

Three identical joints were constructed for each test. This was necessary to achieve fair and reliable results for each test done. The incremental increase in loading was kept at a roughly constant rate.

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3.2 Connection Designs

The stages of construction are explained for each connection. Also shown are the relevant dimensions and sizes of each individual part of the connection.

3.2.1 Bamboo Tenon and Dowel

Figure 27– Bamboo Tenon and Dowel Connection

Figure 28 shows the components of the connection before assembly.

Figure 28 – Bamboo Tenon and Dowel Components

TENON

DOWELS BOTTOM CULM

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Construction:

 Saw a fish mouth at the end of a culm near the node so that it fits securely onto another culm of similar diameter.

 Saw a 200mm culm in half along its length.

 Cut this half culm into a t-shaped tenon with the bottom part of the T being 20mm long. Keep the off cuts.

 Drill a series of small holes into both ends of the culm without the fish mouth so that the tenon is able to pass through the culm.

 Drill a 15mm hole through opposite sides of the fish mouth culm 60mm above the bottom.

 Punch through the diaphragm of the node near the fish mouth so that the tenon is able to pass into the next chamber.

 Assemble the joint and mark the position of the hole to be cut in the tenon  Drill a 15mm hole through the mark in the tenon

 Place the two off cuts together and feed through the drilled holes as a dowel. Use a rat tail file to make sure dowels fit through the hole but ensure the tenon is still causing a tight fit between the culms.

3.2.2 Plywood Gusset Plate

The use of a gusset plate in bamboo connections has been suggested by many people using a range of materials. The shape of the gusset plate was designed to be able to work with a two culm orthogonal joint. Plywood was chosen because of its uniform strength and its workability. The components of this connection not including the steel dowels can be seen in Figure 29. 9mm plywood has been used with 10mm bolts. The shape and dimensions of the plywood gusset plate can be seen in Figure 30.

Figure 29 - Components of Plywood Gusset Plate Connection

TWO GUSSET PLATES

BOTTOM CULM

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Figure 30 – Gusset Plate Dimensions

Construction:

 Mark shape shown in Figure 30 onto the plywood and cut two identical shapes  Measure out position of holes and drill using an 11mm drill bit.

 Temporarily place plywood on the assembled culms, mark the positions of the holes onto the culms and drill one hole through both sides on each culm, again using an 11mm drill bit.

 Cut 10mm thread to the correct length, chamfer the ends of the thread and place through both gusset plates and the culm. Place a washer onto each end (but not between the plywood and the culm) and screw on suitable bolts.

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3.2.3 Concrete Filled Joint

This concrete filled joint is based on research done by Morisco but adapted to be compared with other orthogonal joints. The connection uses a steel eye bolt and steel dowel to initially secure the joint and then the internodes are filled with a mortar. 10mm steel was used for the bolts and dowels. The components of the connection before assembly and mortar injection can be seen in Figure 31.

Figure 31 – Concrete Connection Components before Mortar Injection

The mortar is mixed to have an average compression strength of 45-50N/mm2 and a density of 2380kg/m3. The mix per m3 of mortar consists of: 180kg water, 450kg cement, 990kg 5mm aggregate, 660kg sand and 200ml of plasticiser.

Construction:

 Drill an 11mm hole through both sides of the culm with the fish mouth 100mm from its end.

 Drill a 20mm hole on the side of the top culm between the two 11mm holes but 150mm from the base.

 Drill an 11mm hole through in one side of the bottom culm in the centre.  Drill a 30mm hole directly opposite the 11mm hole.

 Punch through the diaphragm of the node near the fish mouth cut.  Mix raw materials needed for mortar

 Tape the 11mm hole on the bottom culm and fill with mortar through the 30mm hole.  While the mortar is still wet run the eye bolt through the 30mm and locate the 11mm

hole. Secure with a washer and bolt.

 Cut a 100mm length of thread and chamfer.

 Place the top culm over the eye bolt and run the thread through the holes in the culms and through the eye. Secure with washers and bolts.

 Tighten the bottom screw so that the fish mouth is a secure fit to the bottom culm.  Fill the top culm with mortar through the 20mm hole cut in the top culm. Use a

vibrator to ensure the mortar runs through the joint and into the bottom culm.

 When the mortar fills to the 20mm hole tip the joint away from the hole, fill with more mortar, seal the hole with tape and tip the joint back the other way to fill the hole.

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3.2.4 Preformed Steel Tube

This joint is based upon the design by Nienhuys (1978). The Nienhuys joint requires green bamboo to be used to ensure a tight fit to the steel tubing. The green bamboo is cut along its side and then tightened with a jubilee clip. It is not possible to obtain green bamboo in the UK so the design has been adapted to work with dry seasoned culms.

The adaptation of the Nienhuys connection allows for a more secure connection between the steel tubing and the bamboo culms. The adaptation also allows for a larger range of diameter culms to be used. The steel tubing is wrapped in a canvas type material to fit the internal diameter of the culm. This differs from the Nienhuys connection where the bamboo is cut to fit onto the diameter of the steel tubing.

48mm diameter tubing was used with a thickness of 3mm. Each leg of the T-joint extends 80mm. The connection can be seen in Figure 32 with one leg showing just the exposed steel tube, one leg with the material wrapped around and one leg with the bamboo attached. The completed joint can be seen in Figure 33.

Figure 32 – Stages of assembly of steel tube Figure 33 - Completed Steel Tube Connection

Construction

 Cut a 100mm length of the tubing and a 200mm length.

 Saw a fish mouth into the end of the 100mm piece so that it roughly fits onto the other piece of steel tubing.

 Use a file on the fish mouth to make a secure fit onto the tubing

 Weld the fish mouth onto the centre of the steel tubing exactly perpendicular to each other.

 Cut three pieces of bamboo, each with a node at approximately 90mm from the end.  Cut long strips of tough material and wrap around the legs of the steel tubing in a

clockwise direction to fit the size of the internal diameter of the culm.

 Place culms of the steel tubing and twist anti-clockwise to tighten fit between cloth and culm.

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3.3 Expected Results

Calculations to predict the failure loads of the bamboo were carried out using data from Janssen (1995). The strength of a bamboo culm depends on its species, the age of the culm, its moisture content, the part of the culm used, its form and size (e.g. depending on if the culm is perfectly spherical) and the position of the nodes in the joint (Janssen 1981). Janssen found that there was a simplified way to characterise the strength of the culms as there is a ratio that exists between the density and the allowable stresses in a bamboo culm. These strength characteristics are shown for both green and dry bamboo. Many of the connections that are being tested were not able to be predicted due to their composite nature and lack of data for bamboo failure in certain planes.

It should be noted that the data used is not from any design standards but is from the testing of Janssen. The calculations carried out will aim to provide a feasible range of expected failure results to allow for the variations in culm diameters.

Before the calculations can be carried out the density of the Moso bamboo being tested had to be worked out. This was achieved by weighing a sample without any nodes in it and measuring its dimensions to work out the volume. The outer diameter and wall thickness were measured on both ends to achieve an average value. The calculations are shown below:

Height of sample, H = = 192 mm

Average Wall Thickness, w = (6.5+6.4+6.2+6.0+6.2+6.3+6.2+6.0)/8 = 6.23 mm

Average Outer Diameter, D = (64+65+63+63)/4 = 63.8 mm

Volume, V = H x π x (D2– (D - 2w)2)/4 = 216 300 mm3

Mass, m = 151 g

Density, ρ = m / V

= 700 kg/m3

This was checked with another sample and the density remained the same. Area Moment of Inertia =

64 ) ) 2 ( (D4_ _D_ __w 4  = 472 300mm4

Modulus of the Section, S = Area Moment Inertia / Radius = 14800mm3

From Janssen’s studies, the ratio between the density and the allowable bending stress in a dry bamboo culm is given as 0.020.

Allowable bending stress, σ = 0.020 x 700 = 14 N/mm2 Allowable Bending Moment = σ x S

= 207 000 Nmm = 0.21 kNm

This is the value at which the bamboo culm should break under bending if the culm is carrying the entire load.

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3.4 Results

The results are taken from testing each type of connection three times and observing the failure pattern. Also published are the average weights and other relevant practical information to the connection. For each connection the bending strength capacity, shear strength capacity and the tensile capacity were recorded.

The bending moment results show the measured bending moment at the base of the connection against the angle of rotation. The shear test results are shown graphically as the applied shear force against the average displacement of the top part of the joint. Tensile results are shown as the pull-out force applied against the displacement of the top culm.

A test was carried out on the tension test rig to ensure that test results would not be impacted by friction – this proved to be the case.

It should be noted that in some of the tests there is an initial deflection of the top culm with very little force applied. This is because of the bottom culm not being right against the end of the steel circular section at the start of testing.

3.4.1 Bamboo Dowel Connection

Table 1: Bamboo Tenon and Dowel Connection

3.4.1.1 Bamboo Dowel Bending

The bamboo tenon and dowel showed two different failure characteristics over the three tests performed. The initial failure mode of each joint was noted but the testing continued after the initial cracks.

The first bending test initially failed at the bottom of the tenon and this can be seen in Figure 34. The initial failure occurred at a moment of 0.2 kNm and the two corners holding the tenon were sheared off. Until this point the joint showed a linear relationship between the angle of rotation and bending moment carried. The result of this failure was a loss of load carrying capacity. The joint continued to be loaded and reached a value of 0.15kNm before the bottom piece of bamboo cracked on its underside.

Property

Average Weight 0.46 kg

Average Time to Construct 50 mins Tools Required Saw, Drill, Rat tail

TOP LEFT:

Figure 34 - Bamboo Tenon and Dowel Bending Test 1

BELOW LEFT:

Figure 35 – Bamboo Tenon and Dowel Bending Test 2

BELOW RIGHT:

Figure 36 – Bamboo Tenon and Dowel Bending Test 3

Bamboo Connections