Bamboo Connections Student: Chris Davies Supervisor: Pete Walker Department of Architecture and Civil Engineering The University of Bath 2008
AR40223 MEng dissertation
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.
Student Signature:
................................................................................................................
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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 70mm80mm 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. iii
Table of Contents Acknowledgements ................................................................................................................. 1 1. Introduction........................................................................................................................... 2 2. Literature review.................................................................................................................. 4 2.1 Introduction ................................................................................................................................... 4 2.2 Basic Jointing Rules...................................................................................................................... 4 2.2.1 Rule 1 - Construct joints near nodes ...................................................................................................... 4 2.2.2 Rule 2 - Avoid openings in culms .......................................................................................................... 5 2.2.3 Rule 3 - Treat the bamboo culms ........................................................................................................... 5 2.2.4 Rule 4 - Securely fit joints ..................................................................................................................... 5 2.2.5 Rule 5 - Make durable connections........................................................................................................ 5 2.2.6 Rule 6 - Reinforce culms under high point loads ................................................................................... 6 2.3 Tools needed in bamboo jointing .................................................................................................. 6 2.4 Traditional jointing techniques ...................................................................................................... 6 2.4.1 Lashing................................................................................................................................................... 6 2.4.2 Butt Joints .............................................................................................................................................. 9 2.4.3 Splice Joints ......................................................................................................................................... 10 2.5 Modern Connections ................................................................................................................... 12 2.5.1 Gusset Plate.......................................................................................................................................... 12 2.5.2 Wooden insert ...................................................................................................................................... 13 2.5.3 Concrete Infill ...................................................................................................................................... 14 2.5.4 Pin Connection ..................................................................................................................................... 14 2.5.6 Expandable Joints................................................................................................................................. 14 2.5.7 Steel Inserts .......................................................................................................................................... 15 2.5.8 Preformed Tubing ................................................................................................................................ 17 2.6 Bamboo in Timber Connections.................................................................................................. 17 2.6.1 Bamboo Fibre Drift-pins ...................................................................................................................... 17 2.6.2 Laminated Bamboo Lumber................................................................................................................. 18 2.7 Findings....................................................................................................................................... 19
3. Main chapters .................................................................................................................... 20 3.1 Laboratory Testing ...................................................................................................................... 20 3.1.1 The Test Rig......................................................................................................................................... 20 3.1.2 Methodology ........................................................................................................................................ 23 3.2 Connection Designs .................................................................................................................... 24 3.2.1 Bamboo Tenon and Dowel................................................................................................................... 24 3.2.2 Plywood Gusset Plate........................................................................................................................... 25 3.2.3 Concrete Filled Joint ............................................................................................................................ 27 3.2.4 Preformed Steel Tube........................................................................................................................... 28 3.3 Expected Results ........................................................................................................................ 29 3.4 Results ........................................................................................................................................ 30 3.4.1 Bamboo Dowel Connection ................................................................................................................. 30 3.4.2 Plywood Gusset Plate Connection ....................................................................................................... 36 3.4.3 Concrete Filled Connection.................................................................................................................. 40 3.4.4 Steel Tube Connection ......................................................................................................................... 44 4.1 Conclusions from adapted joints ................................................................................................. 46 4.1.1 Bamboo Tenon and Dowel................................................................................................................... 46 4.1.2 Plywood gusset plate............................................................................................................................ 47 4.1.3 Concrete Infill Joint.............................................................................................................................. 47 4.1.4 Steel Tube Joint.................................................................................................................................... 48 4.2 Further Work on an Alternative Connection Solution.................................................................. 49 4.2.1 Initial Reinforced Canvas Joint Design Mark 1 ................................................................................... 50 4.2.2 Improved Reinforced Canvas Design Mark 2 ...................................................................................... 52
5. Final Conclusions.............................................................................................................. 55 5.1 Canvas Joint Mark 2 ................................................................................................................... 55 5.2 Further Studies............................................................................................................................ 56 5.3 Final Conclusions ........................................................................................................................ 56
References ............................................................................................................................. 57
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List of Figures Figure 1 – Simon Velez Cathedral, Columbia Figure 2 – Methods to prevent crushing Figure 3 – Traditional Lashing Techniques Figure 4 - Elastic Band Lashing Figure 5 – Bamboo Scaffolding Figure 6 – Nienhuys Improved Tied Butt Figure 7 – Tongue Joint Figure 8 – ‘Horn’ Connection Figure 9 - Full Lapped (Left) and Half Lapped Joints Figure 10- Butt Joint with Side Plates Figure 11 – Sleeves and Inserts Figure 12 – Gusset Plate Design Figure 13 – ITCR Internal Gusset Plate Figure 14 – Timber Insert Figure 15 – Nienhuys Expandable Connection Figure 16 - Duff Steel Connection Figure 17 – Improved Steel Connection Figure 18 – Modern Variations on steel connections Figure 19 – Steel Tube Connection Figure 20 – Tensile Test Using Bamboo Fibre Board and Drift Pin Figure 21 – Bending and Shear Test Apparatus Figure 22 - Set up for Bending Moment Test Figure 23 – Shear testing from the side and top Figure 24 – Bending testing close up and from the top Figure 25 – Set up for tension test Figure 26 – Tension Rig in action Figure 27– Bamboo Tenon and Dowel Connection Figure 28 – Bamboo Tenon and Dowel Components Figure 29 - Components of Plywood Gusset Plate Connection Figure 30 – Gusset Plate Dimensions Figure 31 – Concrete Connection Components before Mortar Injection Figure 32 – Stages of assembly of steel tube Figure 33 - Completed Steel Tube Connection Figure 34 - Bamboo Tenon and Dowel Bending Test 1 Figure 35 – Bamboo Tenon and Dowel Bending Test 2 Figure 36 – Bamboo Tenon and Dowel Bending Test 3 Figure 37 – Bamboo Tenon and Dowel Bending Results Figure 38 – Bamboo Tenon and Dowel Shear Test 1 Figure 39 – Bamboo Tenon and Dowel Shear Test 2 Figure 40 – Bamboo Tenon and Dowel Shear Test 3 Figure 41 - Bamboo Tenon and Dowel Shear Results Figure 42 – Bamboo Tenon and Dowel Tension Test 1 Figure 43 – Bamboo Tenon and Dowel Tension Tests 2 and 3 Figure 44 – Bamboo Tenon and Dowel Tension Results Figure 45 – Plywood Gusset Plate Bending Test 2 Figure 46 – Plywood Gusset Plate Bending Test 3 Figure 47 - Plywood Gusset Plate Bending Results Figure 48 – Plywood Gusset Plate Shear Test 1 Figure 49 – Plywood Gusset Plate Shear Test 3 Figure 50 – Plywood Gusset Plate Shear Results Figure 51 – Plywood Gusset Plate Typical Tensile Failure Pattern Figure 52 – Plywood Gusset Plate Tension Results Figure 53 – Concrete Filled Bending Test 1 Figure 54 – Concrete Filled Bending Test 1 Side
2 6 7 7 8 9 9 9 10 11 11 12 13 13 15 15 16 16 17 18 20 20 21 21 22 23 24 24 25 26 27 28 28 30 31 31 31 32 32 33 33 34 34 35 36 36 37 37 37 38 39 39 40 40
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Figure 55 – Concrete Filled Bending Test 3 Figure 56 - Concrete Filled Bending Results Figure 57 – Concrete Filled Shear Test 1 Figure 58- Concrete Filled Shear Results Figure 59 – Concrete Filled Connection after Tension Failure Figure 60 – Bamboo Tenon and Dowel Tension Results Figure 11 – Steel Tube Bending Failure Figure 62 - Steel Tube Bending Results Figure 63 – Steel Tube Shear Test Figure 64- Steel Tube Shear Results Figure 65 – Canvas Mark 1 Front Figure 66 – Canvas Mark 1 Back Figure 67 – Canvas Mark 1 Bending Results Figure 68 - Canvas Joint Mark 2 Back Figure 69 -Canvas Joint Mark 2 Front Figure 70 - Canvas Joint Mark 2 Figure 71 – Canvas Joint Mark 2 Failure Figure 72 – Canvas Mark 2 Bending Results Figure 73 – Canvas Joint Mark 3
41 41 42 42 43 43 44 44 45 45 50 50 51 52 53 53 54 54 55
List of Tables Table 1 – Bamboo Tenon and Dowel Connection Table 2 – Plywood Gusset Plate Connection Table 3 – Concrete Filled Connection Table 4 – Steel Tube Connection Table 5 – Summary of Connections
30 36 40 44 46
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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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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 Figure 1 – Simon Velez Cathedral 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 detailing design practice. Traditional methods such as smoking or whitewashing (painting the 2
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 Bamboo culms are strongest near their nodes as at each node there is a diaphragm which adds additional strength and stiffness to the culm. The node should remain as close to the joint as possible to reduce the risk of insects entering inside the bamboo culms and the risk of the 4
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. Figure 2 – Methods to prevent crushing
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. ArceVillalobos 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 Figure 4 - Elastic Band Lashing 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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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, Figure 5 – Bamboo Scaffolding 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 fire damage is often considered too high. Lashed joints would fail before the bamboo culms and cause a sudden and unexpected collapse of a structure. 8
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.
Figure 6- Nienhuys Improved Tied Butt
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.
Figure 10 –Side Plate Splice Joint
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
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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 Figure 12 – Gusset Plate Design 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 manmade 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 Figure 14 – timber insert 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. This connection would be hard to produce with the available resources in the laboratory but ideas from existing expandable plugs used with masonry and concrete could be adapted for 14
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.
Figure 17 – Improved Steel Connection
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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 considerable weight to the connection.
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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.
Figure 19 – Steel Tube Connection
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
Figure 22 – Set up for Bending Moment Test
A test rig was set up to be able to test the shear capacity and moment capacity of each joint by adjusting the height of the load cell. Figures 21 and 22 show the set up for testing the bending 20
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.
Figure 24 – Bending testing close up and from the top
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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.
BOTTOM CULM
TENON TOP CULM DOWELS
Figure 28 – Bamboo Tenon and Dowel Components
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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.
TWO GUSSET PLATES BOTTOM CULM
TOP CULM
Figure 29 - Components of Plywood Gusset Plate Connection
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Figure 30 – Gusset Plate Dimensions
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. 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. Drill the remaining two holes through the culm and repeat the above step.
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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: Saw a fish mouth at the end of a culm near the node so that it fits securely onto another culm of similar diameter. 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 Average Wall Thickness, w Average Outer Diameter, D
= = (6.5+6.4+6.2+6.0+6.2+6.3+6.2+6.0)/8 = (64+65+63+63)/4
Volume, V
= H x π x (D – (D - 2w) )/4 3 = 216 300 mm
Mass, m
= 151 g
Density, ρ
=m/V 3 = 700 kg/m
2
= 192 mm = 6.23 mm = 63.8 mm
2
This was checked with another sample and the density remained the same. Area Moment of Inertia =
(D 4 ( D 2 w) 4 ) 64 = 472 300mm
Modulus of the Section, S
4
= Area Moment Inertia / Radius 3 = 14800mm
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 2 = 14 N/mm
Allowable Bending Moment
=σxS = 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. 29
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 Property Average Weight Average Time to Construct Tools Required
0.46 kg 50 mins Saw, Drill, Rat tail
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. 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 Figure 34
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Figure 35
Figure 36
Figure 35 shows the failure of the second bending moment test. Although the samples aimed to be as similar as possible in design and construction, natural differences in each individual culm produced different results. The second test failed at the top end of the tenon where the dowel passed through the hole. This failure occurred at 0.125kNm and the connection failed to carry much load after this initial failure. Figure 37 shows the comparative failures of each individual connection. The third bending moment test first failed at the bottom of the tenon at a bending moment of 0.05kNm. One side of the tenon was sheared off at this relatively low bending moment. The connection continued to carry moment until the top of the tenon failed around the point of contact with the dowel. A small amount of cracking could be seen in the top culm around the hole although this was not the cause of failure for this test. Bamboo Tenon and Dowel - Bending
Bending Moment at Base (kNm)
0.2500
0.2000 Bending Test 1
0.1500
Bending Test 2 Bending Test 3
0.1000
0.0500
0.0000 0.00
5.00
10.00
15.00
20.00
25.00
30.00
Angle of Rotation (degrees)
Figure 37 – Bamboo Tenon and Dowel Bending Results
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3.4.1.2 Bamboo Dowel Shear The top of the tenon and the top of the bottom culm were under high stresses in the shear test and tended to be the first to yield. The joints tended to find a new failure mechanism after the initial cracking and regain strength capacity. Figure 41 shows the results of the shear tests.
Figure 38 – Bamboo Tenon and Dowel Shear Test 1
The first test failed initially after 1.8kN in the tenon where the cracking could be heard inside the top culm. The joint continued to carry load after this with the broken tenon resting against the top bamboo culm. The stresses on the top culm continued to build until cracks appeared in the top culm around the hole. Eventually the top culm split into pieces at a force of 2.55kN. The post failure behaviour can be seen in Figure 38. The second shear test failed initially around the bottom tenon after a force of 1.3kN. This caused an immediate loss of load carrying capacity but this was restored as the connection found a new failure mechanism. The next part of the joint to break was the top of the tenon around the hole at a force of 2.6kN. The joint continued to be loaded again until it split down the top of the bottom of the culm, this final failure can be seen in Figure 39. The load reached a maximum value of 2.7kN before losing all strength.
Figure 39 – Bamboo Tenon and Dowel Shear Test 2
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The final shear test reached a load of 2.55kN before any failures were noted. At this force the top of the tenon failed around the hole and the connection lost a lot of its load carrying capability. The load was able to be increased again before one side of the bottom of the tenon failed at a force of 3.2kN. When loaded again the connection still managed to withstand a force of up to 2.45kN before the bottom culm finally snapped and the joint lost all load carrying capacity, the is seen in Figure 40.
Figure 40 – Bamboo Tenon and Dowel Shear Test 3 Bamboo Tenon and Dowel - Shear 3.5000
Force Applied (kN)
3.0000 2.5000 Shear Test 1 2.0000
Shear Test 2 Shear Test 3
1.5000 1.0000 0.5000 0.0000 0.00
10.00
20.00
30.00 40.00 50.00 Displacement (mm)
60.00
70.00
80.00
Figure 41 - Bamboo Tenon and Dowel Shear Results
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3.4.1.3 Bamboo Dowel Tension Under the tension test the bamboo tenon and dowel failed in two different ways. The first joint first failed on one side of the base of the tenon at a force of 2.25kN. The joint continued to retain some strength after this and cracking began to appear on the underside of the bottom culm. The tensile force increased to 2.5 kN before the cracking in the bottom culm, seen in Figure 42, caused all loss of load carrying capacity.
Figure 42 – Bamboo Tenon and Dowel Tension Test 1
The second and third tests failed around the hole of the tenon. The maximum load carried in these joints was higher than the first test although the joint failed to have any load carrying capacity after the initial failure. Images of the failure characteristic for the second and third tests can be seen in Figure 43. This failure happened at 4.2kN in the second test and 3.3kN in the third test.
Figure 43 – Bamboo Tenon and Dowel Tension Tests 2 and 3
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Bamboo Tenon and Dowel - Tension 5.0000 4.5000 Force Applied (kN)
4.0000 3.5000
Tension Test 1
3.0000
Tension Test 2
2.5000
Tension Test 3
2.0000 1.5000 1.0000 0.5000 0.0000 0.00
5.00
10.00
15.00 20.00 25.00 Displacement (mm)
30.00
35.00
40.00
Figure 44 – Bamboo Tenon and Dowel Tension Results
3.4.1.4 Bamboo Dowel Notes In all the tests completed the bamboo dowel was never the cause of failure. The failure mode was always brittle and sudden. In most cases the initial failure caused a sudden change in displacement but the connection was able to stiffen and form new load paths. The addition of stronger materials for the tenon would improve the performance of this joint.
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3.4.2 Plywood Gusset Plate Connection Table 2: Plywood Gusset Plate Connection Property Average Weight Average Time to Construct Tools Required
1.1kg 40 mins Saw, Drill
3.4.2.1 Plywood Gusset Plate Bending The plywood gusset plate results all failed in a similar manner under bending and all reached a similar value before the initial failure. In all cases the initial cracking occurred on the bottom culm around the steel dowels. The point of initial cracking can be seen for Figure 47 to occur at between 0.36kNm and 0.33kNm in all cases. After this point of failure the sample continued to be loaded until the sample was unable to carry any more loading. Samples one and two both were able to carry a higher bending moment than at the point of initial cracking with sample two managing to carry a peak moment of 0.59kNm. Sample two sustained a secondary crack in the top culm when loaded to destruction, this can be seen in Figure 45. Sample three failed to regain its moment capacity after the initial failure and the crack pattern can be seen in Figure 46.
Figure 45 – Plywood Gusset Plate Bending Test 2
Figure 46 – Plywood Gusset Plate Bending Test 3
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Plywood Gusset Plate - Bending
Bending Moment at Base (kNm)
0.7000 0.6000 0.5000
Bending Test 1
0.4000
Bending Test 2 Bending Test 3
0.3000 0.2000 0.1000 0.0000 0.00
5.00
10.00
15.00
20.00
25.00
Angle of Rotation (degrees)
Figure 47 - Plywood Gusset Plate Bending Results
3.4.2.2 Plywood Gusset Plate Shear The gusset plate tests showed similar failure modes under shear testing but all varied in their maximum load carrying capacity reached. In all tests the initial cracking occurred in the bottom culm between the steel bolts, this can been seen occurring in Figure 48 on the first shear test. Shear test three performed the best out of the samples and reached a shear force of 6.7kN with only a change in deflection of 3.2mm before cracking occurred. Further loading on test three after the initial failure resulted in the shear force to be able to increase to 9kN but caused cracking in the top culms as can be seen in Figure 49.
Figure 48 – Plywood Gusset Plate Shear Test 1
Figure 49 – Plywood Gusset Plate Shear Test 3
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There were much larger levels of deflection in tests one and two although the shear capacity remained similar. Signs of cracking were heard in the first test at a force of 4.8kN. After this the connection was able to resist the shear force but the crack size increased. The crack ran through to the end of the bottom culm at a force of 6.4kN causing a temporary loss in capacity. The second test did not crack suddenly but instead cracks opened in the bottom culm as the force was increased. This explains the linear behaviour in the graph seen in Figure 50. Plwood Gusset Plate - Shear
Force Applied (kN)
12.00 10.00 8.00
Shear Test 1 Shear Test 2
6.00
Shear Test 3
4.00 2.00 0.00 0.00
10.00
20.00 30.00 40.00 Displacement (mm)
50.00
60.00
Figure 50 – Plywood Gusset Plate Shear Results
38
3.4.2.3 Plywood Gusset Plate Tension The plywood gusset plate acted in a similar way in all the tension tests. The connection failed on the bottom culms around the steel thread, this typical failure pattern can be seen in Figure 51. The first tension test reached a tensile force of 6.5kN before the bamboo initially cracked. Until this point the force and displacement acted linearly. The connection continued to be loaded after the initial failure and managed to carry a tensile force of 7.5kN beyond which the Figure 51 – Plywood Gusset Plate Typical Tensile connection lost all its strength. The small Failure Pattern peaks and troughs on the graph are from when the load cell was being incrementally jacked after initial failure. The second and third tests acted in a similar manner but after the initial failure they failed to regain the same level of tensile force. The worst joint was the second tension test which only carried a maximum tensile force of 4.5kN. Figure 52 shows the comparative failures from all the tensile tests. Plywood Gusset Plate - Tension 8.0000
Force Applied (kN)
7.0000 6.0000 Tension Test 1
5.0000
Tension Test 2
4.0000
Tension Test 3 3.0000 2.0000 1.0000 0.0000 0.00
10.00
20.00 30.00 Displacement (mm)
40.00
50.00
Figure 2– Plywood Gusset Plate Tension Results
3.4.2.4 Plywood Gusset Plate Notes In all tests the plywood gusset plate never failed and showed no signs of failing. The 10mm steel dowels showed little signs of failing although did slightly bend in the bending tests.
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3.4.3 Concrete Filled Connection Table 3: Concrete Filled Connection Property Average Weight Average Time to Construct Tools Required
2.2 kg 25 mins Cement Mixer, Spade, Drill, Saw, File
3.4.3.1 Concrete Filled Bending
Figure 53 – Concrete Filled Bending Test 1
Figure 54 – Concrete Filled Bending Test 1 Side
The concrete filled joints all performed well under bending and in a ductile manner, this is visible from Figure 56. In all cases the experiment was stopped because the load cell ran out of jack, not because the joint failed to retain any moment capability. The first test performed linearly up to a moment of 0.22kNm, at this point the concrete running between the top and bottom culms failed and the steel eye bolt took all the moment. This can be seen on the graph as the point where the line begins to curve. At a moment of 0.46kNm the steel bar yielded and the connection was unable to carry any more moment. The continued loading of the connection caused the steel bolt to fail and for the top bamboo culm to be pulled away from the mortar, this is visible in Figure 53. The loading of the connection after failure also caused the top culm to crack along the side. This was due to the bearing from the top culm onto the bottom culm and can be seen in Figure 54.
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LOCAL CRUSHING
Figure 55 – Concrete Filled Bending Test 3
The other tests performed in a similar manner although they all failed at different moments. The second test was only able to reach a moment of 0.35kNm. There was also a much larger rotation of the top culm before this maximum moment was reached. This was because the concrete between the top and bottom culms failed very quickly and the strength of the connection was dependent on the steel eye bolt from a very early stage. The third test was able to carry a maximum bending moment of 0.46kNm before losing its capability. All tests resulted in the top culm bearing upon the bottom culm at the point shown in Figure 55. This did not cause any cracking or loss of capacity for the connection because the mortar inside the bottom culm stopped any crushing occurring. Concrete Filled - Bending
Bending Moment at Base (kNm)
0.5000 0.4500 0.4000 0.3500 0.3000
Bending Test 1
0.2500
Bending Test 2
0.2000
Bending Test 3
0.1500 0.1000 0.0500 0.0000 0.00
5.00
10.00
15.00
20.00
25.00
30.00
Angle of Rotation (degrees)
Figure 563- Concrete Filled Bending Results
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3.4.3.2 Concrete Filled Shear
Figure 57 – Concrete Filled Shear Test 1
Under shear the concrete filled joint failed in a very similar manner to how it failed under bending and typical failure can be seen in Figure 57. All the tests acted in a linear way before the steel dowel started to bend. The first test was not firmly secured in place so the initial force applied moved the bottom culm to the end of the apparatus, this has offset all the results by 7mm. If this is considered then samples one and two performed in a very similar way at shear forces of around 6kN. An audible sign of cracking was heard at a force of 3kN which explains the short loss of capacity seen in Figure 58. The third test failed slightly earlier than the other samples at 2.4kN. All samples continued to fail in a ductile manner as the steel tenon was bent. The samples all reached over a maximum force of 6kN before the displacement became excessive and the load cell ran out of jack. Concrete Filled - Shear
7.0000
Force Applied (kN)
6.0000 5.0000 Shear Test 1
4.0000
Shear Test 2
3.0000
Shear Test 3
2.0000 1.0000 0.0000 0.00
10.00
20.00
30.00 40.00 Displacement (mm)
50.00
60.00
Figure 584- Concrete Filled Shear Results
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3.4.3.3 Concrete Filled Tension All the tests managed to hold a reasonable load until the jack ran out of stroke. The failure of the joints occurred in the steel eye dowel which opened up. Cracking was evident inside the top bamboo culm in all tests which could be seen when the joint was opened up and this can be seen in Figure 59. All the samples followed a similar curve before the cracking in the mortar of the top culm caused a loss in capacity. The worst performed joint was the first sample tested which only reached a tensile force of 7.1kN before the mortar yielded. Samples two and three were able to carry a tensile force of around 12kN before losing structural capacity. Figure 60 shows the failure points of all the tests.
Figure 595 – Concrete Filled Connection after Tension Failure Concrete Filled - Tension 14.0000
Force Applied (kN)
12.0000 10.0000 Tension Test 1
8.0000
Tension Test 2
6.0000
Tension Test 3
4.0000 2.0000 0.0000 0.00
5.00
10.00
15.00
20.00 25.00 30.00 Displacement (mm)
35.00
40.00
45.00
Figure 606– Concrete Filled Tension Results
3.4.3.4 Concrete Filled Notes In all the concrete tests the steel quickly took the entire applied load. This produced a ductile failure which is more desirable in construction. The tests were unable to be loaded to complete destruction because of the loading cell running out of jack.
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3.4.4 Steel Tube Connection Table 4: Steel Tube Connection Property Average Weight Average Time to Construct Tools Required
0.65kg 90 mins Saw, File, Welding Equipment
3.4.4.1 Steel Tube Bending In all the bending tests the top bamboo culms were the only parts of the connection to fail before the connection lost all structural capability. The failure occurred due to cracking of the top culm on the opposite side of the load cell. The top culms, removed after failure, can be seen in Figure 61. The failures for all tests occurred at an angle of rotation of between thirteen and fifteen degrees and a moment of 0.17-0.28kNm. The curve of all samples up to the point of failure was similar in shape. An initial small force caused five degrees of rotation, this was when the material was being compressed which offered little resistance.
Figure 7– Steel Tube Bending Failure
Steel Tube - Bending
Bending Moment at Base (kNm)
0.3000 0.2500 Bending Test 1
0.2000
Bending Test 2 0.1500
Bending Test 3
0.1000 0.0500 0.0000 0.00
5.00
10.00
15.00
20.00
25.00
30.00
Angle of Rotation (degrees)
Figure 62 8- Steel Tube Bending Results
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3.4.4.2 Steel Tube Shear In shear the steel tube was susceptible to crushing and buckled on the side of the load cell. This test was only performed once due to the shortage of suitable material. The single test has been assumed to be sufficient as the steel tube fails in a predictable manner. Failure occurred at a force of 1.2kN and this caused a loss in capacity of the section, Figure 64 shows this. This loading was able to be restored temporarily before the top bamboo culm started to bend and be pulled away from the steel tube. There was no damage to the bottom tubing but the buckling of the top part of the steel tube can be seen in Figure 63.
Figure 63 – Steel Tube Shear Test Steel Tube - Shear
1.4000
Force Applied (kN)
1.2000 1.0000 0.8000 0.6000 0.4000 0.2000 0.0000 0.00
10.00
20.00 30.00 Displacement (mm)
40.00
50.00
Figure 64- Steel Tube Shear Results
3.4.4.3 Steel Tube Tension The bamboo culms were able to be pulled from the steel tubing by hand without excessive force required. It was decided that the tensile strength of the steel tube joint should be assumed to be zero and testing of the steel tube joints for tension was not necessary. 3.4.4.3 Steel Tube Notes The results from this test were all very predictable and all very similar. 45
4. Initial Conclusions 4.1 Conclusions from adapted joints The studies into the adaptations of existing joints and their testing showed the many different ways that bamboo culms are able to be connected. Each material and design showed different failure characteristics and maximum strengths. The failure patterns of the connection show where additional reinforcement is needed and how the dimensions can be altered to improve the structural performance. Table 5 is a summary of the connections tested. Where strengths are quoted the value refers to the average strength before the initial failure occurred. The difficulty of construction concerns both the quality of labour required and the tools needed. Table 5 – Summary of Connections Bamboo Plywood Tenon and Gusset Plate Dowel Bending Moment Strength(kNm) 0.13 0.35 Shear Strength (kN) 2.5 7.5 Tensile Strength (kN) 3.3 5.6 Retain capacity after failure? No Yes Weight (kg) 0.46 1.1 Time to Construct (mins) 50 40 Difficulty of Construction Med/Hard Easy/Med Aesthetics (0-6) 4 5 Materials used other than bamboo None Plywood, Steel Bolts
Mortar Filled
Wrapped Steel Tube
1.26 6.2 11.0 Yes 2.2 25 Hard 1 Mortar, Steel Bolts
0.22 1.2 0 No 0.65 90 Medium/Hard 4 Steel, Canvas material
4.1.1 Bamboo Tenon and Dowel The tenon tended to be the initial weakness in the joint. In the bending test the initial failure occurred either at the bottom of the tenon or around the hole at the top of the tenon. The joint can be improved by making sure a node is at the base of the tenon. This helps to stop the sides of the T shearing off. The length of the part of the tenon that extends out the bottom could be extended to provide additional strength but this would make the joint less attractive. The other way to improve the strength of the connection would be to make the bamboo tenon longer above the hole. The width of the tenon is limited by the internal diameter of the top culm so extending the length of the tenon is the only way. The dowels provided suitable strength for the connection and should not be changed in design. Ideally the failure of this joint should occur from the base of the bottom bamboo culm cracking. This eventually occurred in the first bending test, the first tension test and the third shear test. The joint was often loose before loading as there is no method of tightening the joint once the pieces have been cut. Skilled labour and time is required to ensure all the parts fit tightly together. Structures using this connection should build in some tolerances to allow for a loose fit and should not rely on the connection to provide rigidity. 46
Another problem with this connection is the need to punch through the first node to let the tenon pass through. This enables insects and water to enter into the first nodal chamber. One solution would be to cut the culm so the node is above the drilled hole but this would result in a loss of strength of the connection. An alternative option, such as filling the hole made in the diaphragm, would be more appropriate. This joint should be tested with the tenon perpendicular to its current position to see how it might be suited to loading in a different direction. With these improvements made this joint can be very useful in an environment where bamboo is the only material available and there is an experienced labour force available. Its naturalness is a very positive aesthetical quality and nothing man made should be added to this joint. The connection could be used in small structures connecting smaller structural members to larger ones, perhaps to connect rafters to a primary beam. 4.1.2 Plywood gusset plate The plywood gusset plate connection proved to be a strong joint in all of the tests. In bending the connection reached moments of 0.35kNm without cracking or considerable rotation. In shear all the bolts were able to take the applied force causing the connection to have a high shear capacity. The joint continued to withstand force after this initial failure which would provide a safer structure. The post-failure capacity often reached a higher value than at the point of failure. This joint could be part of a prefabricated structure where a large amount of gusset plates could be transported to site. The gusset plate would be able to be adapted in size and shape to different connections in joining together more culms or two culms at a different angle. The main disadvantage of this joint is in the amount of other materials used. Plywood plates and steel bolts could be very expensive to buy. An improvement to the joint would be to make the gusset plate shorter and wider and provide three bolts through the bottom culm. The bolts of the culm should be staggered to maximise the bearing area and utilise the strength of the bamboo culms to their capacity. The rigidity and high strength capacity of this connection make it suited for larger structures and for use in a truss. Gusset plates could also be added to a weakened joint to provide extra strength and stiffness. 4.1.3 Concrete Infill Joint The addition of the concrete stopped the local failure of the steel thread on the bamboo culm. This gave the connection a much higher capacity to carry bending moment because the steel eye bolt was able to reach its full capacity without the bamboo cracking. The connection performed well under shear but the top culm tended to bend before any shearing actually happened. Further tests are needed on this connection to find its true shear capacity. The joint could be improved either by increasing the area of contact of concrete between the two culms or by using a thicker bolt to run through the concrete. The tension tests showed that the eye of the tenon was bent out of shape causing the joint to distort. This could be improved by making the eye into a complete loop that was welded closed. Another way to improve the
47
tensile strength of the connection would be to increase the length of the eye away from the bottom culm. This would require more mortar to be used and longer lengths of steel bolts. The joint was able to hold a high tensile load so could be used as part of a truss. This connection should be specified for specific connections of a structure and not applied to every joint. 4.1.4 Steel Tube Joint The steel tube joint performed well in the bending tests and the bamboo culms were always the first to break. The breakage of the bamboo culms was at the expected moment from the calculations carried out. If the joint was subject to shear directly at the joint then additional reinforcement of the tubing would be needed to stop the tube buckling. To improve the bending strength of the connection the length of the legs should be increased. It may be the case that only one of the legs of the T needs to be extended however the extra strength gained from this would probably not be worth the additional expenditure. The connection has a lack of tensile strength and should not be used in trusses or other structures that require such tensile strength from the joints. The addition of a dowel through the steel tube and bamboo culm will bring an element of tensile strength to the joint. This dowel should run perpendicular to the primary direction of loading to minimise interference with the current mode of failure. This joint can be produced to incorporate more than three culms which make it very versatile and simple to use. Green bamboo culms can be used with this connection either by using the same method of wrapping material around the joint or by attaching a jubilee clip to the outside. The shrinking of the green culm should provide additional tensile strength to the joint although signs of cracking should be carefully observed. The addition of the material to wrap around the steel tube makes the joint adaptable for a small range of different diameter culms which was one of the initial requirements for the joint. The steel tube connection does have a limit on the diameters of culms used. Large amounts of material being used increases the deflection of the culm under loading. The ends of the culms remain exposed in the steel tube connection. This facilitates water ingress into the bamboo and would greatly decrease the lifespan of the structural beams and columns. This connection could be used in scaffolding construction where the connections are able to be reused and long term durability of the bamboo culms is not an essential quality. Another use for the steel tube connection would be in immediate shelter construction where the connections could be mass produced and sent to a site. The joints require little skill in construction, few tools and are quick to construct once the joints have been produced.
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4.2 Further Work on an Alternative Connection Solution Further work into the development of a new joint focused on one particular aspect of bamboo construction and be designed especially for this. No individual bamboo connection can be suited for every purpose as there are so many factors to consider. Each joint tested had an application for use in construction depending on the available resources. Some joints were suited to temporary construction and others more permanent. The different connections were also suitable for a range of loads and different sizes of construction. Much work has been done on temporary shelter after a disaster but these tend to require materials and tools that may not be available on site. Development of the study into existing joints has provided a new connection suitable for use in disaster relief shelter or quick build structure. The assumptions made for this joint are that:
No skilled labour is available Tools are limited to just a machete Only green bamboo is available It can be mass produced off site and delivered at the time of need It can be used on a small short term shelter where loads are small
The connection should be designed to be flat packed and shipped to an area in large quantities. The basic concept uses wrapped canvas to hold the culms in place and a steel T bar to provide further support against loading. The canvas can be tightened to fit the size of any culm. The connection can be tightened regularly to allow for any shrinkage of the green bamboo culms. This can be done without the need to dismantle any of the structure.
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4.2.1 Initial Reinforced Canvas Joint Design Mark 1 The first design was made from paper to find suitable dimensions and then cut from canvas. The dimensions and design of the joint can be seen in Figure 65 and Figure 66. The blue represents the rougher “hook” part of the material and the orange represents the softer “loop” part of the material. The steel bar used is 4mm thick and 25mm wide. The canvas used should be unidirectional.
Figure 65 – Canvas Mark 1 Front
Figure 66 – Canvas Mark 1 Back
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4.2.1.1 Construction Construction: Saw two steel sections, one of length 300mm and one of length 480mm. Saw a 10mm deep and 25mm wide hole into the centre of the 480mm steel strip and file down to enable a loose fit of the 300mm strip into the hole. Weld the 300mm steel strip in place perpendicular to the 480mm strip Cut the canvas to the shape and dimensions shown in Figures 65 and 66. Sew edges of the canvas over and reinforce corners with additional canvas as shown. Sew on hook and loop strips in positions shown in Figures 65 and 66. Place steel “T” on the back of the canvas in the location shown on Figure 66, cover with strips of 50mm wide canvas and sew in place so that the steel “T” is not able to move inside the canvas sleeve. Cut culms to desired lengths and place together at right angles. Wrap strip C around the bottom culm keeping the loop material on top. Wrap strip A around the top culm tightly using the hook and loop to keep in place Wrap strips D and B around the bottom culm as tightly as possible. 4.2.1.2 Canvas Mark 1 Results The testing showed that the connection was not bound tightly enough to the bamboo culms. This caused the entire canvas joint to slide along the bamboo until the metal T rested against the supports. This can be seen on Figure 67 to happen at a bending moment of 0.02 kNm. The reason for the change in gradient is the change from when the joint was sliding along the culm and when the steel bar was pushing against the support. This made the results unreliable but did show how poorly the joint would perform in a shear test. The test did show some other weaknesses in the joint aside from its poor grip to the bottom culm. The connection was subject to twisting in loading because of the steel bar only being placed on one side. This led to the failure being in the steel bar which was not expected. The connection is assumed to have no tensile capacity as the top culm can be removed without excessive force exerted. Canvas Mark 1- Bending
Bending Moment at Base (kNm)
0.1800 0.1600 0.1400 0.1200 0.1000 0.0800 0.0600 0.0400 0.0200 0.0000 0.00 -0.0200
5.00
10.00
15.00
20.00
25.00
30.00
Angle of Rotation (degrees)
Figure 67 – Canvas Mark 1 Bending Results
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4.2.2 Improved Reinforced Canvas Design Mark 2 The improved design used the same basic concept but provided additional strength in the areas of need. The problems with the first canvas joint were noted as:
Little frictional resistance between the bottom culm and the canvas Too much hook and loop made the joint hard to tighten to the bamboo without distorting its shape Connection was subject to rotational failure Steel T-bar does not perform well when loaded along its wide face Joint was unnecessarily long along the culms requiring a large amount of steel to be needed. Steel bar was imbedded in the joint and could not be removed to be replaced without cutting the canvas. The top culm was not long enough so the load cell was bearing onto the canvas and not the bamboo culm.
Steel T bars were placed on both sides of the joint. Only one leg of the steel bars was sewn in to provide additional flexibility of the joint. The canvas joint was shortened in length. This resulted in less steel being used which was important when considering the cost of each joint to produce. Much less of the hook material was used in the modified design to allow for each strip to be pulled tighter before being attached. The overlapping strip C of the canvas was modified in its design. This strip provided more support than was previously thought. The strip was extended to be able to pass around the entire joint and attach on the opposite side through the hook and loop material. The corners of the additional strip were also reinforced to the same degree as the other side. Both lengths of bamboo culm were also cut much longer than in the previous testing. This allowed for the canvas joint to slide further along the bottom culm if needed and not touch the testing rig apparatus. The top culm was longer to allow for the load cell to be placed further away from the base of the connection and not interfere with the canvas. The design of the modified canvas joint can be seen in Figure 68 and Figure 69. Figure 70 shows the connection at different stages of assembly.
Figure 68 - Canvas Joint Mark 2 Back
52
Figure 69 -Canvas Joint Mark 2 Front
Figure 70 - Canvas Joint Mark 2
53
4.2.1.2 Canvas Mark 2 Results The failure of the joint in bending occurred through the ripping of the canvas as the steel T rotated. The initial failure was at a moment of 0.21 kNm. The joint was still able to carry load after this and reached an angle of rotation of 46 degrees before the steel T-section finally completely ripped through the canvas. The final failure of the connection can be seen in Figure 71. The joint was able to carry a considerable bending moment but was subject to a large amount of rotation under loading. Figure 71 – Canvas Joint Mark 2 Failure Canvas Mark 2- Bending
Bending Moment at Base (kNm)
0.2500 0.2000 0.1500 0.1000 0.0500
-10.00
0.0000 0.00
10.00
20.00
30.00
40.00
50.00
-0.0500 Angle of Rotation (degrees)
Figure 72 – Canvas Mark 2 Bending Results
54
5. Final Conclusions 5.1 Canvas Joint Mark 2 The mark two canvas joint performed well under bending and the initial failure bending moment was at the value that the bamboo culm would break if the connection was completely rigid. There was a considerable amount of rotation in the top culm before the failure which ideally should be avoided. Because of this the connection should always be treated as a pin connection and secured at both ends. There was a linear relationship between the rotation and the bending moment before failure and a large amount of flexibility in the connection. The improved joint could be modified further by increasing the length of strips B and D to the same length as strip C as is shown in Figure 72. This would allow for more loops of the canvas to be wrapped around the culm and for the steel to withstand more of the loading without being torn through the canvas. The steel T bar should be sewn in place to try to stop the excessive rotation of the top culm. This would also help to protect the steel bar from corrosion.
Figure 72 – Canvas Joint Mark 3 The effects of rain of the canvas have not been explored and this could result in a considerable loss of structural capacity. The same tests should be carried out with a saturated canvas connection. Other materials should also be compared to find the most suited one. To give the connection some tensile support a hole could be drilled through the top culm just above where the connection ends. Tab C could then be adapted to pass though this hole before it is passed around the bottom culm and strapped up. This would not give very much strength and a new solution would be needed if the connection was to be used as part of a truss.
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5.2 Further Studies Further work on this project could be carried out in testing a wider range of joints using the same testing rig. Additionally new apparatus could be set up to test other mechanical properties of the joint. These could include tests loading perpendicular to the bottom culm, loading the bottom culm instead of the top culm or performing torsion tests. Over time it would be possible to collect results for every joint bamboo joint designed. This would help with the development of a code for bamboo structures and to find where each joint was best suited. One particular joint could be focused on for further studies. The optimum shape and size for all the elements in this connection could be worked out through a series of similar tests and repeated modification. This connection could be developed to suit different types of construction and for a range of applied loads. The development of the canvas joint to incorporate other connections other than orthogonal ones would allow for a more practical use of the joint to be tested. L-shaped steel bars could be looked at in addition to T-shaped bars. The possibility of combining more than two culms in one joint would also show the versatility of the connection. Other options for further studies might include looking into other uses of bamboo construction, such as bamboo scaffolding, long term bamboo structures, bamboo bridges or bamboo structures under large applied loads. Suitable connections for each purpose could be designed and tested in a similar way to the work done with the canvas connection. 5.3 Final Conclusions The use of bamboo as a serious structural material is hindered by its weaknesses in connections. The use of other materials would bring more standardisation into bamboo joints and result in more reliability, but it would impact on two of the main strengths of bamboo structures - its naturalness and adaptability. It must also be questioned whether the additional strength gained by using unnatural materials justifies the additional expenditure and time required.
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References ARCE, O. 1993. Fundamentals of the Design of Bamboo Structures. PhD Thesis Eindhoven. University of Technology. CHAN S.L. AND XIAN X.J. 2002. Engineering and Mechanical Properties of Structural Bamboo. Research Centre for Advanced Technology in Structural Engineering, Hong Kong. CHAVES. A.C. GUTIERREX, J. 1988. The Costa Rican Bamboo National Project. Proceedings of the international bamboo workshop, Cochin, India, November 14-18. CHUNG, K, CHAN, S, YU, W. 2002. Recent developments on bamboo scaffolding in building construction. Advances in Building Technology. Esevier Science Ltd. Kinglington. pp 629-636. CHUNG, K, CHAN, S, YU, W. 2002. Practical Design of bamboo scaffolds. Proceedings of the international seminar on bamboo scaffolds in building construction. International network for bamboo and rattan and Hong Kong polytechnic University. Hong Kong. 2002 DUFF, C.H 1941. Bamboo and its structural use. Engineering Society of China. Session 1940-41, Institution of Civil Engineers of Shanghai FARRELLY, D., 2003. The Book of Bamboo. San Francisco: Sierra Club Books GRATANI, L, CRESCENTE, M, VARONE, L, FABRINI G, AND DIGIULIO, E, 2008. Growth pattern and photosynthetic activity of different bamboo species growing in the Botanical Garden of Rome, Flora 203: 77-84 GUTIERREZ, J. 1998. The development of non-traditional bamboo technologies in Costa Rica. Bamboo for sustainable development. Proceedings of the Vth International Bamboo Congress and the VIth International Bamboo Workshop, San José, Costa Rica, 2-6 November 1998 HERBERT, M.R.M. and EVANS, P. 1979. The development of structural connections for bamboo. N143/79. Building Research Establishment, Watford UK. HIDALGES, O. 1974. Bambu, su cultivo y aplicaciones, Estudios Tecnicos Colombianos Limitada, Colombia. HONG KONG BUILDING DEPARTMENT. 2002. Guidelines on the Design and Construction of Bamboo Scaffolds. Building Department, Hong Kong 2006 INOUE, M, TANAKA AND K, TAGAWA, Y. 2008 Application of bamboo connector to timber structure – Introduction of construction and dismantlement of Japanese government pavilion Nagakute in Expo 2005 Aichi, Japan. Modern Bamboo Structures. October 2008. ICBS p191-200 JANSSEN, J. 1984. Better Bamboo Trusses, Building Research & Information,12:6, 369-372
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JANSSEN, J. 1995. Building with bamboo, a handbook.2nd Ed. London, UK: Intermediate Technology Publications. JANSSEN, J. 1999. Bamboo Structural Design. International Network for Bamboo and Rattan. Beijing, China JAYANETTI, L. AND FOLLET, P. 1998. Bamboo in construction – An Introduction. TRADA Technology Ltd and INBAR and DFID LAWSON, A. 1968. Bamboo – A Gardener’s Guide to their Cultivation in Temperate Climates. UK: Faber and Faber Limited MORI, T, UMEMURA, K AND NORIMOTO, M. 2008. Manufacture of drift pins and boards made from bamboo fibre for timber structures. Modern Bamboo Structures. October 2008. ICBS p129-138 MORISCO AND MARDJONO,F. 1995. Filled Bamboo Joint Strength. Fifth International Bamboo Workshop. Bali (unpublished) NIENHUYS, S. 1978. Bambu cana guadua. Recommendations para el uso en la construccion. Instituto Ecuatoriano de Normalizacion INEN. RAMANATHAN, M. 2008. Hong Kong – bastion of bamboo scaffolding. Proceedings of the ICE. Civil Engineer 161 November 2008, Pages 177-183 SONTI, V. 1990. A workable solution for preserving round bamboo with Ascu. Bamboos: Current Research. Proc. International Bamboo Workshop. KFRI/IDRC, pp 207-208 SPOER, P. 1982. The use of Bamboo in Space Trusses. M. Sc. Thesis, Technical University of Denmark. STULZ, R. 1983. Appropriate Building Materials. SKAT and Intermediate Technology Publications Ltd. St Gallen, Switzerland. TANAKA, K, SHIRAKAWA AND GUAN, Z. 2008. Reinforcement using bamboo board and road around bolt hole at fastener joint in timber structure. Modern Bamboo Structures. October 2008. ICBS p139-149 VAESSEN, M, AND JANSSEN, J. 1997. Analysis of the critical length of culms of bamboo in four-point bending tests. Heron, vol. 42 no 2, pp 113-124. VELEZ, S. 2000. Grow your own house = Simón Vélez und die bambusarchitektur. Weil am Rhein : Vitra Design Museum ZHANG, D, FEI, B, REN, H AND WANG, Z (2008) The research of joint composed by laminated bamboo lumber. Modern Bamboo Structures. October 2008. ICBS p181-190
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