Sustainable Built Environment in Tropical Hemisphere Countries
2. Development of bamboo connection system
High strength bamboo material cannot be fully utilized due to the constraints of the connection system. Splicing or coupling the whole members is usually done conventionally by using rope, nails and pins. Connection with nails or pins cause tears to the member where the fibers are aligned so that the low shear strength of bamboo easily exceeded. The connection with a rope relies on the strength of the friction of the rope, rope with bamboo or bamboo with bamboo.
Connection with a rope on bamboo truss structure is presented in Fig. 1. Restraint rope shown in Fig. 1 influenced to the strength the connection. Due to the changes in temperature, bamboo could shrink and cause the rope slack. This is the problem of the conventional bamboo connection that generally produces very low strength. In addition, the formula for calculating the strength of the rope connections was difficult to be formulated. Due to geometric reasons, bamboo construction often requires extension to prolong the member and connection for joining some members in a gusset or joint. Bamboo new connection systems that had been developed by previous researchers and demonstrated better strength connections are described as follows:
1. Connection with bolts, steel gusset plates and cement mortar or wood filling
Fig.1 Connection with (a) ijuk rope and; (b) Rattan rope in bamboo truss structure (Source: Fieldwork 2011)
Research to improve the strength of the connection on a bamboo truss structure has been done by Morisco et al4. Species of bamboo used in this study was Gigantochloa atroviolacea. The method used bolts, steel gusset plates, and cement mortar or wood filling as shown in Fig. 2. Bamboo truss structure was made to demonstrate the joint strength as shown in Fig. 2. The loading was applied using a concrete block with a total weight of 40 kN. This jointing method has been made on the structure of the bamboo bridge with span length of 12 m using Gigantochloa atroviolacea about 7 cm in diameter.
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Fig.2 (a) Connection with steel gusset plate4; (b) Experimental set up of testing truss structure4
The results of test show that strengths of the jointing method under investigation is quite high, and an estimated strength of the connection can be formulated4. Nevertheless, despite the high strength resulted from this connection, the use of steel for gusset plates and relatively heavy infill material has made this connection system is less desirable because of the significant increase of structure weight and construction costs that make it uneconomical.
2. Connection with PVC joint
Research to maintain the lightweight nature of a bamboo connection with PVC material has been done by Albermani et al5. Species of bamboo used in the study were Phyllostachy Bambusoides and Phyllostachy Pubescens.
The culms used were from 3-6 years old, with outer diameters ranging from 50-65 mm. The joint design was based on preserving the good tensile and compression strengths of bamboo culms without weakening them through cleavage or splitting Fig. 4 shows a prototype of the joint system.
Fig. 4 (a) Basic components PVC joint system for bamboo5; (b) assembled joint5
The joint hub itself is composed of two identical parts that are connected together by a 20 mm diameter bolt. The ends of the bamboo culms are encased inside the cylindrical connector with a mega epoxy grouting material. Three tests were conducted on the PVC material which gave tensile yield strength of 45 MPa and elasticity modulus of 3000 MPa.
The testing of the PVC joint was conducted under compression, tension and bending. The PVC component failed under 24 kN load in compression, 9 kN load in tension and 3 kN load in bending. Bamboo double layer grid (DLG) was made to demonstrate joint strength as shown in Fig. 5. The module has been made on DLG with 2.6 x 2.6 m in plane and 0.9 m deep. The loading was applied using a timber pallet loaded with concrete mix bags with a total weight of 10 kN.
The highest compression was close to 3.5 kN in web member and the highest tension force was close to 1.8 kN in bottom layer. Disadvantages of using this connection method is that natural bamboo cross section that are not symmetrical complicate the process of installation, construction prices become more expensive and can only be used for medium span bamboo structures.
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Fig. 5. Experimental setup of DLG6 3. Connection with nails, plywood and hard wooden planks
Research to improve the strength of the connection on a bamboo truss structure has been carried out by Mishra6. Species of bamboo used in the truss structure was Bambusa tulda with a diameter of not less than 8 cm, and thickness of approximately 9 mm. Strong joints are made by placing 25 mm thick pieces of hard wooden planks or 12 mm thick structural plywood shaped according to the configuration of the joint, on both faces of the joint. The joint is composed of hard wooden planks that are connected together by a 3.5 mm diameter nails as shown in Fig. 6.
Fig. 6 Connectionsystemonthe trussstructure7
There are weaknesses in the system connections using wooden gusset plate and nails that if the bamboos diameter being connected are not uniform, it will cause gap between plate and bamboo that weakened connection. In addition, the use of grafting nails proposed by Mishra6 has no formula for calculating connection strength.
4. Connection with nails, plywood and hard wooden planks
Research to improve the strength of the connection on a bamboo truss structure with wooden board gusset plate been done by Gunawan7. Species of bamboo used in the truss structure was Dendrocolamus asper. The bamboo used in this study was made in the form of blade. Resin consists of two components, namely adhesive and hardener catalyst that should be mixed before use with a ratio set by the resin manufacturer. The resin used in this study was Ponal epoxy.
Bamboo truss structure was made to demonstrate the joint strength with 6 m in span. The loading was applied using a concrete block with a total weight of 14 kN. The disadvantages of using this connection system are thick and the diameter of the bamboo which varied causes trouble to get constant look. While the curved surface of the bamboo which varied causes difficulty in providing wood filler to match with bamboo blade. Another weakness of the system is weathering since the greater opening the greater contact area exposed to the environment.
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Page | 82 3. Research method
The purpose of this study was to develop a bamboo connection system that possesses lightweight nature but higher strength and lower cost while keeping the form of the bamboo being connected remains natural. The proposed connection system consists of bolts, wooden gusset plates and special wooden clamps that have been adjusted with the shape and dimension of the bamboos being connected. The wooden clamps were placed between the bamboo and wooden gusset plates and tightened by the bolts as shown in Fig. 8. The wooden plates were Dipterocarpaceae which has sufficient strength. This effort was done because the wooden gusset plate is much lighter and the price is much cheaper than that of steel. Meanwhile, the wooden clamps provide contact area that capable of mobilizing its friction capacity to transfer the applied load for stronger and reliable connection. The special wooden clamps can also make connection among various diameters of bamboo at the joint easier to construct.
The study was conducted experimentally in two phases of testing. In the early stage of the research preliminary testing on physical and mechanical properties of the materials used have been conducted. The second phase of the research involved designing and fabricating several types of connections with full scale sizes and tested experimentally in the laboratory. Comparison with other available connection systems then could be done to show the superior of the proposed system. Flow chart of the research steps is presented in Fig. 7.
Fig.7. Flow chart of the implementation of research
Design and manufacture of test specimen connection system Preliminary testing of the basic properties of materials
Analysis of data specimen connection system
Making preliminary test specimen
Bamboo Wood board Wood
Preparation of materials
Bolt
The behaviour of a review and discussion of the connection system
Finish Start
Analysis of data basic material properties
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Page | 83 4. Material
As stated previously, the proposed connection system consists of wooden gusset plates, bolts and wooden clamps as shown in Fig. 8. The type of bamboo used as the main structural material in this study was Gigantochloa atroviolacea. The bamboo was taken from Purwodadi area in the province of Central Java. The average diameters of the bamboos used were 75 mm, while the age of the bamboo varies form 3 to 5 years. The gusset plates ware made of Keruing wood (Dipterocarpaceae), while Mahoni wood (Swietenia Macrophylla Kings) was used for the clamps. Screw type of bolts with a diameter of 12.2 mm was used in this connection.
Fig.8. (a) Connectionsystemonbamboo trussstructure; (b) Basic components of joint 5. Specimen preparations
The specimens of physical and mechanical properties such as (moisture content, density, compressive strength, shear strength, tensile strength and elastic modulus) were prepared based on ISO N22157-1 2004 standard for bamboo and ASTM D 143-94 for wood. Bolt specimens were made according to ASTM standard F1585-03-2008. Based on the results of preliminary material testing, the connection specimens were designed and fabricated accordingly. Tastings were conducted to study the influence of wooden clamp shape on the strength of the connection system. The details of connection system for joint with three variations of clamp developed by authors were shown in Fig. 9. The figure depicts three variations of wooden clamps which are distinguished by their ring angles, i.e. α = 600, 900, and 1200, respectively. The difference of moisture content in the test specimens should be minimized because it can affect distribution of test data.
Fig. 9 (a) Clamp variation in the form of angles (α); (b) Clamp angle 60o; (c) Clamp angle 90o; (d) Clamp angle 120o
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Page | 84 6. Test set-up
The testing method of basic material properties was based on ISO N22157-2 for bamboo and ASTM D 143-94 for wood. The testing method of bearing strength of bamboo was adopted from ASTM D 5764 standard test method for evaluating dowel bearing strength of wood and wood based products. Bearing strength test has been carried out on bamboo and wood with deformed bolt diameter of 12.2 mm. Material were tested under compression and tensile using Universal Testing Machine which was integrated with data logger. The test set-up for tensile test for proposed various connection systems and the configuration of specimens are shown in Fig.10. Tensile load was applied gradually by a tensile testing machine. The relative displacement of joint was measured by two displacement transducers.