Bamboo Connections
1
Student : Chris Davies Supervisor : Pete Walker The intent of this study has been to review a range of existing bamboo connections used across the world and publish reliable data on the strength of these joints. The author has used the knowledge learnt from this study to develop a new connection that would be suitable for use in a quickly constructed temporary bamboo shelter after a natural disaster. 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 properly through. 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 work might not be suitable because of the lack of standardisation in bamboo culms. The tapered nature of bamboo is very important and the jointing techniques need to accommodate a range of diameters. 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 very popular. This brings flexibility into the structure but also needs a high level of skilled labour which is not always available. 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 culm. The culms all ranged within a 70mm-80mm diameter. THE INITIAL CONNECTIONS BAMBOO TENON AND DOWELL The following connection is a traditional joint using only bamboo and used in a situation where bamboo is the only the material available. The connection can be made with green or dry bamboo culms but using dry seasoned culms provides a stronger connection. An alternative option would be to use a hardwood for the tenon and dowel which would make the connection simpler to construct as no curves would need to be drilled. The components of the bamboo tenon and dowel and the completed connection can be seen in the images below. PLYWOOD GUSSET PLATE The use of a gusset plate in bamboo connections has been tested by others using a range of materials. The shape of the gusset plate was designed to be able to work with a two culm orthogonal joint. This connection was made using two 9mm plywood plates and 10mm steel bolts. Plywood was chosen because of its uniform strength and workability. The individual parts of the connection and the assembled joint can be seen in the images below. FURTHER WORK ON ALTERNATIVE CONNECTION 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 has 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. The dimensions and design of the first version of the joint can be seen in the images on the right. 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. IMPROVEMENTS TO FIRST CANVAS JOINT 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 did not grip well to the bottom bamboo culm causing it to slip. The changes made for the second version of the connection were: • Steel T bars placed on both sides of the connection • Only one leg of the T bar was sewn in place • Each leg of the connection was shortened in length • Overlapping strip C increased in length to wrap around entire connection and to the opposite side of the top culm. • Less hook material used to allow for each strip to be tightened more The design of the second version can be seen on the right and images of the connection being tied can be seen below. CONCLUSIONS FROM CANVAS CONNECTION The failure of the joint in bending occurred through the ripping of the canvas as the steel T rotated. A graph showing the bending moment capacity of the modified canvas connection can be seen below as well as an image of the final failure mode. The improved joint could be modified further by increasing the length of strips B and D to the same length as strip C to allow for more loops of the canvas to be wrapped around the culm. The steel T bar should be sewn in place to try to stop the excessive rotation of the top culm and protect against corrosion. 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. TESTING APPARATUS The bamboo joints are held in place through specially welded steel circular sections which are clamped to the floor through 30mm bolts. Displacement transducers placed at heights of 180mm and 40mm above the top culm recorded the displacement and rotation. The test rig seen in the images on the left 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. The load cell acts at a height of 180mm in the bending tests and a height of 40mm in the shear tests. A steel section was placed between the load cell and the top culm in the shear tests to increase the amount of pure shear the base of the connection was subject to. The images above show the set up for the tension tests. A tensile force was applied to the top culm using a welded square steel section with a part cut out to insert the joint. THE RESULTS CONCRETE FILLED The connection uses a steel eye bolt and steel dowel to initially secure the joint and then the internodes are filled with concrete. This connection was adapted from previous work done on concrete filled connections into an ortogonal connection. 10mm steel was used for the bolts and dowels and the concrete is mixed to have a compression strength of 45-50N/mm 2 . In the images below the connection can be seen before it is screwed together and filled with concrete, also shown is the completed concrete filled joint. STEEL TUBE This joint is based upon a design for use with green bamboo but adapted to be able to be used with dry seasoned culms. The steel tubing is wrapped in a canvas type material to fit the internal diameter of the culm. The adaptation also allows for a larger range of diameter culms to be used and ensures a tighter fit between the culm and tubing. 48mm diameter tubing was used with a thickness of 3mm and each leg of the T-joint extends 80mm. The completed steel tube connection and the different stages of construction can be seen in the images below. The results are taken from testing each type of connection three times and observing the failure pattern. 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 tube at the start of testing. The strength of each connection can be seen from the graphs shown below. To the right of each graph an image of a typical failure is shown. No tension test was performed for the steel tube as it is assumed to have no tensile strength. Bamboo Tenon and Dowel Plywood Gusset Plate Mortar Filled Wrapped Steel Tube Moment Strength 0.13 kNm 0.35 kNm 1.26 kNm 0.22 kNm Shear Strength 2.5 kN 7.5 kN 6.2 kN 1.2 kN Tensile Strength 3.3 kN 5.6 kN 11.0 kN 0 kN Retain capacity? No Yes Yes No Weight (kg) 0.46 1.1 2.2 0.65 Time to Construct (mins) 50 40 25 90 Difficulty of Construction Med/Hard Easy/Med Hard Medium/Hard Aesthetics (0-6) 4 5 1 4 Other materials used None Plywood, Steel Bolts Mortar, Steel Bolts Steel, Canvas material The tenon tended to be the initial weakness in the joint and cause the failure of the connection. This could be strengthened by providing a node around the base of the tenon and increasing its length into the top culm. The joint was 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. There should be suitable tolerances in the design to allow for loose connections. This connection should be tested with the tenon perpendicular to its current position to see how it might be suited to loading in a different direction. Overall this connection would be very useful in an environment where only bamboo is available and there is an experienced labour force to construct the connection. 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 and the connection also performed well in shear and tension. An improvement to the joint would be to make the gusset plate shorter and wider and to provide three bolts through the bottom culm. The bolts of the culm should be staggered to maximise the bearing area. 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 or failing joint to provide extra strength and stiffness. 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. 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. Using a complete loop at the end on the eye bolt and extending this bolt further into the top culm would improve the tensile strength of the connection. This connection could be used in a trusses or for strenghtening joints under high stresses. The steel tube joint performed well in the bending tests and the bamboo culms were always the first to break. 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 could be increased. The connection has a lack of tensile strength but the addition of a dowel through the steel tube and bamboo culm will bring an element of tensile strength to the joint. 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. SUMMARY OF RESULTS FROM CONNECTIONS
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Analysis and testing of existing bamboo connection types and development of a new one.
Transcript of Bamboo Connections
Student : Chris Davies Supervisor : Pete WalkerThe intent of this study has been to review a range of existing bamboo connections used across the world and publish reliable data on the strength of these joints. The author has used the knowledge learnt from this study to develop a new connection that would be suitable for use in a quickly constructed temporary bamboo shelter after a natural disaster.
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 properly through.
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 work might not be suitable because of the lack of standardisation in bamboo culms. The tapered nature of bamboo is very important and the jointing techniques need to accommodate a range of diameters.
Standardised joints will not fi t in every situation unless a certain amount of fl exibility is introduced. This is why the traditional method of bamboo connections being cut and constructed on site to fi t is still very popular. This brings fl exibility into the structure but also needs a high level of skilled labour which is not always available.
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 fi lled joint and a steel tube joint. The tested samples were all made into orthogonal joints that connected together two pieces of Moso bamboo culm. The culms all ranged within a 70mm-80mm diameter.
THE INITIAL CONNECTIONSBAMBOO TENON AND DOWELL
The following connection is a traditional joint using only bamboo and used in a situation where bamboo is the only the material available. The connection can be made with green or dry bamboo culms but using dry seasoned culms provides a stronger connection. An alternative option would be to use a hardwood for the tenon and dowel which would make the connection simpler to construct as no curves would need to be drilled.The components of the bamboo tenon and dowel and the completed connection can be seen in the images below.
PLYWOOD GUSSET PLATEThe use of a gusset plate in bamboo connections has been tested by others using a range of materials. The shape of the gusset plate was designed to be able to work with a two culm orthogonal joint. This connection was made using two 9mm plywood plates and 10mm steel bolts. Plywood was chosen because of its uniform strength and workability.The individual parts of the connection and the assembled joint can be seen in the images below.
FURTHER WORK ON ALTERNATIVE CONNECTIONDevelopment 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 atthe time of need• It can be used on a small short term shelterwhere loads are small
The connection has be designed to be fl at 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 fi t 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.
The dimensions and design of the fi rst version of the joint can be seen in the images on the right. 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.
IMPROVEMENTS TO FIRST CANVAS JOINT
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 did not grip well to the bottom bamboo culm causing it to slip. The changes made for the second version of the connection were:
• Steel T bars placed on both sides of the connection• Only one leg of the T bar was sewn in place• Each leg of the connection was shortened in length• Overlapping strip C increased in length to wrap around entireconnection and to the opposite side of the top culm.• Less hook material used to allow for each strip to be tightenedmore
The design of the second version can be seen on the right and images of the connection being tied can be seen below.
CONCLUSIONS FROM CANVAS CONNECTIONThe failure of the joint in bending occurred through the ripping of the canvas as the steel T rotated. A graph showing the bending moment capacity of the modifi ed canvas connection can be seen below as well as an image of the fi nal failure mode.
The improved joint could be modifi ed further by increasing the length of strips B and D to the same length as strip C to allow for more loops of the canvas to be wrapped around the culm. The steel T bar should be sewn in place to try to stop the excessive rotation of the top culm and protect against corrosion.
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 fi nd 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.
TESTING APPARATUS
The bamboo joints are held in place through specially welded steel circular sections which are clamped to the fl oor through 30mm bolts. Displacement transducers placed at heights of 180mm and 40mm above the top culm recorded the displacement and rotation.The test rig seen in the images on the left 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. The load cell acts at a height of 180mm in the bending tests and a height of 40mm in the shear tests. A steel section was placed between the load cell and the top culm in the shear tests to increase the amount of pure shear the base of the connection was subject to.The images above show the set up for the tension tests. A tensile force was applied to the top culm using a welded square steel section with a part cut out to insert the joint.
THE RESULTS
CONCRETE FILLEDThe connection uses a steel eye bolt and steel dowel to initially secure the joint and then the internodes are fi lled with concrete. This connection was adapted from previous work done on concrete fi lled connections into an ortogonal connection. 10mm steel was used for the bolts and dowels and the concrete is mixed to have a compression strength of 45-50N/mm2.In the images below the connection can be seen before it is screwed together and fi lled with concrete, also shown is the completed concrete fi lled joint.
STEEL TUBEThis joint is based upon a design for use with green bamboo but adapted to be able to be used with dry seasoned culms. The steel tubing is wrapped in a canvas type material to fi t the internal diameter of the culm. The adaptation also allows for a larger range of diameter culms to be used and ensures a tighter fi t between the culm and tubing. 48mm diameter tubing was used with a thickness of 3mm and each leg of the T-joint extends 80mm.The completed steel tube connection and the different stages of construction can be seen in the images below.
The results are taken from testing each type of connection three times and observing the failure pattern. 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 defl ection 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 tube at the start of testing. The strength of each connection can be seen from the graphs shown below. To the right of each graph an image of a typical failure is shown. No tension test was performed for the steel tube as it is assumed to have no tensile strength.
Bamboo Tenon and Dowel
Plywood Gusset Plate
Mortar Filled Wrapped Steel Tube
Moment Strength
0.13 kNm 0.35 kNm 1.26 kNm 0.22 kNm
Shear Strength 2.5 kN 7.5 kN 6.2 kN 1.2 kNTensile Strength 3.3 kN 5.6 kN 11.0 kN 0 kNRetain capacity? No Yes Yes No
Weight (kg) 0.46 1.1 2.2 0.65Time to
Construct (mins) 50 40 25 90
Diffi culty of Construction Med/Hard Easy/Med Hard Medium/Hard
Aesthetics (0-6) 4 5 1 4Other materials
usedNone
Plywood, Steel Bolts
Mortar, Steel Bolts
Steel, Canvas material
The tenon tended to be the initial weakness in the joint and cause the failure of the connection. This could be strengthened by providing a node around the base of the tenon and increasing its length into the top culm. The joint was 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 fi t tightly together. There should be suitable tolerances in the design to allow for loose connections.This connection should be tested with the tenon perpendicular to its current position to see how it might be suited to loading in a different direction. Overall this connection would be very useful in an environment where only bamboo is available and there is an experienced labour force to construct the connection.
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 and the connection also performed well in shear and tension.An improvement to the joint would be to make the gusset plate shorter and wider and to provide three bolts through the bottom culm. The bolts of the culm should be staggered to maximise the bearing area.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 or failing joint to provide extra strength and stiffness.
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.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. Using a complete loop at the end on the eye bolt and extending this bolt further into the top culm would improve the tensile strength of the connection.This connection could be used in a trusses or for strenghtening joints under high stresses.
The steel tube joint performed well in the bending tests and the bamboo culms were always the fi rst to break. 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 could be increased.The connection has a lack of tensile strength but the addition of a dowel through the steel tube and bamboo culm will bring an element of tensile strength to the joint.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.
SUMMARY OF RESULTS FROM CONNECTIONS
