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The Study of Design and Structural Potential of Bamboo Practical Joints and Frame Truss System
Pilot Project of ‘Kashiihama House for All’
PHANRATANAMALA Susira
1. INTRODUCTION
1.1 Background
The use of bamboo in architecture has recently regained new value because of sustainability. Bamboo, as a natural, versatile, and renewable, is generally regarded as sustainable material. There is potential that bamboo can be used more extensively in all parts of a building, as architectural and structural elements.
One of the most critical issues of bamboo being used in a building structural system is durability. Currently there is no building standards or regulations regarding its performance and maintenance. Bamboo is relatively vulnerable to insect and fungus attack. More importantly, it has a tendency to crack easily around the joint/connection parts. In most cases, bamboo may be more suitable for temporary structure, rather than permanent structure. In term of bamboo structural design, the joint systems are the most significant aspects, as they have a direct impact on building appearance and construction techniques. In fact, the weaknesses of bamboo structure are their joint systems and their inherent insufficient material strength. In other words, if the joint system does not work properly, the overall structure would have to be demolished earlier than its expected lifespan. ‘Kashiihama House for All Project’ is a pilot project in Kyushu University, in which a temporary architecture is constructed for the purpose of investigating the Mousou bamboo engineered joints and frame truss system. To set up the project ‘Kashiihama House for All’, the researchers faced many specific problems that were similar to the real constructions: such as budget, construction period, self-construction, detachable structure, laws & regulations, limited area and transportation. The research project intended to address these problems, especially the safety concern.
The research investigated the bamboo architectural design and construction possibilities, by utilizing the inherent structural property of Mousou bamboo, application of effective practical connections, and application of frame truss system. The selected joints came from fundamental technologies commonly used in modern bamboo architecture. ‘Bolted Joint’ and ‘Mortar-Injection’ can be used along with bolted joint, to fix a bolt inside bamboo, preventing it from crushing against perpendicular force. The advantages from using both of these joints include convenience, effectiveness, workability, etc. If bolted joint is used along with mortar injection, the bamboo structure would become permanently fixed and undetachable. In this case, the requirement of the ‘Kashiihama House for All’ or ‘KHFA’ project program would not be met, and transportation would become an issue. The commonly-used post-tension construction connection, ‘Sheath Steel Tube’, would be inserted through the drilled hole to prevent mortar concrete from solidifying the joint.
1.2 Objectives
There are four objectives of this research. 1) A bamboo structural and joint system was investigated. 2) The overall structural design was improved through material basic strength experiment, joints and frame experiments. 3) Potential light-weighted detachable building components were examined. 4) A pilot project ‘Kashiihama House for All’ was constructed with all the construction term issues resolved.
1.3 Research Methodologies
Initially, this research addressed the construction issues in
‘Kashiihama House for All’ project. Because of its durable
strength and low cost, ‘Mousou’ bamboo (Phyllostachys pubescene) was used as the structural material. All bamboo used in this research were harvested at the Ito Campus, Kyushu University. To complement the inherent properties of bamboo, a specific structural system for bamboo was developed, during the design solution and development stage, followed by the experiments and construction. The overall research methodology is shown in Fig.1.
2. Bamboo Joint Connections
2.1 General Information
Traditional bamboo joinery techniques, using ropes or ties with buckets, can be simply constructed by common people, but it is inadequate from structural point of view. It also reduces joint strength capability between bamboo culms. According to Widyowijatnoko (2012), modern building construction is categorized based on joinery techniques. Bolted joints with or without concrete-mortar infill are categorized as ‘Engineered Conventional Bamboo Constructions’. They are based on the bamboo characteristics by employing modern tool electric drill machines, and they are commonly applied in modern architecture and practical for self-construct. They offer many advantages, depending on the number of joint points filled by mortar. However, the structure would become relatively heavy for overall building and material itself. Such construction also increases the on-site construction work, to fill all the joints with mortar and put the bolt. Consequently, in the ‘Kashiihama House
for All’ project, the mortar techniques were only applied
selectively at certain joint points, to make a sufficiently effective rigid structure.
2.2 Joint Connections and Structural Systems Consideration
Construction process and safety were significant for ‘KHFA’ project because it was self-constructed. Various bamboo effective joints were used but some of them required specific tools and skills of expert craftsmen. Therefore, the design and construction process was aimed to be simple and effective. Moreover, transforming bamboo shape for connections was not recommended for this project, as it usually required specific tools and craftsmanship. As a result, the most practical, effective, appropriate for amateur builders was the bamboo joinery system, which perform full of bamboo culm for construction.
Fig.1 Project Processes for Research Framework
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3. Bamboo Structural Frame System Proposal
3.1 Frame Truss System Proposal for KHFA
The structural design principle for bamboo is to use it for tension and compression. Nevertheless, from joints concern and project limitations, the structural system was limited to the two-dimensional frame behavior. Thus, achieving with a triangular pattern or trusses system would provide stability by making the entire structure more rigid. Further, it would keep the bamboo pole from buckling, bending and shear.
3.2 Weakest Joint Points at KHFA Structural Frame Truss
The weakest joint points are illustrated in Fig.3. It is necessary to fill concrete-mortar at these points, and joint type would be tested.
4. Experiments
The objective is to study the strength of specific bamboo joints and structural behaviors in ‘Kashiihama House for All’ project. The experiments were divided in 3 sections: 4.1)Bamboo Material Strength 4.2)Joints and 4.3)Full-scale prototype frame.
4.1 Bamboo Material: Experiments
To understand the basic mechanical properties of bamboo and the failure characteristics when the applied force is larger than what it can endure.
4.1.1 Compression Test (Parallel to the Fibers)
The bamboo specimens were cut in 40 mm length All test pieces were tested by 50 kN loading machine.
4.1.2 Tension Test (Parallel to the Fibers)
The bamboo specimens were cut in the “dog bone-shape”. All test pieces were tested by 10 kN loading machine.
4.1.3 Material Basic Strength Tests Results
The compression test of bamboo showed that its average compression strength was 80 N/mm2, and tensile strength was 180-200 N/mm2. The results revealed that bamboo had an
inherent high tensile strength in parallel direction, in comparison to compression. However, the results from each specimen were varied even if they were taken from the same bamboo piece. In fact, the mechanical property of bamboo varies according to its age, species, location, or the specific portion of bamboo used.
4.2 Bamboo Joint System: Experiments
The significant point in bolted joint connecting bamboo was the tensile force from bolt, applied towards the end of the bamboo joint, resulting in shear. (Janssen, 2000). Thus, to examine the possibility of the construction joints, joint tests were conducted to investigate the critical load impact, durability of bamboo, and bolted joint (with/without concrete-mortar). A Sheath Steel Tube of ø18-19 mm (inner size of ø16 mm) was used to put through the drilled hole. A ø16 mm Steel Bolt(M16) was used to connect bamboo altogether. The purpose is to enable the joint to be detachable from the concrete-mortar infill.
Concrete-mortar was infilled into the bamboo after sheath steel tube was inserted. Plugging plasticine clay was then applied around the drilled holes to prevent mortar leakage.
The experiments were divided in 2 groups; Joint in Parallel and Perpendicular Direction, separated in two types: ‘Joint With Nothing Infill Bamboo(NFB)’ and ‘Joint With Concrete-mortar Infill Bamboo(CFB)’ for tolerance comparison. The results were recorded from testing each type of connection at least 3 times. This research aimed to obtain allowable strength possibility from experiments instead of finding maximum load. Since bamboo is raw material, which is elastic-like, the tolerance level of displacement was not supposed be excess than 15 mm. This number is acceptable for testing bamboo as structural material.
4.2.1 Group 1–Bamboo Bolted Joint in Parallel Direction
Two bamboos were placed overlapped in parallel direction, and connected altogether by ø16mm of steel bolt set. Both were gripped by U-shape steel plate, and tightened by ø12mm of steel bolt sets as shown in Fig.7. The purpose was to investigate the impact of bolted joint on bamboo connected in parallel direction.
1. Drilling ø19mm of the hole on bamboo. 2. Removing the diaphragm inside the bamboo. And inserting the ø18-19mm of the sheath steel tube through the drilled hole. 3. Pasting plasticine clay around the drilled hole. And putting concrete-mortar into the bamboo. 4. After concrete-mortar setting, removing plasticine clay, and inserting ø16mm steel bolt through sheath steel tube, assembling members, and tightening with steel washers and nuts.
Fig.6 Concrete-mortar infill the bamboo process
1 2 3 4
Fig.7 Group 1 – Bamboo joint in parallel direction.
Fig.5 Tensile Stress Loading Effects and Setup Material Tests
Before After
Fig.4 Compressive Stress Loading Effects and Setup Material Tests
Before After
Fig.3 Frame Proposal and Critical Weakness Point Probability Neutral Point
Not Require CFB but Requiring Joint Test
Weakest Point Requiring CFB and Joint Test Weak Point Requiring CFB and Joint Test
27-3 The experiments demonstrated that all typical failure in NFB and CFB were similar, mainly at the bamboo itself. Failure of NFB occurred earlier than that of CFB. Fig.9 showed the longitudinal crack from the joint point to the end of culm. The crack was particularly noticeable since bamboos moved vertically from the joint point. Washers were then slipped out, and drilled hole could be seen. There was no damage on bolt and sheath steel tube unless it was angular displaced. Furthermore, the concrete-mortar in CFB case showed different kind of results. Some of them did not have any damage, some were lightly cracked, and some were broken in 2 pieces but it did not slip out.
The average of tolerance strength level of this joint group could be assumed that 1.66 kN for NFB, and 3.70 kN for CFB.
4.2.2 Group 2–Bamboo Bolted Joint in Perpendicular Direction
In this experiment, 3 bamboos were connected altogether. One was orientated in vertical direction between another two bamboos, which were orientated in horizontal direction. The purpose was to investigate the impact of bolted joint on bamboo in perpendicular direction.
The post-failures joint in NFB occurred on both of horizontal bamboos, which were cracked in parallel. The friction from washers caused indented trace on bamboo. Meanwhile, the CFB case initially occurred on bolt. It was severely curved and bended upward but was not torn apart. The second damage occurred on vertical bamboo around the hole of bolted joint. The result was a longitudinal crack on bamboo, but only few damages occurred on the horizontal bamboos except for slight longitudinal crack, and nothing happened on perpendicular to grain. The results of concrete-mortar inside bamboos were the same as Group 1. Sheath steel tube had less damage than expected.
The average of tolerance strength level of this joint group could be assumed that 0.715 kN for NFB, and 2.42 kN for CFB.
4.2.3 Results
The experiment demonstrated that using bamboo bolted joint with concrete-mortar infill was very effective. Such joint could make the bolts to endure the load capacity, and made bamboo more rigid, and acted as bonding to tightly attached bamboo and fastener altogether. However, bolted joint in Group 2 was weaker than Group 1. Steel bolt could not endure the compression load. Although, sheath steel tube was not effective on strength, it could be the alternative method to apply for detachable system.
4.3 Bamboo Structural Frame: Experiments
This experiment conducted the pulling test in Vertical and Lateral Direction. The purpose was to analyze the potential strength, and impact of joint on overall bamboo frame for building structures. The experiment also investigated different structural behavior when each load was activated on the frame.
4.3.1 Experimental Setup
The prototype frame was constructed with concrete-mortar infill at the selective joint points. The frame was laid down on ground, and its bottom part was fastened by steel pipes. The load was applied by the use of a manual winch anchored to an external reaction frame. The maximum load could increase up to 350 - 400 kg. The strength has to meet the load to be carried with sufficient safety and the displacements were recorded.
4.3.2 Vertical Load Test Performance and Investigation
Fig.10 Group 2 – Bamboo joint in perpendicular direction.
Fig.11 Experiment set up for joint in Group 2 – Compression Load Test The compression load was applied on the loading machine, which was the same as that in Group 1. Putting the load on the vertical bamboo. Both of horizontal bamboos were placed on blocks. Two displacement transducers were placed under the movable plate. Fig.8 Experiment set up for joint in Group 1 – Tensile Load Test
The Tensile load was applied on the loading machine from both sides, upper and lower head. The machine was able to produce the maximum load of 200 kN. 2 displacement transducers were placedat the steel strap, which were attached on the U-shape steel plate, to record the displacement and rotation of specimen. The specimen was lopsidedly set up on load machine due to the machine limitation, but it did not influence the strength.
CFB
Fig.9 Comparison of Typical Failures of Group 1 Joint – NFB and CFB Type
CFB
NFB
Fig.12 Comparison of Typical Failures of Group 1 Joint – NFB and CFB Type NFB Ave. 3.70 kN Ave. 1.66 kN Ave. 2.42 kN Ave. 0.715 kN
27-4 To investigate the dead load effect from the rooftop, the vertical load was applied as shown in Fig.13. However, this project was decided to use fabric membrane for covering the whole structure, thus the load application number was considered to be acceptable and adequate for such light-weight structure. Most importantly, the post-failure did not occur on the prototype frame.
4.3.3 Lateral Load Test Performance and Investigation
To investigate the lateral force such as wind load resistance, the lateral load was applied as shown in Fig.14. The load application number for one frame to be acceptable and adequate for this structure was considered at 1.1 kN (100 kg).
The slight parallel crack occurred around the holes with long bolted joint. The bolt length might cause effect on bamboo but they were not considered to be severe problems.
4.3.4 Results and Conclusions of Frame Experiment
The frame prototype behavior was relatively rigid at the joint, especially infill with concrete-mortar as expected. There is no structural failure occurred after finishing the frame tests process.
4.4 Conclusion
The results from material basic strength test were still preliminary, and could not clearly identify the exact strength of bamboo material itself. However, the results from specific joints and frame experiments showed the potential of bamboo as structural material. It could confirm that this structural system was effectively rigid and firmly safe for temporary building.
5. The Processes of KHFA Project 5.1 Pre-Construction
5.2 On-Site Construction
All consist of 8 bamboo culms/frame connected by bolted joint with and without CFB system. All included was 9 frames for whole building. The building could be separated in 3 units, which comprised 3 frames/unit, connect each unit by bamboo beam.
6. Conclusions and Suggestions
Using bamboo as structural material for KHFA project by utilizing bolted with/without concrete mortar infill provide strength rigidity. Meanwhile sheath steel tube can improve detachability. This system does not require for shape deformation of bamboo that can provide workability for unskilled labors. Further, the frame truss system is certainly appropriate for bamboo as structural material since it effectively prevent bamboo from bending and buckling in the middle of the culm. Nevertheless, this system has not been improved in term of force resistance in perpendicular direction to the frame (such as wind blowing against the end of the building). The two dimensional frame trusses only can take force in their own plane. The solutions possibility basically is adding extra bracing between trusses. Or it could be: 1) Performing rigid component at the end of building. 2) Applying with 3 dimensional truss system. References
1. WIDYOWIJATNOKO, Andry. Traditional and Innovative Joints in Bamboo Construction. The faculty of Architecture, RWTH Aachen University. Dissertation 2012
2. KASSA Z.,Bewketu. Bamboo: An Alternative Building Material for Urban Ethiopia. The faculty of Architecture, California Polytechnic State University, 2009
3. SHARMA,Bhavna. Seismic Performance of Bamboo Structures. Faculty of Engineering, University of Pittsberg, 2010 4. SHARMA,Bhavna/ HARRIES A., Kent/ GHAVAMI, Khosrow. Work in Progress – Pushover Test of Bamboo Portal Frame Structure. University of Pittsberg. ASEE North Central Sectional Conference, March 26-27, 2010 5.KOBAYASHI, Hirohide. Design and Construction Process of Trial Bamboo Greenhouse. Graduate School of Global Environmental Studies, Kyoto University. Japan Bamboo Society, Bamboo Journal No.26. March 26, 2009. 6. DAVIES Chris. Bamboo Connections. Department of Architecture and Civil Engineering, The University of Bath,2008 7. JANSSEN, Jules J.A. Building with Bamboo: A Handbook. Practical Action Publishing, UK, 1995
Vertical Test Setup
The vertical load application was designated to be stopped at 1725.97 N (~176 kg), at the displacement 21.8 mm.
The lateral load application was applied and stopped because of out of load at 3189.96 N (~325 kg) at the displacement 164.8 mm. The damages were found as (1) and (2)
Fig.13 Vertical Test Setup and Load Application
Fig.14 Lateral Test Setup and Load Application
(2)
(1) Fig.16 On-Site Construction Processes
Lateral Test Setup
Damage Points (1) (2) Load: 3189.96 N at 164.80 mm Load: 1725.97 N at 21.80 mm
Fig.15 Pre-Construction Processes
(1) Frame Assembling - To position drilled holes.
(2) Frame Disassembling – To prepare concrete-mortar infill process. Drilling
ø5mm of the hole on bamboo to infill mortar.
(3) Transporting – After all concrete-mortar was setup itself. They were all ready
to be carried to the site.
(1) Frame Assembling and Setup – Put the frames on bricks. Setup in parallel. (2) Floor Installation – Two plane of wooden floor were set on bamboo frame. (3) Wall Installation – Brick dry process system was used for wall component. (4) Covering Installation – Installing bamboo lintels for supporting fabric
