27-1

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

Fig.2 Engineered Conventional Bamboo Construction often used

27-2

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 18-19 mm (inner size of 16 mm) was used to put through the drilled hole. A 16 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 1Bamboo Bolted Joint in Parallel Direction

Two bamboos were placed overlapped in parallel direction, and connected altogether by 16mm of steel bolt set. Both were gripped by U-shape steel plate, and tightened by 12mm of steel bolt sets as shown in Fig.7. The purpose was to investigate the impact of bolted joint on bamboo connected in parallel dire