Bamboo Structures

Bamboo Structures Ammar Patel

2015

Supervisor: Dr Lee Cunningham

School of Mechanical, Aerospace and Civil Engineering

Ammar Patel

Bamboo Structures

Abstract Bamboo is receiving increased interest as a sustainable construction material due to its high strength-to-weight ratio, flexibility, excellent mechanical properties and speed of growth. Furthermore, the need for carbon-neutral methods of construction is a growing concern with ever growing emphasis being placed on sustainability. This project investigates the applicability of bamboo as a mainstream construction material, primarily in the developing world but also in the developed world alongside steel and timber. Standards have been reviewed and designs looked at. The result is a design of a transitional shelter to be used after a natural disaster.

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Contents Abstract

2

Introduction

5

Chapter 1: Why bamboo?

8

1.1: The Bamboo plant

8

1.2: Use of bamboo in construction

10

1.3: Sustainability

12

Chapter 2: Mechanical Properties

16

2.1: Competitiveness of Bamboo

18

2.2: Microstructure

20

2.3: Flexural Strength

21

2.4: Compressive Strength

21

2.5: Tensile Strength

22

2.6: Shear Strength

23

2.7: Buckling

23

2.8: Moisture Content

24

2.9: Fatigue in bamboo

25

2.10: Failure behaviour

26

Chapter 3: Preservation of bamboo

28

3.1 Preservation

29

3.2: Traditional methods

29

3.3: Chemical methods

30

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Chapter 4: Jointing and Connections

32

Chapter 5: Codes and Standards

37

5.1: ISO (International Standards Organisation)

39

5.2: Indian Standard

41

5.3: The future of standardisation

44

5.4: Timber and bamboo

45

Chapter 6: Ideas and Design

47

6.1: The Transitional Shelter

47

6.2: Calculations

50

6.3: About the design

55

Conclusion Suggestions for further work

57 59

References

60

Closing note on Nepal

63

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Introduction Bamboo is a plant that has been used for centuries for a variety of purposes, ranging from its use as a food source to the making of furniture, fabrics and musical instruments (BambooGrove, 2008) to construction, with the lattermost use being the focus of this report. Bamboo has been used in construction for centuries, with some South American structures – where bamboo grows in abundance – dating back to thousands of years. Given the fact that bamboo has been proven to possess remarkable qualities pertaining to structural performance, such as its high elasticity and strength-to-weight ratio, it may come as a surprise that modern day architects and engineers remain reluctant to work with it. This is largely owing to the lack of standardisation and codification existing for bamboo, given that most designers have used traditional methods and relied on the natural insight they gained into its behaviour by their prolonged exposure to it. This does not, however, mean that information on this natural construction material is scarce. Over the years, bamboo designers, architects,

engineers

and

enthusiasts

alike

have

researched

and 5

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documented a huge amount of valuable information about bamboo’s origin, taxonomy, uses, properties and preservation techniques as well as supplying future designers with useful hints and tips. The objective of this report is to explore bamboo as a natural solution to the ever growing emphasis being placed on sustainability by studying its relevant properties and any existing knowledge pertaining to bamboo as a structural material. It further aims to evaluate any existing design codes or standards and investigate the potential for any further research that may be required. After looking at existing bamboo structures which exemplify the use of these codes, if any, the ultimate aim is to design a new structure using bamboo as the primary material and thereafter to check its capacity and adequacy using relevant calculations. The structure of this report is as follows: 

Chapter 1 will introduce bamboo to the reader, in terms of its biological origin, its use as a construction material and the reason why it must be explored for sustainable development.



Chapter 2 will discuss the mechanical properties of bamboo which make it a credible alternative to the more widely accepted materials like steel, concrete and timber.



Chapter 3 will explore the need for preservation in bamboo since its natural durability is not great.



Chapter 4 will look into the different types of joints and connections for bamboo members.

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Chapter 5 will list all the codes and standards existing for testing procedures and designing with bamboo and critically compare some of them. In addition, the path towards further standardisation will be explored.



Chapter 6 will introduce a possible design solution for bamboo after having discussed possible ideas as to where the potential of bamboo would be realised to its fullest. Thereafter, calculations will be conducted to test this design, after which engineering drawings will be given.

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Chapter 1 – Why Bamboo? Contrary to popular belief, bamboo is actually a grass and not a wood. It belongs to the Poaceae (also known as Gramineae) family of bush-like flowering grasses. More than one thousand species have been identified globally, with Europe being the only major continent where bamboo is not found, (Trujillo, 2007) as can be seen from Figure 1 below.

Figure 1: Regions of the world where bamboo grows (Bambus, 2002)

The Bamboo Plant An in depth review into the anatomy of bamboo is beyond the scope of this project; however it is important to have an idea about the appearance of bamboo and its method of growth in order to understand

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the problems that will be addressed later, such as mechanical properties. Bamboo predominantly grows in sandy and loamy soils and prefers well drained soils for growth but is also known to have survived in wet and marshy places (Bambus, 2002). Bamboo plants grow from seeds or rhizomes in their natural habitat and the rhizomes provide the foundation for growth since the bamboo plant does not possess a trunk like in trees (Janssen, 2000). Although bamboo does produce dense foliage, much like trees, the area of primary focus for construction purposes is the culm, which can be likened to the trunk of a tree.

Figure 2: The anatomy of the bamboo stem (culm) (Schröder, 2011)

The feature of the bamboo culm that is of most interest is its hollow nature. However, it is not hollow throughout its cross-section. Every so often, there is a node which can be likened to a diaphragm, separating the spaces in between which are called internodes. The internodal length towards the base of the culm is shorter than that towards its tip 9

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(Schröder, 2011); and this is of particular importance because the nodes provide the strength to the culm. The outside of the culm wall has high silica content (Janssen, 2000) which provides a smooth and shiny surface and one that protects the interior from rain, much like the waxy cuticle on the upper surface of a leaf. Bamboo is predominantly propagated by the well-known process of cuttings (Janssen, 2000) as opposed to the usual route taken by plants of multiplying via flowering to produce seeds. This is because bamboo flowers very rarely throughout its lifetime, sometimes after 100 years which is not a sensible amount of time to wait for the propagation of a useful plant.

Use of Bamboo in Construction Bamboo has been used in construction for many centuries by the local people in the regions where it was native to. Throughout history, the plant has commonly been seen as a symbol of friendship and longevity to the local people due to the benefits it brought to them (Laroque, 2007), with some even making folksongs about their particular species of bamboo (Trujillo, 2007). Bamboo has been widely used for many forms of construction, in particular for housing in rural areas (Hussain, 2013). However, this has inadvertently led to bamboo gaining something of a mis-reputation in

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some parts of the world like Latin America and India as “the poor man’s timber” (Janssen, 2000 & Trujillo, 2007). In an interview, Oscar Hidalgo, a long time bamboo architect and enthusiast, explained that the Chinese had invented suspension bridges using tension cables stretching up to 120m long. They, along with the South Americans, have also used bamboo historically for the construction of buildings (Adams, 1997). A more well-known use is in scaffolding in Hong Kong; a use which has advanced considerably in the last century due to the advancement of the Hong Kong skyline and the need for a cheaper alternative to steel. The five main species for building with bamboo are Dendrocalamus Strictus, D Atter, D Asper, Gigantalochia Apus and Guadua Angustifolia (Janssen, 1988).

Figure 3: Bamboo scaffolding in Hong Kong (Bambus, 2002) 11

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Sustainability “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” – (Brundtland Commission Report, 1987) Climate change is a very real global problem that is, to some extent, beginning to be addressed at different levels around the world. Many climate scientists are adamant that the widely accepted 2⁰C target for temperature rise form pre-industrial (pre-1990) levels is still not entirely safe; however there is a unanimous agreement among them that adopting this policy has the potential to avoid the worst impacts of climate change (Climate Energy Institute, 2013). Added to this is the issue of the Earth’s depleting resources. It is estimated that the world needs to achieve an annual decarbonisation rate of over 5% for the next few decades (Johnson, 2012) in order to have any chance of reaching the 2⁰C target and governments are being put under increasing pressure to force legislative changes towards that. In terms of major industries, the construction industry is one that is known to have plenty of room for improvement in this regard. As well as behavioural changes post-construction – which is not of primary interest here – sustainable changes can be made before and during construction to help alleviate the stress on the planet. The most commonly known construction materials in the urban world are steel and concrete, both of which have high embodied energies. Timber follows closely in terms of popularity. With regards to concrete, a 12

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Chemistry World article (2008) stated that “concrete is the single most widely used material in the world – and it has a carbon footprint to match.” It continues to point out how 5% of annual human-caused CO2 production is due to concrete production, simply because of the huge quantities produced. One advantage steel has over concrete in this matter is that it possesses recyclable potential. Table 1 below uses data from the Inventory of Carbon and Energy prepared by the University of Bath to compare the three materials (Hammond & Jones, 2006). Embodied energy in Material mega-joules per kg

Density kg per Carbon kg CO2 per kg

m3

Steel

20.1

1.37

7800

Concrete

1.11

0.159

2400

Timber

8.5

0.46

480-720

Table 1: Embodied energies and carbon of three main materials (Hammond & Jones, 2006)

Timber seemingly poses a more viable alternative since it is renewable. However, the felling of trees provides the problem of removing a source of carbon intake. Furthermore, trees take extremely long to mature to the strength required for construction. This is where bamboo comes in. Bamboo, in general, is a fast growing plant. Trujillo (2007) states that bamboo can grow up to 25m in six months, although it would take another three to five years to mature. Compared to the century that species of tree like English Oak are known to take to mature, this is still incredibly fast. Added to this is the fact that bamboo does not grow like 13

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trees where each individual is its own organism; rather bamboo develops as an underground network, allowing for increased protection and safer harvesting. Being one of the most rapidly renewable construction materials available (Murphy et al, 2004) makes bamboo a lot more sustainable than timber, simply because the issues of deforestation are lessened so dramatically. If bamboo is harvested and used locally, emissions related to transport of materials will also be low. Bamboo has the ability to decrease soil erosion, aid biomass regeneration and increase carbon sequestration. However, it would be imprudent to consider only emissions and environmental

issues

when

discussing

sustainability.

Sustainable

development has been described many times, by Richard (2013) and others, as a “triple bottom line” balancing between environmental, social and economic issues, as shown in Figure 3 below. The ideal would be to achieve a system that ticks all three boxes, thus landing in the centre of the diagram.

Figure 4: The triple bottom line of sustainable development (Taste of Sustainability, 2012) 14

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A UN Habitat document promoting the use of bamboo for disaster relief (2008) listed advantages of bamboo across the three dimensions of sustainability. For economic, there was the growing market demand for bamboo, caused by innovations in bamboo construction and increasing awareness of the material. Being relatively cheap, using bamboo over steel, concrete and timber saves on costs and allows money to be spent in other areas. Furthermore, bamboo construction can stimulate local economic development by creating employment opportunities. China’s bamboo industry, whether in construction or otherwise, is reported to employ 5 million people (UN Habitat). For the social and cultural aspect, bamboo has the potential to assist its producers and workers in developing sustainable livelihoods. Since the process of erecting a bamboo structure includes so many varied stages such as harvesting, treatment and construction, there are opportunities for people with varying levels of skillsets. Finally, the document talks about how bamboo allows for the promotion of local methods of construction rather than employing foreign and new methods which reinforces the connection of the local people with their history and culture (UN Habitat).

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Chapter 2 – Mechanical Properties The commendable features of bamboo discussed previously, like its aesthetically pleasing nature, its rapid growth and its sustainable potential, amount to nothing for its use as a construction material if it does not perform well structurally. Architects, and in particular engineers, will remain reluctant to work with bamboo if it is not proven to possess the necessary mechanical and physical properties they have come to expect of the more widely used materials like steel, concrete and timber. Janssen (1988) states that the mechanical properties of bamboo, in theory, depend on a few key points: 

The species of bamboo



The age at which the bamboo plant has been cut prior to use



The moisture content (quantity of water contained within the bamboo culm wall)



The position along the culm, since bamboo culms are usually tapered (larger diameter at the bottom)



The relative positions of nodes and internodes along the culm

With that in mind, coupled with years of research, Janssen came up with a set of ratios as shown in Table 2 to relate the density of a piece of 16

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bamboo to its most important allowable stresses, namely compressive, bending and shear. Compression

Bending

Shear

Dry bamboo

0.013

0.020

0.003

Wet/Green bamboo

0.011

0.015

Table 2: The ratio between the density, ρ in kg/m3 and the allowable stresses in N/mm2 (Janssen, 1988)

So for example, if dry bamboo had a measured density of 700 kg/m 3, one could very easily calculate the allowable bending stress as 0.020 * 700 = 14 N/mm2. Obviously, this is only straightforward under the assumption that the density is known, but Janssen argues that the density is a value that can be much more easily determined – by cutting from a culm wall and measuring dimensions - than stresses at failure. The density of bamboo typically varies between 500 and 800 kg/m3, with the most common range for structural applications starting at 700 kg/m3 (Richard, 2013). In another publication called Mechanical Properties of Bamboo, Janssen (1991) has collected and compiled the research of over thirty authors who have carried out extensive research on certain properties of bamboo and how they relate to others. This includes growth and anatomy, thermal expansion, elasticity, bending, compression, shear and tension.

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Competitiveness of Bamboo The reason steel is so widely used in industry today is because of its controlled manufacturing process (which vastly reduces the occurrence of bad specimens) and hence extremely predictable and optimal behaviour. This means that in design, the allowable stress can be very close to the ultimate stress at failure (Janssen, 2000), which is attractive to engineers. Since bamboo and timber are natural materials, this implies a very wide variety of stresses around the mean stress at failure; even more so for bamboo, due to the smaller industry and currently less controlled harvesting process. This uncertainty also leads to a distance between the allowable stress and ultimate failure stress, one which is of orders of magnitude larger than that of steel. Concrete lies somewhere between steel and timber in terms of safety. From this, one could argue that the use of steel is the most economical because of the smallest distance between allowable and ultimate stresses. On the other hand, bamboo would appear to be the least economical choice for engineers, owing to the large gap between stresses, signifying that bamboo is less optimal. In his conclusion to this topic, Janssen (2000) provides a counter argument in favour of bamboo, stating that the above is only correct in normal circumstances. However, in the case of natural disasters when forces are multiplied dramatically, stresses in steel will venture into the area of failure, due to its small room for error. This will not be the case, or at least is less likely to be the case, for timber and bamboo. Therefore, a bamboo residence is actually a safer place to be in during an

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earthquake, provided the structure has been designed and built with due regard for the rules and codes (Janssen, 2000). Janssen (2000) also compared the relative strength and stiffness of the four materials as a function of density and produced a graph, as shown in Figure 5 below.

Figure 5: Strength and Stiffness comparison of four materials (Janssen, 2000)

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Microstructure The wall of the bamboo culm contains cellulose fibres which are distributed densely in the outer layer and more sparsely in the inner layer, as shown in Figure 6.

Figure 6: Cross-section of the bamboo culm (Amada & Untao, 2001)

These cellulose fibres constitute about half of the cross-sectional area of the culm and they act as reinforcement, which Janssen (2000) has likened to the steel rebar reinforcement in concrete. The formation of lignin in the cell walls of bamboo is not dissimilar to that in species of timber, thus providing a similar texture also. The difference is that whereas timber has a hard centre and gets progressively weaker towards the outer edge, the opposite can be said for bamboo, which allows for much more stable construction (Bambus, 2002).

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Flexural Strength Although Table 2 shows estimated ratios of the allowable stresses found by Janssen (1988), he has also estimated stresses at failure. The bending stress at failure for air-dry bamboo has been estimated as 0.14 times the density, which as a rule of thumb is 7 times greater than the allowable stress of 0.02 (Janssen, 2000). However, the typical mode of failure in bending tests that have been performed by researchers is longitudinal splitting of the material rather than fracturing of the fibres (Richard, 2013). The cause of this is shear (VQ/It) which overcomes the lesser capacity of the weaker lignin. In his calculations using Poisson’s coefficient of 0.3 and the modulus of bamboo, Janssen (2000) calculated a critical longitudinal strain of 0.0037 and an ultimate bending stress of 62 N/mm2, which he describes as “a typical outcome”.

Compressive Strength As with bending strength, the ultimate compressive strength of air-dry bamboo was estimated by Janssen (2000) as being 0.094 times the density. During compression tests, the bamboo specimen often bulges laterally, much like a wooden barrel, due to the development of vertical cracks (Richard, 2013). Under compression, critical tangential strains are induced in the culm as a result of tangential expansive forces; lignin has a major role to play in this (Richard, 2013).

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Tensile Strength It is the high tensile strength of bamboo which has given it the description of “nature’s version of steel”, although the values vary widely from species to species. Tensile strength is influenced greatly by the fibre volume ratio (Janssen, 1981). Bamboo is able to resist more in tension than in compression (Bambus, 2000). This is because of the microstructure discussed earlier; the high volume of cellulose fibres along the outer edge provide tensile resistance to bamboo just as steel rebars do to reinforced concrete. However, all is not good for the tensile strength of bamboo. Although it performs well in the direction of the fibres (parallel to grain), it has been reported as having performed very poorly perpendicular to grain, so much so that some authors have disregarded this value altogether and simply described it as negligible. Only the lignin matrix is able to act in resistance to the tension applied in this direction (Richard, 2013)., which ultimately leads to splitting and cracking failures. Richard (2013) has cited the works of Arce-Villalobos (1993) when describing studies that have shown that the critical transverse strain of bamboo is 0.001. Hence this is the value that must be used as a limiting value for design.

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Shear Strength Shear strength is of particular importance when considering the design and construction of the bamboo joint (Bambus, 2002). This could be a beam to beam connection, a beam to column connection or a column to foundation connection among others. When comparing to timber, Richard (2013) has stated that the hollowness of the bamboo culm gives it less cross-sectional area to resist shear than timber, although timber has defects such as knots which are not present in bamboo. Using his estimation techniques, Janssen (2000) predicts the critical shear stress of a bamboo culm in flexure to be 2.2 N/mm2. However, bamboo has two asymmetric shear planes due to the fibres’ orientation in only one direction (Richard, 2013).

Buckling Janssen (2000) concludes that the Euler relation can be applied to bamboo for buckling, which is the instability that occurs in slender columns under axial loading (Jassen, 2000). Euler’s equation requires that the moment of inertia I be known, but this could prove difficult for a bamboo culm that is tapered, has nodes and whose modulus varies along its length. The solution was to express the factors as correction-factors to Euler’s equation (Janssen, 2000 & Arce-Villalobos, 1993).

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Moisture Content Figures 7 and 8 below show that bamboo does not perform well in contact with water, as the mechanical properties begin to deteriorate due to the level of moisture content increasing. Figure 8 (NBCI, 2005) actually describes this decrease as an exponential one. This can be understood considering bamboo is a natural material, and as already shown in Table 2, green bamboo has lower strength values than dry bamboo; the primary difference being the higher moisture content.

Figure 7: Effect of water absorption on mechanical properties, (Janssen, 1991)

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Figure 8: Relationship between moisture content and maximum stress in a species of bamboo (NBCI, 2005)

Fatigue in bamboo Bamboo culms display fatigue failure when loaded in compression across their diameters but fatigue behaviour does not occur under axial compression (Keogh et al, 2015). In the first test on fatigue on bamboo of its kind, Keogh et al (2015) discovered that cracking in one location tended to increase stresses at other locations, which can be seen in Figure 9.

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Figure 9: a) Maximum principal stress contours for a plain sample, b) defected sample, c) plot of stress as a function of distance around the outer circumference (Keogh et al, 2015)

Failure behaviour Janssen (2000) relates that failure in bending of bamboo is not actually a failure. In other materials like timber, a crack first appears after a certain amount of bending, some time after which the beam breaks in two. Tests performed at the Technical University of Eindhoven in the 1980s showed that creep, a natural phenomenon in timber, does not occur in bamboo. At failure, all fibres along the length of the bamboo culm continue to hold strong without damage (Janssen, 2000), which can be partly seen in Figure 10. Bamboo will return to its original form as soon as the load placed on it is removed because the appearing cracks along the culm are stopped by regions of strength in the microstructure (Bambus, 2000). This is excellent for bamboo structures subjected to forces like in earthquakes, as they will

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remain standing even after failure. The damaged elements can be replaced at one’s own convenience. Amada & Untao (2001) performed experiments to determine the fracture properties of bamboo and concluded that the fracture toughness of the culm is much higher in the outer surface layer (where the fibres are held together) than the inner layer.

Figure 50: Failure in bamboo culm due to bending (Bambus, 2002)

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Chapter 3 – Preservation of Bamboo Since bamboo is a natural product, its durability is naturally very low as it possesses very little resistance to biological agents. Without proper preservation techniques being applied to it, a bamboo structure that had the potential to survive for decades would be unusable and unsafe for structural use in a matter of months. Although bamboo is known to be rich in silica, and is known to have small amounts of resins and waxes, none of these have enough toxicity to contribute towards natural durability in a manner that timber does (Kumar at al., 1994). Conversely, the presence of high amounts of starch in bamboo makes it a source of attraction for unwanted pests and fungi. When exposed to moisture, such as when in contact with the ground, studies have shown that the durability of bamboo diminishes even further (Kumar et al., 1994). Janssen (2000) explains that the hollow cross section of bamboo, rather than its chemical composition, is something that compounds its low natural durability in comparison to timber. This is because an attack to a depth of, say 2 mm, in timber would be almost negligible, but an attack on bamboo of the same depth is a quarter of its thickness, which could

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prove to be detrimental. The soft inner layer also provides a suitable hiding place for these biological agents (Janssen, 2000). Added to this is the problem that high humidity brings in regions where bamboo can be used. Without preservation techniques being applied, Janssen (2000) estimates that untreated bamboo will survive for: 

About one year in the open and even less when in contact with soil



Up to six years when under cover and not in contact with soil



Up to fifteen years with very good storage and use

Preservation Since the natural structure of bamboo is such that it does not allow rainwater to enter, via its high silica outer layer and waxy inner layer, this means that bamboo is set up to be impermeable to preservatives as well (Janssen, 2000). The vessels within the walls of the culm are the only other route, but these close up 24 hours after harvest, which shows how small the window for action is.

Traditional methods Janssen (2000) states that many of the traditional methods of preservation - such as smoking, curing, seasoning and soaking - can be applied by local people with low skill sets and without much investment. However, the actual effect of these methods has not been recorded. A few benefits of 29

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these methods include reducing of the starch content and forming of protection against insects and fungal attack. A traditional method that Kumar et al. (1994) have recorded is drying, whether that be kiln drying or air drying. The latter of the two can take up to twelve weeks and cause collapsing because of the build-up of stresses due to non-uniform shrinkage in the culm.

Chemical methods For large-scale projects and industrial use, chemical methods must be used. Although there are plenty of methods and chemicals that can be used for this purpose, which have been documented by Kumar et al. (1994) and others, many of them pose health and environmental risks, such as the use of arsenic (Janssen, 2000). Since this, by ceasing to fulfil the triple bottom line of sustainable development, would defeat the purpose of using bamboo in the first place, these methods will not be discussed here. Safer

chemicals

include

the

element

Boron.

Examples

of

preservatives involving Boron include Copper-Chrome-Boron (CCB), boric acid and borax which are plentiful, cheap and effective. Janssen (2000) states that one major advantage of using certain Boron-based fertilisers introduced in Costa Rica is that they can be recycled after preservation as fertiliser.

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One method of introducing chemicals into the bamboo culm is the Boucherie process, which Janssen (2000) describes as the passing of the preservation through the culm vessels under pressure until it appears at the other end. Kumar et al. (1994) state that the preservative can be pushed through by gravity. This method is only used for whole green culms.

Figure 61: Boucherie treatment of green bamboo culms using a foot pump to create pressure (Kumar at al., 1994)

Another method is dip diffusion where the culm is immersed in the preservative, after which a penetration process will take place. This method can only be applied to split bamboo strips (Janssen, 2000).

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Chapter 4 – Jointing and Connections Janssen (2000) has illustrated many different types of connections that can be used in bamboo structures to connect members to each other. Connection detailing is important in design with any material, but more so for bamboo because bamboo is hollow and has nodes which can appear at varying distances. The general rule for joints in bamboo is that they should be made at nodes or as close to nodes as possible. Janssen (2000) has very conveniently classified the different types of bamboo joints into groups based on the axis of joining and whether the joint is perpendicular or parallel: 

Group 1: full cross-section



Group 2: from the inside of the culm to a parallel element



Group 3: from the cross-section to a parallel element



Group 4: from the cross-section to a perpendicular element



Group 5: from the outside of the culm to a parallel element



Group 6: for split bamboo (not round) These groups have been briefly summarised in the following pages,

using figures from Janssen (2000) to provide extra clarity. 32

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Figure 72: Group 1 connections (Janssen, 2000)

Figure 12 shows joints involving the full cross-section of the bamboo culm (Group 1). Lashings and ropes are perhaps the most prevalent and traditional method of jointing around the world; they fall into this category (Janssen, 2000). Figure 13 shows Group 2 joints, involving the inside of the culm connected to a parallel element. It is possible to fill the void with a material like mortar or even wood and thereafter a steel bar (Janssen, 2000), so that the transfer of forces happens via the secondary material. 33

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Figure 13: Group 2 connections (Janssen, 2000)

Figure 14: Group 3 connections (Janssen, 2000)

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Figure 14 shows connections from the cross-section of the culm to a parallel element. Pins are introduced in this group and they are usually made of wood or steel. Figure 15 below shows similar connections to Figure 14, but the difference in Group 4 connections is that they are to perpendicular elements. Pins and bolts are used more extensively in this group (Janssen, 2000).

Figure 15: Group 4 connections (Janssen, 2000)

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Figure 16: Group 5 connections (Janssen, 2000)

Figure 16 shows a modern advancement of Group 1’s traditional lashings. Usually, steel wire is tightly wrapped around bamboo to produce a strong and simple connection (Janssen, 2000).

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Chapter 5 – Codes and Standards The major proportion of what is known about bamboo in construction is based on the experiences and individual expertise of a few people who have spent time researching and working with bamboo. For instance, in the previous chapter, a lot of the property values were based on rules of thumb and generalisations (Richard, 2013). Not only would standards provide safety for occupants of structures built using bamboo, but they also have the potential to take bamboo from its position as a lower class material in certain third world areas to new international heights amongst more widely used materials. Janssen (2000) reiterates that a standard would primarily be for the protection of the consumer, as there have been many cases where non-engineered dwellings have been demolished for corporate gains under the excuse of them not meeting the acceptable standards. Standards

provide

a

baseline

reference

and

a

benchmark

requirement (Richard, 2013). International standardisation would promote structural bamboo as a renewable structural product worldwide (Gatoo et al., 2014). There are some codes and standards existing around the world.

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These have been summarised by Gatoo et al. (2014) in an Institution of Civil Engineers (ICE) article, as shown in Table 3 below.

Table 3: Summary of existing standards and codes for structural bamboo (Gatoo et al., 2014)

It is clear from Table 3 that some of the countries where bamboo grows and has grown for centuries have taken the initiative and have begun to develop standards for the species that they are accustomed to. They are able to do this by collaborating the works of experienced bamboo architects and engineers to produce a set of regulations.

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ISO (International Standards Organisation) There currently exists one set of international standards on bamboo; the ISO 22156 and ISO 22157 series, both of which were published in 2004. ISO 22156 was created to centralise the suggestions of researchers for structural design (Myers, 2013). It contains suggestions for design and drawing details. ISO 22156 is backed up by ISO 22157, which contains a set of guidelines on how to determine the physical and mechanical properties of bamboo. Research and testing is still required to use these test methods to produce a universal set of results of mechanical and physical properties that can be used worldwide. The ISO 22156 document begins in the usual manner of any standard, by outlining the scope, terms and definitions and symbols. It is worth noting that the normative references listed towards the start of the document include ISO 6891 and 12581, both standards for timber structures. This immediately gives the impression that the standard was developed using the standard for timber as a base or reference. After briefly mentioning design concepts, the document goes on to structural design of bamboo, firstly using limit states; states beyond which the structure will no longer satisfy the design performance requirements. As with steel, concrete, timber and composites etc., limit states can be Ultimate Limit State (ULS) or Serviceability Limit State (SLS). Reference is then made to ISO 22157 in which mechanical properties of bamboo can be tested, and how to use these results to produce design values.

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In the next section, the standard gives the option of adopting the allowable stress method instead of the limit state design procedure and gives a formula for this:

 allowable  Rk * G * D / S where 

G is a modification value; default = 0.5



D is the duration of load modification value: 1.0 for permanent load, 1.25 for permanent plus temporary load, 1.5 for permanent plus temporary plus wind load.



S is the factor of safety; default = 2.25 The next sections are beams and columns. There are a set of

guidelines which should be followed during the calculations required for beam and column design. For beams the following should be calculated: the moment of inertia I, the maximum bending stress, the deflection and the shear stress in the neutral layer. For columns, there is a choice of using a full scale buckling test on the same species of bamboo or using calculations. These must include: the moment of inertia I, the bending stresses due to initial imperfections, buckling according to Euler and combined bending and compression. The following two sections contain detailed advices on joints, most of which are highlighted in Chapter 4, and assemblies (trusses). This precedes two short sections on panels such as plybamboo and particleboard and the use of bamboo as reinforcement in concrete and soil. The document then gives pointers regarding durability and 40

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preservation, most of which is covered in Chapter 3 of this report. This is followed by advice on fire protection and quality control.

Indian Standard (National Building Code of India (NBCI)) Gatoo et al. (2014) provided a brief analysis of the Indian standard for bamboo, by saying it provides strength limits for three classes of bamboo as a representation of species common to India. Similar is the case of the Columbian, Ecuadorian and Peruvian standards; they focus only on local species. The reason the Indian standard has been chosen for description here is that; firstly it is the only other one widely available in the English language and secondly it relates to a species of bamboo which will be of particular interest in design later on. The document begins with the usual definition and symbols, after which there is an introduction to the material bamboo. Sixteen species of bamboo were found to be suitable for structural applications and these were divided into three classes. The limiting strength values of these three groups are given as ranges in a table. The next table in the standard is taken directly from Janssen (1988). This is Table 2 in Chapter 2 of this report - Mechanical Properties. In the standards document, two large tables are given which highlight some of the most important properties and safe working stresses of sixteen to twenty of the most prominent species of bamboo.

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Table 4: Properties of full-culm Indian bamboos (NBCI, 2005) 42

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Table 5: Safe working stresses of Indian bamboos, separated by class, for structural design (NBCI, 2005)

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Beyond this, the Indian standard for bamboo is almost a direct copy of the ISO standard. The rest of the sections include durability and preservation, design considerations (beams, columns, trusses and joints) and storage. The joints section of the Indian Standard is, however, much more extensive than that of ISO. It includes detailed images of many jointing techniques, which signifies advancements in that field of research in India. The species of importance in the above table is Dendrocalamus Strictus, (xvii) in Table 4 and (ii) in Table 5. This is a strong and durable species that has been used historically for construction purposes in large regions of India. It has a local nickname, the English translation of which is Male Bamboo (NBCI, 2005). This further signifies its widespread use in India. Its diameter at full size, ready for structural use, is reported to be about 80mm, but it is possible to acquire culms of smaller diameter also.

The future of standardisation Since many of the existing codes and standards for bamboo, including the National Building Code of India one reviewed above, reference the ISO standards extensively, the most logical step is to use the ISO standards as a base from which to conduct further research. Gatoo et al. (2014) are of the opinion that the ISO standards are of insufficient calibre for widespread use, which can be understood considering many of the items within the document contain phrases like “in accordance with the

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applicable national standards.” (ISO 22156, 2004) However, the ISO standards do provide a foundation which was greatly needed. Another pathway Gatoo et al. (2014) have identified is to create standards that are directly analogous to those of timber. If timber based test methods are used for design, it may be an easier method to attract the attention of architects and engineers.

Timber and Bamboo To begin to understand how timber standards can be used for bamboo, the differences between the two must first be determined. Janssen (1988), while listing a few states that bamboo has no rays and knots like wood. An obvious difference is that bamboo is hollow, while wood is usually dense. The outer skin of bamboo does not have any bark like that of trees. Instead, the external layer has high silica content, and this can cause traditional tools to become dull and lose their sharpness (Janssen, 2000). For limit state design, timber design standards indicate that certain ‘k values’ must be incorporated into the formulae to account for modification factors, strength factors and depth factors. SLS deflection checks include creep, which according to Janssen (1988) does not occur in bamboo. However Arce-Villalobos (1993) performed tests which showed that a small amount of creep does in fact occur and these can be accounted for in a manner similar to timber.

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The area where bamboo and timber can be most readily compared is in their engineered products, such as plywood and ply-bamboo but this requires further research and testing.

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Chapter 6 – Ideas and Design Many types of successful bamboo structures have been designed over the years. Dwellings, footbridges and shelters are just a few examples. Some rather magnificent structures which showcase the potential of bamboo on an aesthetic level have been designed by experienced architects such as Velez and Hidalgo. One such example is the ZERI (Zero Emissions Research Initiative) Pavilion, shown in Figure 16.

Figure 86: An interior view of the ZERI roof pavilion (Trujillo, 2007)

The Transitional Shelter The need for transitional shelters is self-evident since shelter is a basic human need. There can be a period of time after a natural disaster or war 47

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when people are stranded with no place to live, and the authorities or those in charge cannot provide them with a new residence at such short notice. Therefore, an ideal shelter is one which can be erected relatively quickly (in a few days), but can survive structurally and protect those living inside it for over a year. In the meantime, more permanent forms of housing can be constructed to meet the needs of the people. The relevance of bamboo in this regard is that bamboo naturally grows in many regions of the world where earthquakes are common; one of those being India. This is why the Indian standard was especially chosen for review in the previous chapter and also the reason why the species Dendrocalamus Strictus was specified. Ideas from the Red Cross document on emergency shelters were used to arrive at a final design for a shelter that could house a small family in case of displacement (IFRC, 2011). All the material can be locally procured. The main frame is whole bamboo culms with connections at nodes or as close to the nodes as possible. An initial sketch of the design is shown in Figure 17 with an extra view of the roof truss in Figure 18.

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Figure 17: Sketch of the bamboo transitional shelter

Figure 18: Sketch of the truss under the roof shown in Figure 13

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Calculations Calculations were carried out according to Janssen (2000) who used simple bending calculations for a bamboo beam. The estimated external loads upon the structure were as follows: 

Wind load = 1 kN/m2



Snow/rain = 0.6 kN/m2



Cover and cladding = 0.3 kN/m2

The specific properties of Dendrocalamus Strictus, taken form Tables 4 and 5 in Chapter 5, as found in NBCI (2005) are shown in Tables 6 and 7 for clarity. Density (kg/m3)

728

Modulus of

Modulus of

Max Compressive

Rupture

Elasticity *103

Stress (N/mm2)

(N/mm2)

(N/mm2)

119.1

15.00

69.1

Table 6: Properties of Dendrocalamus Strictus in air-dry conditions

Extreme Fibre Stress in

Safe Modulus of

Allowable Compressive

Bending (N/mm2)

Elasticity *103 (N/mm2)

Stress (N/mm2)

18.4

2.66

10.3

Table 7: Safe working stresses of Dendrocalamus Strictus for Structural Design

The first step is to work out the allowable bending stress of the bamboo by using Janssen’s rules of thumb (shown earlier in Chapter 2, Table 2).

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One major assumption is that the self-weight of bamboo has been neglected, which is not unsafe since bamboo is a lightweight material, unlike steel and timber. 0.020 * density = safe allowable stress, σallow 0.020 * 728 = 14.56 N/mm2 The value of 14.56 N/mm2 will be used as opposed to the extreme fibre stress shown in Table 7 because 14.56 < 18.4. A regular beam in this design has a span, L of 3m. The bamboo species chosen has a diameter, D of 80mm with a wall thickness, t of 6mm as a minimum. The uniformly distributed load, q (N/mm) that one bamboo culm can carry can be calculated from 8 * M / L2, where M is moment. D = 80 mm and d = 80 – 6 = 74 mm. Hence the section modulus can be calculated as 17800 mm3. Thus M = σallow * section modulus M = 14.56 * 17800 = 259000 Nmm. Hence q = 8 * M / L2 = (8 * 259000) / 30002 = 0.230 N/mm The total load this beam could experience is (1.5 * (1 + 0.6)) + (1.3 * 0.3) = 2.79 kN/m2, which becomes 0.00279 N/mm2. When this is multiplied by the ‘length’ of load on element will take, the load = 0.22 N/mm 0.23 > 0.22, therefore it is safe to use one bamboo culm per element.

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Janssen (2000) also calculates using safe deformations, which can be considered to be the serviceability limit state calculation. The formulae used are: s = (5 * q * L4) / (384 * EI), or (F * L3) / (48 * EI) Assuming E = 20000 N/mm2, the following equation is formed: s = (10 * L2 * σallow) / D, hence the deformation, s = (10 * 32 * 14.56) / 80 = 16.38 mm Any span larger than this creates deflections in excess of 50 mm which are too high. Janssen (2000) states that a good guideline in practice is to limit the deflection or deformation to 1/300 of the span. Therefore, we must limit the stress to σallow = D / (3 * L) which equals 8.89 N/mm2. M = σallow * section modulus = 8.89 * 17800 = 158222 Nmm. This corresponds to a q value of 0.141 N/mm. For a load of 0.22 N/mm, two or three bamboo culms will be required to counteract the force but these will have to be tied together well to act as one beam. This will only be needed at the bottom of the structure. AutoCAD drawings of the plan and elevation of the structure are given here, in Figures 19 and 20.

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Figure 15: A plan view of the floor at the ground

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Figure 16: An elevation view of the longer 6m length side

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About the design As mentioned earlier, the idea for the design was acquired from IFRC (2011) and modifications were made according to the advices by engineers in their document for transitional shelters. It is estimated that the design will take three to four people about three days to build and the anticipated lifespan is anything between one and five years (IFRC, 2011). Since the optimum age of bamboo for construction is estimated to be between three and six years, bamboo of this age should be used for the structure. Evidently, all bamboo should have undergone the preservation process to lengthen the lifespan of the structure. Because this is only a transitional shelter, traditional methods may be used. However, if the bamboo culms are going to be used for a more permanent structure later on, chemical treatments must be used according to the Indian standard.

Figure 21: Bamboo cast in various concrete foundations (Credit: CBTC)

The foundations are of concrete cast in place. There are four of them at the corners and one in the centre of the structure. The relatively large foundation dimensions used in the design are to provide sliding resistance under excess loads, such as wind or seismic loads. Seismic loads 55

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are possible in a post-tremor. It must be borne in mind that bamboo must never come into direct contact with the concrete as the alkalinity could adversely affect the bamboo. To protect the bamboo, bitumen can be used or if possible, steel as shown in Figure 21. Half of the floor has been raised to form a sleeping area. The floor of this part of the structure consists of whole or half bamboo culms, which will rest on a triple-culm beam underneath it. To provide anchorage, this beam will be connected to a fifth concrete foundation in the centre of the structure. The columns at the edges consist of triple-culms like the bottom beam. In the IFRC (2011) document a single culm of a larger diameter was utilized, but this has been altered to allow for one universal bamboo size. The X bracing systems are to provide lateral stability to the structure in the case of strong winds. An alternative to this would be to use an engineered bamboo panel to reduce the number of connections needed, but this requires further research. The IFRC document shows that terracotta tiles have been used for roofing but this causes unnecessary loading on the members underneath (IFRC, 2011). The roofing material to be used in this structure can be bamboo matting, which is lightweight, cheap and easily manufactured.

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Conclusion Bamboo has the potential to be a leading worldwide construction material. Many engineers and architects like Jules Janssen have dedicated their time and efforts to realise its potential and capacity. Bamboo works in favour of the triple bottom line of sustainable development, namely environment, society and economy, especially for developing nations in Asia and South America which incidentally is mainly where bamboo grows. Its mechanical properties make it comparable to the leading materials like steel, concrete and timber and in many cases, even more desirable. In fact, in situations like earthquakes, it has been found that one would much rather be in a bamboo structure than a steel or concrete one

due

to

its high

elasticity. However,

its poor

performance

perpendicular-to-grain is something that needs to be worked around. Since bamboo is a product of nature, it is a source of attraction for other organisms of nature. To protect and preserve its structural capability, many methods have been devised both traditionally and after research. For wide-scale production, chemical preservation using Boronbased chemicals should be seen as a must.

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Jointing and connection is where structural design of bamboo is critical. If designed correctly, according to tried and tested methods, connections can allow structures of many interconnecting members to perform. On the other hand, poor jointing will inhibit the structural ability of even simple structures. The golden rule is to apply connections at nodes or as close to them as possible. This can pose a problem for culms where the internodal distances vary and members may have to be moved to accommodate. The path to an international standard for bamboo, similar to that of steel and timber, is under way thanks to the increased research in recent decades. However, it is by no means complete. Researchers continue to look for new methods to attract the attention of architects towards bamboo, such as adopting codes analogous to those of timber which has many similarities. Finally, designing a structure out of bamboo is not a simple task. Ideally, it should only be done by a person who has been exposed to the material and has experience in testing its properties. Nevertheless, simple structures like shelters can be erected in times of need based on the designs and calculations in this piece of work. Calculations for bamboo structural design follow the common pattern of ultimate limit state followed by serviceability limit state and deflections. If possible, bamboos of larger dimensions can be used if required, otherwise the solution is to use more than one culm tied strongly together to act as one member.

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Suggestions for Further Work The connection detailing of bamboo has not been researched in detail for this piece of work, hence this is an area that requires more work. The path to standardisation and codification is a long one and much research and testing is needed in this regard. A possible solution is to separate bamboo according to species and to give the responsibility of a few species to certain researchers. The use of bamboo as an engineered product rather than wholeculm bamboo is a vast new area of research. For bamboo to take its place on the world-wide mantel, it may well need the ability of engineered bamboo. The lack of hollowness not only improves structural performance, but also workability, especially in jointing. Finally, more aesthetic designs using bamboo should be thought of, such as the use of arcs. Although current standards are reluctant to allow for bending of bamboo, due to the extra stresses this would induce, Hidalgo claims to have devised a way forward in this regard; by creating a formwork whose shape the bamboo will grow into (Adams, 1997).

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References Adams, C. (1997) Bamboo Architecture and Construction with Oscar Hidalgo - Natural Building Colloquium. Available at: http://www.networkearth.org/naturalbuilding/bamboo.html (Accessed: 25 April 2015) Amada, S. and Untao, S. (2001) ‘Fracture properties of bamboo’,Composites Part B: Engineering, 32(5), pp. 451–459. doi: 10.1016/s1359-8368(01)00022-1 Arce-Villalobos, O. A. (1993) ‘Fundamentals of the design of Bamboo Structures’, Eindhoven University of Technology, Bamboo as a building material (2002) Bambus, pp. 1 – 16 Available at: www.bambus\new\eng\reports\buildingmaterial\buildingmaterial.html. CBTC (no date) Affordable Bamboo Housing in Earthquake Prone Areas Climate Energy Institute (2013) 2 degrees C target, climate change policy, dangerous climate change, climate change mitigation. Available at: http://www.climateemergencyinstitute.com/2c.html (Accessed: 21 April 2015) Gatóo, A., Mulligan, H., Sharma, B., Bock, M. and Ramage, M. H. (2014) ‘Sustainable structures: bamboo standards and building codes’,Proceedings of the ICE - Engineering Sustainability, 167(5), pp. 189–196. doi: 10.1680/ensu.14.00009 General Uses For Bamboo (2008) Available at: http://www.bamboogrove.com/general-uses-for-bamboo.html (Accessed: 20 April 2015)

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Green Materials: Bamboo - bamboo for disaster relief (no date) UN Habitat, Hammond, G. P. and Jones, C. I. (2006) ‘Embodied Energy and Carbon Footprint Database’, Department of Mechanical Engineering, University of Bath, UK, Hussain, S. (2013) ‘Bamboo as a cost effective Structural Material in Buildings’, International Journal of Engineering and Technical Research (IJETR), 1(9), pp. 45–49. IFRC (2011) ‘Transitional Shelters - Eight designs’, International Federation of Red Cross and Red Crescent Societies, ISO 22156: Bamboo - Structural Design (2004) Geneva, Switzerland: ISO 22157: Bamboo - determination of physical and mechanical properties (2004) Geneva, Switzerland: Janssen, J. J. A. (1988) Building with Bamboo: A Handbook. United Kingdom: Intermediate Technology Janssen, J. J. A. (1991) Mechanical properties of bamboo. Dordrecht: Kluwer Academic Publishers Janssen, J. J. A. (2000) Designing and Building with Bamboo. Edited by Arun Kumar. International Network for Bamboo and Rattan (INBAR) Johnson, L. (2012) ‘Too late for two degrees? Low carbon economy index 2012’, PwC (www.pwc.co.uk), Keogh, L., O’Hanlon, P., O’Reilly, P. and Taylor, D. (2015) ‘Fatigue in bamboo’, International Journal of Fatigue, 75pp. 51–56. doi: 10.1016/j.ijfatigue.2015.02.003 Kumar, S., Shukla, K., Dev, T. and Dobriyal, P. (1994) ‘Bamboo Preservation Techniques: A review’, International Network for Bamboo and Rattan a n d Indian Council of Forestry Research Education,

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Laroque, P. (2007) ‘Design of a low cost Bamboo Footbridge’,Massachusetts Institute of Technology, Mechanical Properties of Bamboo - Bambus (2002) Bambus, pp. 1 – 11. Murphy, R. J., Trujillo, D. and Londoño, X. (2004) ‘Life Cycle Assessment (LCA) of a Guadua House’, Simposio Internacional Guadua 2004, Pereira, Colombia, pp. p235–244. Myers, E. T. (2013) ‘Structural Bamboo in East Africa’, Department of Architectural Engineering and Construction Science College of Engineering KANSAS STATE UNIVERSITY, Part 6: Structural Design, 3B Bamboo - (NBCI) National Building Code of India pp474 - Bureau of Indian Standards (2005) Richard, M. J. (2013) ‘Assessing the performance of bamboo structural components’, The Swanson School of Engineering, University of Pittsburgh, Schröder, S. (2011) ‘Bamboo Stem Anatomy’, Guadua Bamboo, Available at: http://www.guaduabamboo.com/identification/bamboo-stemanatomy Taste of Sustainbility (2012) Available at: http://www.tasteofsustainability.com/ (Accessed: 23 April 2015) The Concrete Conundrum (2008) Chemistry World [Available at: www.rsc.org], Trujillo, D. (2007) ‘Bamboo structures in Columbia’, The Structural Engineer (IStructE), World Commission on Environment and Development. (1987) Our Common Future: World Commission on Environment and Development. Oxford: Oxford University Press

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Closing note on Nepal My thoughts and prayers are with the thousands of people affected by the recent earthquake in Nepal. It is always devastating to hear the news of an earthquake and the 7.9 Richter Scale one which hit Nepal and parts of India on Saturday 25 th April 2015 was no different. Events like this reiterate the need for engineers to put aside their differences and work together to produce sustainable solutions for the betterment of the welfare of the people. Even though this piece of work is not likely to make a huge difference today, I sinceriously hope that one day, works of engineers will contribute towards achieving safety in structures and infrastructure, so that even if we cannot avoid natural disasters, we have a proper way to combat them and help save lives. I firmly believe that this is the age of the engineer. Thank you. Ammar Patel

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