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17 / 11 / 2016
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The Mechanical Properties

from Bamboo

What are the mechanical properties of bamboo? It is probably one of the most frequently asked questions by architects, engineers and building owners who are interested in using bamboo as a building material. The material properties for wood, steel, concrete and even bamboo semi-finished products such as flooring and bamboo panels have been available for a long time, but this is not the case for bamboo poles. Why?

"The strength properties of bamboo are often two to three times better than those of conventional wood. However, legal uncertainties surrounding building codes and standards make it difficult to use bamboo poles as a structural building material in Europe."

Bamboo is a collective name for more than 1575 different plant species belonging to the family of grasses (Gramineae). Each of these bamboo species has different structural and mechanical properties, just like conventional wood species such as teak, oak, balsa, etc. In addition, the mechanical properties of a single bamboo species can also vary greatly. These differences are due to the age of the bamboo stem, the moisture contentthe growth conditions (climate, altitude, soil conditions), and on which part of the bamboo stem the tests are carried out.

Since the most suitable bamboo species for construction purposes originate from tropical countries, it is often difficult to find out the exact growing conditions and to obtain good test material. As a result, different studies often produce divergent results and the behaviour of the material in certain situations is not yet known with certainty.

Why are mechanical properties important?

In Europe there is no standard (Eurocode) for bamboo poles as a constructive building material. The lack of such a bamboo building code makes it difficult for those who want to build with the material to do so, so there is an urgent need for clear rules and standards.

Tests already carried out on bamboo for compression strength, tensile strength and bending strength are promising and usually show much better results compared to conventional building materials. However, there are other factors that need to be investigated before a legal building code can be issued for bamboo poles as a constructive building material. These other properties are:

  • Sustainability
  • Fire safety
  • Environmental impact
  • User friendliness
  • Energy efficiency

Especially in the areas of fire resistance and durability, further research is still required. Nevertheless, significant progress has been made in recent years with the introduction of an international ISO 22157 standard.

bamboo bending test
Bend test | Photo: Pablo Acevedo

The International Organisation for Standardisation (ISO)

In 2004, the International Organisation for Standardisation (ISO) drew up its own standard for determining the mechanical properties of bamboo. In the ISO 22157 standard, not only are deflection, compression, tension and shear cited as important properties, but moisture content is also a relevant parameter.

In this article, we present some test results from different sources and of different types of bamboo. It is important to note that not all tests were carried out according to the ISO 22157 standard but it does give a good general picture. For the Guadua angustifolia bamboo species, better known as the strongest bamboo in the world, tests were carried out in Colombia according to the ISO 22157 standard. You can read the results a little further on.

For the future, we refer to tables with information about the tests. These tables can be found in the PDF accompanying this blog.


Compressive strength

In order to meet the European regulations, two types of compressive strength must be tested according to the ISO 22157 standard, namely compressive strength parallel to the fibre direction and compressive strength perpendicular to the fibre direction. Strangely enough ISO 22157 only describes the test method parallel to the fibre direction, and does not yet provide a methodology for compressive strength perpendicular to the fibre direction.

Due to the natural shape of the bamboo stem, not one sample of a stem will be tested, but three different samples per stem. A sample is taken from the lower part of the log, from the middle part and from the top of the log. This is necessary because a bamboo stem does not have a continuous cross-section and there are differences in structural properties between the lower section, which has a larger diameter, and the upper section, which has a smaller diameter.

bamboo trunk construction
The construction of a trunk

The specimen must not contain any knots, because the results on these specimens would not give a true picture. The knots are in fact the strongest zones (when subjected to pressure) because of the fibre structure. A sample is always taken between two knots, as this is the weakest zone of the bamboo stem.

When bamboo poles are used as columns or beams, only the lower, middle and upper sections can be used for these constructive applications. The head and the pipe of the trunk are not used for constructive applications due to the small outer diameter.

Compressive strength and modulus of elasticity of Guadua Bamboo (G. angustifolia)

In 2010, the Los Andes University in Bogota, Colombia The mechanical properties of the bamboo species Guadua angustifolia were tested according to the ISO 22157 standard. To investigate the influence of the age of the bamboo stem on its mechanical properties, bamboo stems of two, three, four and five years old were tested.

The respective moisture contents of the bamboo stems were 59.3%, 56.5%, 56.7% and 65.2%. It is important to mention these moisture contents because the moisture content is an important parameter for the strength properties of the bamboo stem. The average outer diameter of the lower part of the bamboo stem was 13 cm, of the middle part 12 cm and of the upper part 10 cm.

The maximum compressive strength for Guadua angustifolia (in the green state) was observed in 3-4 year-old logs with an average compressive strength of about 40 N/mm² . The compressive strength of 5-year-old Guadua decreased by approximately 16% compared to the compressive strength of 3-4-year-old Guadua Bamboo.

bamboo compressive strength testing
The Mechanical Properties of Bamboo 6

Comparing the compressive strength of different bamboo species

The following table gives an overview of the compressive strength of different types of bamboo. It is important to note that not all results were obtained using the test method prescribed in the ISO 22157 standard. The results should be seen as average values, whereby the difference in age or height of the bamboo stem is usually not taken into account.

Conclusion

The compressive strength of the various bamboo species lies largely between the 40 and 80 N/mm². This is twice to four times higher than the compressive strength of most commercial timber species. The difference in results can be explained by the different test methods and samples used. However, it is clear that the age and moisture content of bamboo poles has an important influence on their compressive strength. Bamboo piles with a low moisture content have a higher compressive strength than bamboo poles with a high moisture content.


Tensile strength

The maximum tensile strength of bamboo is determined by testing the fibres (bamboo strips) and not the entire bamboo stem. As with compressive strength, the ISO 22157 standard provides guidelines for tensile strength parallel to the fibre direction, but not for tensile strength perpendicular to the fibre direction.

To measure the tensile strength of bamboo, three test strips are tested per bamboo stem, taken from the lower, middle and upper part of the stem. Each bamboo strip is 100 mm long, 10 to 20 mm wide, and its thickness is equal to the thickness of the bamboo stem.

Each test piece must contain a node, because the fibre direction of the node is opposite to the fibre direction of the trunk. The node is therefore considered to be the weakest point when loaded in tension (the reverse is true when testing compressive strength). The moisture content of each of these test pieces must also be determined as prescribed in the ISO 22157 standard.

The following table shows the tensile strength of the most commonly used species of bamboo for structural applications.

Conclusion

The average tensile strength of different bamboo species is largely around the 160 N/mm². This is three to four times higher than the tensile strength of most commercial woods.


Shear stress

The maximum shear stress that a bamboo stem can withstand is important for the development of connection systems. Here too, the ISO 22157 standard provides guidelines for the shear stress parallel to the fibre direction, but not for the shear stress perpendicular to the fibre direction.

As in the previous tests, three samples per bamboo stem are tested, taken from the lower, middle and upper part of the stem. The difference with the pressure test is that in this test half of the test pieces must contain one node.

Before the test starts, the specimen is measured in detail. The height of the test piece and the thickness of the stem are measured at the four shear points. This is important because the thickness of the bamboo stem may vary and the specimen may not be completely straight.

Shear stress of Guadua Bamboo (G. angustifolia)

At the University of Los Andes in Colombia, the maximum shear stress was also determined on the same bamboo stems from which the compressive strength was determined, using the discussed test method according to ISO 22157. As with the compressive strength, the table shows that the middle section of the bamboo stem exhibits the best values at an age of three to four years, and at an older age the maximum shear stress decreases again. Guadua Bamboo has a maximum shear stress of about 8 N/mm² (at a moisture content of approximately 56.6%).

bamboo shear strength testing
The Mechanical Properties of Bamboo 7

Comparing the shear stress of different bamboo species

The shear stresses of different bamboo species are shown in the table below. Again, the results should be considered as average values as we cannot confirm that all tests were performed according to ISO 22157 standard.

Conclusion

The tension at which shear occurs, parallel to the fibre direction, is about 10 times lower than the compressive strength and even up to 20 times lower than the tensile strength of the same bamboo species. Nevertheless, the shear strength of most bamboo species is approximately twice the shear strength of tropical woods (3 to 6 N/mm²).


Bending strength

The deflection of a bamboo stem has a direct influence on the behaviour of the built-up structure, so it is necessary to be able to predict the deflection of each element in a structure before it is used. The most commonly used method for determining the deflection of a beam or column is the four-point deflection test. In the ISO 22157 standard, this test is used to determine the bending strength of a bamboo stem.

Bending strength of Guadua Bamboo (G. angustifolia)

The four-point bending test prescribed in the ISO 22157 standard was performed on the University of the Los Andes in Colombia. The bending test was conducted on the same bamboo stems as the tests to determine compressive and shear strength.

During the four-point bending test, the modulus of elasticity and, of course, the bending strength were determined. The results of that test are shown in the following table.

The modulus of elasticity of Guadua angustifolia is highest in the lower and middle part of the trunk at an age of four to five years. This is not the case for the upper part of the trunk, where the greatest modulus of elasticity is reached at an age of three to four years, just as in the determination of the modulus of elasticity in the compression test.

The bending strength of the bamboo stem is also not the same for all three parts; the upper part of the bamboo stem has a greater bending strength than the lower part. The bending strength increases with the height of the trunk. The bending strength of Guadua angustifolia is approximately 100 N/mm².

Comparing the bending strength of different bamboo species

The bending strength of Guadua angustifolia is in the same order of magnitude as the bending strength of other commonly used bamboo species. The following table shows a summary of the bending strength of some other bamboo species. Not all results were obtained by the four-point bending test prescribed in the ISO 22157 standard. These values may therefore show a small deviation, but they do give an idea of the order of magnitude of the bending strength of different bamboo species.

Conclusion

The bending strength of most bamboo species lies between 50 and the 150 N/mm² what about is twice as high as most conventional woods. The deviations in the test results in the above table are probably due to the moisture content of the tested bamboo stems and to the different test methods. For example, a bamboo log with a low moisture content is more likely to have a lower bending strength than a bamboo log with a higher moisture content.

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