The mechanical properties of bamboo
What are the mechanical properties of bamboo? This is probably one of the most frequently asked questions by architects, engineers and builders interested in using bamboo as a building material. The material properties of wood, steel, concrete, and even semi-finished bamboo products such as flooring and bamboo panels have long been known, but not those of bamboo canes. And why?
"The strength properties of bamboo are often two to three times better than those of conventional wood. However, legal uncertainties regarding building codes and standards make it difficult to use bamboo poles as a building material in Europe."
Bamboo is a collective name for more than 1575 different plant species that belong to the grass family (Gramineae). Each of these bamboo species has different structural and mechanical properties, just like conventional wood species such as teak, oak, etc. In addition, the mechanical properties of a single species of bamboo can also vary greatly. These differences are due to the age of the bamboo trunk, moisture content, growing conditions (climate, altitude, soil conditions) and the part of the bamboo trunk on which the tests are performed.
Since the most suitable bamboo species for construction purposes come from tropical countries, it is often difficult to determine the exact growing conditions and obtain good test material. As a result, different studies often come to different conclusions, which means that the behavior of the material in certain situations is not yet completely certain.
mechanical properties important?
In Europe, there is still no standardization (Eurocode) for bamboo canes as a building material. The lack of such a building regulation for bamboo makes it difficult for those who want to build with this material, so there is an urgent need for clear rules and standards.
The tests already conducted on the compressive, tensile and flexural strength of bamboo are promising and generally show much better results than conventional building materials. However, there are other factors that need to be investigated before a legal building code can be issued for bamboo piles as a building material
These other properties are:
- Fire safety
- Environmental sustainability
- Ease of use
- Energy efficiency
More research is needed, particularly in the areas of fire resistance and durability. Nevertheless, significant progress has been made in recent years with the introduction of the international standard ISO 22157.
The International Organization for Standardization
In 2004, the International Organization for Standardization, or ISO, created its own standard for determining the mechanical properties of bamboo. ISO standard 22157 not only lists bending, compression, tension and shear as important properties, but moisture content is also a relevant parameter.
In this article, we present some test results from different sources and from different types of bamboo. It is important to note that not all tests were performed according to ISO 22157 standard, but it gives a good general picture. For the bamboo species Guadua angustifolia, better known as the strongest bamboo in the world, tests were conducted in Colombia according to ISO standard 22157. You can read the results a little further down.
For the rest, we refer to the tables with information about the tests. You can find these tables in the PDF file that accompanies this blog.
In order to comply with European regulations, the ISO 22157 standard requires that two types of compressive strength be tested, namely compressive strength parallel to the grain direction and compressive strength perpendicular to the grain direction. Strangely, ISO 22157 only describes the test method parallel to the grain direction and does not yet provide a methodology for compressive strength perpendicular to the grain direction.
Due to the natural shape of the bamboo trunk, not one sample of a trunk is tested, but three different samples per trunk. One sample is taken from the lower part of the log, from the middle part and from the upper part of the log. This is necessary because a bamboo trunk does not have a continuous cross-section and there are differences in structural properties between the lower part, which has a larger diameter, and the upper part, which has a smaller diameter.
Compressive strength and modulus of elasticity
The sample should not contain any knots, as the results on these samples would not give a correct picture. This is because the knots are the strongest zones (under compressive load) due to the fiber structure. A sample is always taken between two branches as this is the weakest zone of the bamboo stem.
When bamboo poles are used as columns or beams, only the bottom, middle, and top can be used for these structural applications. The head and tube of the log are not used for structural applications due to their small outside diameter.
The compressive strength of different bamboo species.
The following table provides an overview of the compressive strength of the different bamboo species. It should be noted that not all results were determined according to the test method prescribed in the ISO 22157 standard. The results should be considered as average values, where the differences in age or height direction of the bamboo stems have generally not been taken into account.
The compressive strength of the various bamboo species is predominantly between 40 and 80 N/mm2. This is two to four times higher than the compressive strength of most commercial wood species. The different results can be explained by the different test methods and specimens used. However, it is clear that the age and moisture content of bamboo poles have a significant effect on the compressive strength of bamboo. Bamboo poles with low moisture content have higher compressive strength than bamboo poles with high moisture content.
The maximum tensile strength of bamboo is determined by testing the fibers (bamboo strips) rather than the entire bamboo stem. As with compressive strength, the ISO 22157 standard provides guidelines for tensile strength parallel to the grain direction, but not for tensile strength perpendicular to the grain direction.
To measure the tensile strength of bamboo, three specimens are tested per bamboo log, taken from the lower, middle and upper parts of the log. Each bamboo strip is 100 mm long, 10 - 20 mm wide, and the thickness is equal to the thickness of the bamboo stem.
Each sample should contain a knot because the direction of the knot's grain is opposite to the direction of the stem's grain. Therefore, the knot is considered to be the weakest point under tensile loading (for compressive strength testing, it is the other way around). The moisture content of each of these test specimens should also be determined according to the ISO 22157 standard.
The average tensile strength of the various bamboo species is, for the most part, 160 N/mm2. This is three to four times higher than the tensile strength of most commercial wood species.
The deflection of a bamboo stem has a direct influence on the behavior of the built structure. Therefore, it is necessary to be able to predict the deflection of each element of a structure before it is used. The most common method for determining the deflection of a beam or column is the four-point deflection test. Therefore, the ISO 22157 standard suggests this test for determining the bending strength of a bamboo log.
Bending strength of Guadua Bamboo
The four-point bending test prescribed in ISO 22157 was conducted at the University of Los Andes in Colombia. The bending test was performed on the same bamboo logs as the compressive and shear strength tests.
The four-point bending test determined the modulus of elasticity and, of course, the flexural strength. The results of this test are shown in the table below.
The modulus of elasticity of Guadua angustifolia is highest in the lower and middle part of the trunk, and this is when the trunk has an age of 4 - 5 years. This is not the case for the upper part of the trunk, where the highest modulus of elasticity is obtained when the trunk has an age of 3 - 4 years, just like when the modulus of elasticity is determined in the compression test.
Also, the bending strength of the bamboo log is not the same for all three parts, the upper part of the bamboo log 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 about 100 N/mm2.
Comparison of 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 gives an overview of the bending strength of some other bamboo species. Not all results were obtained using the four-point bending test as prescribed in the ISO 22157 standard. Therefore, these values may have a small variation, but they give an idea of the magnitude of the bending strength of different bamboo species.
The maximum shear stress that a bamboo stem can withstand is important for the development of joining systems. Again, ISO 22157 provides guidelines for shear stress parallel to the grain direction, but not for shear stress perpendicular to the grain direction.
As in the previous tests, three test specimens are tested per bamboo log from the lower, middle and upper parts of the log. The difference between this test and the specimens in the compression test is that half of the specimens in this test must contain a knot.
Before the test begins, the specimen is measured in detail. The height of the specimen and the thickness of the log are measured at the four zones where shear forces occur. This is important because the bamboo stem does not have the same thickness everywhere and the specimen may not be perfectly straight.
Shear stress of Guadua bamboo
At the University de Los Andes in Colombia, on the same bamboo logs whose compressive strength was determined, the maximum shear stress was also determined using the test method described in ISO 22157. As with the compressive strength, the table shows that the middle part of the bamboo stem has the best values at an age of 3 - 4 years, with the maximum shear stress decreasing again at higher ages. Guadua bamboo has a maximum shear stress of about 8 N/mm2 at 3 - 4 years of age (at a moisture content of about 56.6%).
Comparison of shear stress of different bamboo species.
The shear stress of different bamboo species is 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 the ISO 22157 standard.
The stress at which shear occurs parallel to the grain 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 about twice the shear strength of tropical woods (3 to 6 N/mm2).