properties

We know that the mechanical properties of timber are changing with the age, date of harvest, degree of humidity and climate, soil conditions, location, etc. Furthermore, there are differences over the length of the trunk or cross-section, and also, whether the loads are parallel or perpendicular to the direction of the fibers.
The deviation of characteristic strength values is almost even bigger in bamboo, showing very different density in the cane wall section and also its diaphragms.

Since the bamboo canes differ from each other and can not be cut like timber, they cannot be standardized. Moreover, the mechanical qualities of bamboo depend on the botanical species, its habitat, the age of the cane at the date of harvest, the moisture content and of course on diameter and wall thickness. So it is recommended to sort the culms accordingly.
Standards about building with bamboo are still missing, which impedes the application of bamboo in countries with strict technical regulations. But there exists a draft of the ISO about bamboo testing methods: Determination of physical and mechanical properties of bamboo

In a cross-section of a bamboo culm it has to be distinguished between a dark outer zone (approximately 30 percent) with closely packed fibers and a white inner zone (about 70 percent), which is rather porous. With increasing culm height the share of percentage of the dense exterior fibers in relation to the cross-sectional area increases and therefore slim canes are superior to thick-walled bamboo.
The accumulation of the rigid fibers in the edge zone improves the values of elasticity as well as tension, shear, and bending strengths. As of timber, the modulus of elasticity (MOE) of bamboo is reduced with increasing stress (5-10 percent). A MOE of 2.000 kN/cm² can be assumed to calculate constructions.

The bamboo tube is stiffened by the nodes by preventing buckling of the culm wall: The fibers are curved towards the culm axis, similar to a multiple-curved surface. Their MOE is 40 percent less than that of the internodes.

The tensile strength of the outer zone is two to three times higher than that of the inner zone. The tensile strength in the nodes is only moderate because the fibers there run disorderly. Nodes reduce the tensile strength of the culm. The tensile strength decreases in culms of over 5-6 years of age.

The compression strength, however, increases with the age. In a test the 6 year old culms showed 2,5 times the compression strength than the one year old ones. Cane sections with nodes show approximately 8 percent higher compression strength values parallel to the fiber compared to sections without nodes. In tests with compression load perpendicular to the culm axis, the nodes cause strength values up to 45 percent higher than in tests with tube sections without nodes. The sea level, at which the plant grows, as well as the age of the poles have a big influence on the silicification of the vessels, which leads to a considerable increase of compression strength.

The shear strength of thin bamboo canes is higher than that of thick-walled tubes, caused by the ratio of the rigid fibers to the sectional area. The shear strength of node material is about 50 percent higher than that of internode material.

'Atrops' examined bamboo that is usual for building materials: Tube diameter 70-100 mm, wall thickness 6-12 mm, used for a span of 3,60 m. The elastic deflection was min=1/25,9; max=1/16,1; on average 1:20,1 of the span.
If deflection is not desired, the green bamboo culms can be bended to the opposite before installation (superelevation).

The bamboo tubes behave well at compression load. No sudden buckling nor cracking of the tube wall occurs. Shear fractures are the most frequent failures in bending tests. These are favored by drying cracks parallel to the culm axis. The internodes in which high shear stresses occur should therefore be filled with concrete. In case of shear failure, there will always remain a load-carrying capacity of the two halves.

The fracture behavior of conventional timber differs from the fracture behavior of bamboo. Cracking of individual fibers does not lead to a spontaneous break of the whole cane. The appearing cracks are immediately redirected into the fiber direction and therefore affect less the stressed part. The input of energy is retarded. The reinforcement nodes (diaphragms) prevent the emerging longitudinal cracks from spreading over the entire tube length.
Particularly the compression, shear, and ply bond strength are increased by the node material. Such symptoms are called factors of increase of the fracture toughness. In the research of modern composite materials, the attention is focussed rather on preventing further crack spread by a specialized material structure than on preventing cracks themselves.

On the basis of the high concentration of silicic acid in the "bark" and the high density, bamboo is classified, according to the DIN 4102 (Burning behavior of building materials), as flammable but hardly combustible. The ignition susceptibility depends particularly on the position of the component, so horizontal components are less susceptible as diagonal or vertical components. On a horizontal bamboo cane, the flame spreads annularly to the next knot point (node). There the fire dies down because the flame cannot pass easily the hardly combustible node (diaphragm) to the next segment (internode).
If the internode bursts showing longitudinal and transversal cracks, the combustion is faster. Moreover, transversal cracks decrease the load-carrying capacity significantly (fatigue strength).
A bamboo tube, that is filled with water, can stand up to 400°C at the bottom side, while the water cooks in the tube.

Bamboo material (with its physical properties) is more than a match for timber; but only proper handling and application bring these advantages into effect.
Still in the eighties the dutchman Jules Janssen compared the level of knowledge about mechanical and technical properties of bamboo with the situation of timber construction approximately 100 years ago, as a technical application based on tradition led to stable, however, often very complicated and inefficient constructions.
The step of a Low-Tech-Material to an innovative building material, that wood already carried out, is still to be done by bamboo. An increase of research efforts leads to lower material demand and development of standards for a meaningful application of this new material.