1, water content

The strength of wood is greatly influenced by water content. When the moisture content of wood changes above the fiber saturation point, only the free water changes, which has no effect on the wood strength; When the moisture content of wood changes below the fiber saturation point, with the decrease of moisture content, the adsorbed water decreases, the cell wall tends to be dense and the wood strength increases. On the contrary, the strength of wood decreases. Moisture content has different effects on various strengths of wood. It has a great influence on the compressive strength and bending strength along the grain, a little influence on the shear strength along the grain and a little influence on the tensile strength along the grain.

2. Ambient temperature

Environmental temperature has a direct influence on the strength of wood. When the temperature of wood increases, the components that make up the cell wall will gradually soften and the strength will decrease. Under normal climatic conditions, the increase of temperature will not cause the change of wood chemical composition, and when the temperature decreases, the wood will recover its original strength. But when the wood is at 40℃~60℃ for a long time, the wood will slowly carbonize; When the wood is kept at 60℃ ~ 100℃ for a long time, it will cause the evaporation of wood moisture and volatiles. When the temperature is higher than 100℃, wood begins to decompose into its chemical elements. When the ambient temperature drops below 0℃, the moisture in the wood will freeze and the strength will increase, but the wood will become brittle. Once thawed, the strength of wood will be lower than that without freezing.

3. Loading time

The length of time that the load acts on the structure has a great influence on the strength of wood. The maximum stress that wood can bear under long-term load is called the durable strength of wood, which is only 50% ~ 60% of the ultimate strength of wood under short-term load. This is because wood will have great creep under long-term load, and with the increase of time, it will produce a lot of continuous deformation and damage. Wood structure is generally under long-term load, so the design of wood structure is usually based on the durability of wood.

4, the defects of wood

Wood defects, such as knots, twill, cracks, moth-eaten, rot, etc. , it will cause the discontinuity and unevenness of wood structure, thus reducing the strength of wood.