WPS 234 Factors affecting timber strength

Dead load & live load

• Dead load is self-weight (permanent).

• Live load is everything supported by, but not a permanent part of the structure.

Duration of load

• Significant loss of strength and stiffness when loaded over a long period of time.

• Wood members under load may lose about 40% of its strength and some wood based panel products about 80% of its strength over a ten-year period.

• DOL Clearwood: The load at which clear wood will fail after 10 years, is 58% of the short term strength.

• DOL Structural timber: Less affected. Affected by moisture content & cross-sectional area.

DOL cause

• Reduction in strength is caused by slippage in relative position of the long-chain cellulose molecules in the cell wall.

• The OH bonding sites that hold water or are mutually satisfied in dry wood, may shift or slide with respect to adjacent molecules when the whole matrix is placed under stress.

• The more water present within the wall, the more easily this slippage can occur.

Moisture content effect

• Compression strength most affected by moisture.

• Clearwood: Compression strength increase by nearly threefold from fibre saturation point to completely dry. Above fibre saturation point, which is around 28% moisture content, the strength stays relatively constant.

• Effect is highly species dependant.

• Structural timber: Less affected. Only compression strength significantly affected in 5th percentile.

Moisture content affect cause

• Above fibre saturation point water is present in the both the cell cavities and cell walls of wood. As the wood gets drier water is first removed from the cell cavities. This water is not bound to the cell structure and is called free water. There is no effect on the strength when removal of water takes place from cell cavities.

• Water in cell walls is chemically bonded (hydrogen bond) with the OH groups of non-crystalline cellulose, hemicellulose and the lignin matrix constituents. When the moisture content drops below the fibre saturation point, these hydrogen bonds are broken, the water is removed and the microfibrils come into closer proximity. Interfibrillar bonding then occurs between the available OH groups of different fibres. These bonds increase the strength of the wood material.

• This process is nearly completely reversible.

Pressure treatment affect

• Pressure treatment with waterborne chemicals like CCA affects the strength of wood and timber negatively.

• Some of the OH groups of the cellulose and hemi-cellulose that was bound to water lose its ability to bond with each other after the wood has been treated and dried.

Stressed volume or size effect

• Weakest link theory: when in tension, a chain is only as strong as its weakest link.

• For 2 pieces of timber that are identical, but have different lengths, the longer piece is more likely to have a major defect, therefore it is weaker.

• Depth (height) also displays size effect. Thickness does not.

• 5th percentile timber:

  • Bending: length

  • Tension: length & depth

  • Compression: similar to tension

Load sharing

• A load is seldom carried by a single member in a system even if it is placed right on top of that member.

• For 5th percentile wood, since there is a reasonably good relationship between elasticity and strength in timber it means that the weaker members will deflect more than the stronger members. In effect the stronger members will then carry a larger portion of the load than the weaker members.