WPS 234 Other factors affecting timber

Dimensional stability: shrinking / swelling of timber

• Wood is a hygroscopic material, meaning that it takes up or sheds water to the air in order to stay in equilibrium with the surrounding atmosphere.

• As soon as timber is dried to a MC below the fibre saturation point it will start to shrink.

• Rate of shrinkage is roughly linear from the FSP up to 0% moisture content.

• Nearly completely reversible.

Reason for shrinking / swelling

• The same mechanism that causes strength increase with moisture content decrease (below FSP) is responsible for the shrinkage and swelling of timber.

• Above FSP water is present in the cell cavities and cell walls. The water in the cell walls is chemically bonded (hydrogen bond) with the OH groups of the noncrystalline cellulose and lignin matrix constituents. The water in the cell cavities is so-called free water and will be removed before the water in the cell walls. When the MC reaches the fibre saturation point, all the water has been removed from the cell cavities. The hydrogen bonds of the water in the cell walls are broken, the microfibrils come into closer proximity, and interfibrillar bonding occurs. This also causes shrinkage.

Amount of shrinkage / swelling

• Density & microfibril angle are biggest factors. Higher density causes higher deformation. Higher MFA causes less tangential/radial shrinkage, but more longitudinal shrinkage.

• Tangential shrinkage: 4-10%. Roughly linear over MC. Radial shrinkage is a little less.

• Longitudinal shrinkage: 0.1%. Insignificant.

• SA Pine and Eucalyptus grandis has highest tangential and radial shrinkage.

Practical implications of shrinkage / swelling

• EMC varies between geographic regions and seasons. Subsequent shrinking or swelling may weaken joints or cause cracks in products like furniture or structural products.

• In large constructions like roofbeams, shrinkage also needs to be taken into account – especially when wet, unseasoned timber is used.

• eg. For a hanger the bolt should be placed at the bottom so that if shrinkage occurs, the bolt will not carry the weight of the beam.

Durability

• Timber degrades over time if designed incorrectly.

• Caused by:

  • Biological agents (insects, fungi). Most important.

  • Light

  • Abrasion

  • Chemical agents

• Requirements for fungal growth: MC > 20%, oxygen, moderate to warm temperature. Moisture easiest to control:

  • Most important is to eliminate timber contact with stationary water, especially end-grain contact.

  • High roof overhangs, ventilation, no small openings trapping water, no ground water, low interior condensation, slopes, and caulking greatly reduce degradation.

• Chemical and surface treatments prevent degradation, but less important than design.

Creep

• Increase of deformation over time. Not same as elastic deformation.

• Closely related to the duration of load effect.

• Partly recoverable.

• High or cyclic moisture conditions increase creep. Cyclic more important.

• Recovery increased by moisture cycling.

• Reducing creep: reduce MC variation, load, and MC.

Fire

• Timber can be safer than steel in fires.

• Affects of heat and fire:

  • Timber heated to 100-200C decomposes wood components slowly.

  • Heating without oxygen is called pyrolysis, which forms H20, CO, CO2. Accelerated between 200-350C, which produces flammable gases.

  • Fire is self-sustaining above 270C.

  • Char produced is good thermal insulator making heat produced at some stage insufficient to produce flammable gasses and the will flame die.

Increasing fire resistance

• Heavy timber greatly increases resistance. Lower char rate. Wood burns longer making enough time for char to have affect.

• Managing light timber sections.

• Fire walls: seperate buildings.

• Fire blocking: filling gaps between pieces of timber to eliminate oxygen.

• Draftstopping.

• Firecuts: cut that causes timber to break before causing surrounding structures to break.

Extra factors

• Fatigue

• Sound insulation

• Thermal insulation

• Vibrations of timber floors

• Finishes and maintenance of timber structures