Designing for Durability - Introduction
Durability can be defined as the capacity of a timber product, component, system, building or structure to perform its function for a specified period of time.
Durability of timber and durability classes
In general, there are two main factors that influence the durability of timber in service. The first is the natural durability of the particular species. The second is the type and degree of hazard to which the timber is exposed. Classes 1-4 are described.
Design for Durability - Fastener Selection
Fastener selection is important when detailing joints for durability.
Design for Durability - Glued Products
To achieve good durability from glued products, the species of timber used must have appropriate durability and a suitable adhesive must be used.
Durability and the control of moisture change in timber structures
If a structure is to be durable, the design of all timber joints and surfaces needs to recognise the factors that influence the moisture content of timber and changes that occur due to variations in that moisture content.
Termites are a major destroyer of timber structures in Australia. This article shows three means of excluding them from buildings.
Design for Durability - Shielding
Timber exposed on the outside of a building can be protected by any means that reduces extremes of sunlight, temperature, wind and moisture, and encourages regular ventilation. These include natural vegetation and physical shields such as a roof or flashing.
Design for Durability - Type of Member
The type of member selected for a particular application can be a deciding factor in the durability of structures. For example, glued laminated timber or sheet products exposed to the weather will shrink, swell and undergo checking along the glue lines unless adequately protected from the weather.
Design for Durability - Isolation
Timber must be physically isolated from concrete or masonry as they can act as sources of moisture.
Design for Durability - Moisture Traps and Contact Areas
Moisture traps should be avoided, particularly where connections and joints are exposed to the weather.
Ventilation of internal spaces
To avoid problems with fungal decay, high humidity air within the building envelope must be dispersed by ventilation.
If warm, high humidity air from within a building is allowed to move into colder areas within the building envelope, condensation may result. If this condensation occurs on timber, then the timber will be susceptible to fungal decay.
Design for durability - Shrinkage Restraint
Shrinkage restraint at joints needs careful consideration, particularly where unseasoned timber is used. If shrinkage restraint occurs, stresses may be induced perpendicular to the grain, causing splitting and subsequent moisture ingress or loss of structural integrity.
Design for Durability - Corrosion Resistance of Fasteners
The interaction of moisture and chemicals on metals can cause breakdown of wood fibres around metal fasteners. This breakdown can result in additional moisture traps and loosening of joints with a propensity for decay.
Allowance for movement due to shrinkage
Allowance often needs to be made for shrinkage and differential movement at connections and in construction.
Design for Durability - Finishing
Paints, stains and water repellents protect timber from moisture as they act as vapour barriers.
Durability - Timber Grades and Size
Australian Standard grading rules usually provide a range of grades that can be selected to suit specific applications.
Design for Durability - Maintenance
All structures require maintenance, unless designed and specified for very specific purposes or for a short life span. This is particularly relevant for the external envelope of a building or structure.
Design for Durability - Specifications
Following is a check list of some items which should be included in the specifications, considered at the design stage, or indicated on drawings.
Hazard Conditions and Protection - Chemical
Some softwoods such as cypress pine are highly resistant to a wide variety of chemicals in reasonable concentrations at ambient temperatures and are widely used for chemical storage and processing.
Lyctid borers are small wood-eating beetles that infect the starch rich sapwood of some hardwood timbers. They are also known as "powder post borers". The most common lyctid in Australia is Lyctus brunneus, but not all lyctids are of the Lyctus genus.
The Anobium borer (furniture beetle)
Most instances of attack by the Anobium borer in Australia occur in old furniture and old Baltic pine flooring.
Hazard Conditions and Protection - Introduction
Timber is a permanent structural material given proper design, construction, use and maintenance. Protected from weathering, moisture, insects and strongly corrosive chemicals, timber has performed satisfactorily for centuries.
Weathering of timber
Weathering is the breakdown of the exposed surfaces of a piece of timber due to action of sunlight, rain, wind, dew, frost and fumes. These cause deterioration and erosion of the surface cells through degradation of the lignin that binds the wood cells together and sugars, starches and extractives are all washed away from exposed timber surfaces.
Damage to timber by insects
Timber structures are best protected from damage by insects through proper design and construction procedures, and accurate specification including species selection and preservative treatment where necessary.
Fungi can be broadly grouped into two main types, based on whether they affect the strength properties of timber. Moulds and stains (i.e. blue stain) usually only mar appearance (blue stain can effect impact strength) whereas rot fungi (white rot, brown rot and soft rot) can significantly effect strength properties as they penetrate or break down the cell walls of wood.
Chemical resistance of timber
Timber offers considerable resistance to attack from a wide variety of chemicals including organic materials, dilute acids and hot or cold solutions of acid or neutral salts. Direct contact with caustic soda should be avoided. Strong acids and alkalis will slowly destroy timber.
Marine hazards for timber
Marine piles are subject to three zones of durability hazard. The bottom ends are usually embedded in mud and are free from hazard while the parts above the high water mark have to withstand weathering and abrasion. The greatest hazard occurs in the zone between the mud line and the high water mark where marine organisms are active.
Hazard levels and the durability of timber
To enable selection of appropriate natural durability and preservative treatment, hazard levels have been generalised and defined by AS 1604.
Design Life and Reliability
The design life required by clients for various structures or structural elements will vary considerably depending upon the nature, use and cost of replacement of the building or structure.
Performance Requirements for Durability - Costs
When deciding upon durability performance requirements, initial material and finishing costs must be balanced against long term maintenance and/or repair and replacement costs.
Preservative Treatments - Copper azole
Developed as an environmentally friendly alternative to CCA treatment for less demanding applications, this treatment uses copper and an organic azole co-biocide as active ingredients.
Uses of Treated Pine
Treated pine is used extensively above and below ground for many uses. Some of these are listed in this article.
Introduction to Preservative Treatments
Timber is an organic product and in the presence of sufficient moisture and air will be attacked by fungus, causing decay. Preservative treatments can be introduced into the cellular structure of timber to protect it from fungi and insects. Preservative treatments may also include fire retardant chemicals. These are frequently applied to linings, joinery and cladding.
Is Preservative Treatment Necessary?
When deciding whether to specify preservative treatment, certain factors must be considered.
Preservative Treatment Requirements
Research and in-service trials have shown that different standards of preservative treatment are required to prevent degrade in various hazard conditions. When deciding upon the level of preservative treatment, hazard levels should be specified in accordance with AS 1604.
Use of preservative treated timber
Use of preservative treated timber requires special care during construction to maintain the efficiency of the treatment.
Preservative Treatment of Reconstituted Timber Products
Many reconstituted timber products can have preservative chemicals incorporated in the manufacturing process. These include plywood, LVL, particleboard, and MDF