A common problem faced by engineers when designing structures is identifying relevant information from standards and applying them to their projects. Each design standard contains a vast amount of information; therefore, time and resources may be wasted in searching and selecting bits that are useful for design. This article aims to aid engineers in such a situation by providing a guideline on the design procedure of timber structures according to the Australian Standards, namely AS1648, and AS1720. Fundamental equations, calculation methods for different timber materials, and important design parameters are explained in the following sections. Ultimate strength and serviceability limit state designs are extensively discussed, and any shortfalls in the standards that need improvement are pointed out.
Australian Standards also present guidelines to be followed during the manufacture of timber materials. These documents set standardized testing and grading methods and outline minimum requirements for material properties (such as moisture content, modulus of elasticity, etc.). All timber materials manufactured in Australia must satisfy conditions outlined in the standards and be appropriately graded before they apply to the actual design of timber structures:
- AS 2082/2858 - Visually graded sawn timber
- AS/NZS 1748 - Mechanically graded timber
- AS 3519 - Proof-graded timber
- AS/NZS 2269 - Structural plywood
- AS/NZS 4357 - Structural laminated veneer lumber
- AS/NZS 1328.1 - Glued laminated timber
- AS 3818.3/3818.11 - Round timber
Ultimate Strength Limit State (ULS)
AS 1684 and AS 1720 provide detailed, comprehensive guidelines for ULS design. The calculation methods for design bending moment, shear, and bearing capacities of structural timber members, joints, and fasteners are robust enough to account for different scenarios and environmental factors. According to AS1720 Clause 2.1.2 and 2.1.3, the design capacities of members and joints must satisfy the following limit:
Design Capacity Rd and Design Action R*
Rd is the design capacity of the timber member/joint, or the maximum amount of load that a particular element can take before failure. Rd must be larger than R*, the design action, or the maximum load that the element is expected to experience during its usage. The design action R* is calculated following load combinations outlined in AS1170, which accounts for different types of loads acting on the structure, such as wind, snow, live and dead loads.
Capacity Factor ϕ
Capacity factors are used to upscale or downscale the design capacity of structural elements. In AS1684 and AS1720, the ϕ values for timber members/joints are selected based on their application. Higher values are used for structural elements deemed to cause less danger upon failure.
Modification Factor Kmod
the mod is the product of a combination of modification factors (k1 x k2 x … kn) that affect the capacity of the member/joint. These various factors depend on the placement and orientation of the element, loading conditions, and material properties. The engineer should select and change k values as necessary for their design.
Modification factors in AS1684 and AS1720 account for a large range of variables and scenarios, which allows flexible adaptation of parameters.
Characteristic Capacity f’oX
f’oX is the characteristic capacity for the relevant design action in question (bending, tension, compression, shear, etc.). For example, when designing for bending moment capacity, f’o = f’b (characteristic value in bending for the section size) and X = Z (section modulus of the beam about the bending axis).
Characteristic material strength is defined as the strength below which no more than 5% of the results are expected to fall, as assessed by a standardized test. When timber is manufactured, it undergoes tests, and grades are allocated according to the results. In Australian standards, timber is graded by its strength in shear, bending and tension, and compression parallel to the grain.
A unique property of timber is that strength varies depending on the direction of the applied load. Due to the cellular structure of wood, engineered wood products are stronger when loaded parallel to the grain compared to perpendicular. AS1720 Section 3 accounts for this phenomenon through the provision of design tension, compression, and torsion capacity formulae in both directions.
Serviceability Limit State (SLS)
The major shortfall in AS1684 and AS1720 is the lack of comprehensive consideration for deflection and vibration control. Timber is lighter and proportionally less stiff compared to reinforced concrete and steel; hence, it is extremely prone to deflections and vibrations when subject to similar load patterns. Timber’s lighter weight leads to oscillations that can cause discomfort to occupants, and lower stiffness leads to larger deflections. Additionally, timber is an environment-sensitive material that is heavily influenced by temperature and moisture content in the atmosphere. Therefore, timber structures must be constructed to ensure satisfactory performance and safety throughout their service life, accounting for variance in material properties over time. In fact, serviceability is often the governing limit state for timber elements rather than ultimate strength.
For the calculation of deflections, AS1720 suggests the use of upper-bound estimates obtained through elastic analysis methods. The upper-bound deflection limit is found using the lower 5th percentile estimate elasticity of modulus, E0.05 (generally, a range of E values are attained whilst grading through tensile tests). In cases where this information may be unavailable, the standard recommends calculating it as a proportion of the average value of the modulus of elasticity:
While these estimates may be useful for smaller projects like residential complexes, they are not appropriate for large-mass timber structures. Upper-bound estimates tend to greatly under-estimate the limits, and engineers may design conservatively, leading to unnecessarily thicker and stronger elements. This can result in dramatically increased cost and time demands on a project.…even more than usual!
In contrast, AS1684 provides tables with “structural models” that are used to determine span-relative deflection limits. The structural models are categorized according to the load type (e.g. distributed or concentrated, live or permanent, etc.), and then modification factors and deflection limits are selected based on the load category. Section 2 of AS1684.1 covers deflection limits for the design of timber roof members in residential buildings, which includes roof battens, rafters, underpurlins, strutting beams, ceiling battens, and joints, as well as hanging, counter, and verandah beams. An example of structural models and load categories for serviceability design from AS1684 Clause 126.96.36.199 is shown below:
Vibration control measures are covered briefly in AS1684 and AS1720. Vibrations induced by machinery and human activities (walking, running, dancing, etc.) cause the structure to sway and oscillate, which may be fatal if the frequency of this oscillation matches the natural frequency of the structure. The standards state that ‘the dynamic response of floor systems, including frequency of vibration, should be considered…’ but no equations or guidelines are provided.