Timber Design Standards - AS1720 and AS1684

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 (residential timber code), and AS1720 (general structural timber code). 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. 
Guidelines for the manufacture of timber materials
This design guide relates to our Timber Beam Calculator to AS1720.1 and Timber Column Calculator to AS1720.1.
Timber PropertiesAustralian Standards set out guidelines and minimum requirements for material properties, design parameters, design procedures and calculation methods. Here, we explain some pertinent timber properties that should be considered carefully when undertaking timber design calculations. 
☀️ Seasoning
🌳 Types
🎓 Stress Grade
Ultimate Strength Limit State (ULS)AS1684 and AS1720 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:
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Rd>R∗ WhereRd=design capacityR∗=imposed design actionR_d>R^* \\\ \\ \text{Where} \\ R_d= \text{design capacity} \\ R^* = \text{imposed design action}Rd​>R∗ WhereRd​=design capacityR∗=imposed design action
Explore the toggles below to read more about the ULS design parameters.
Design Capacity ﻿Rd​
 and Design Action ﻿R∗
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Capacity Factor ﻿ϕ
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Modification Factors ﻿kmod​
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Characteristic Capacity ﻿fo′​
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Geometric Properties ﻿X
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Compression & Tension Capacity ﻿Nd,c​
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Bending & Shear Capacity ﻿Md​
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Permissible allowable stress limits or design limit capacities of timber beams are then calculated for each check by multiplying the relevant characteristic properties, modification factors and geometric properties together:
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Design capacity in bending: Md=ϕk1 k4 k6 k9 k12 fb′ ZDesign capacity in shear: Vd=ϕk1 k4 k6 fs′ As\text{Design capacity in bending: } M_d = \phi k_1 \space k_4 \space k_6 \space k_9 \space k_{12} \space f'_b \space Z \\\text{Design capacity in shear: }  V_d = \phi k_1 \space k_4 \space k_6  \space f'_s \space A_s Design capacity in bending: Md​=ϕk1​ k4​ k6​ k9​ k12​ fb′​ ZDesign capacity in shear: Vd​=ϕk1​ k4​ k6​ fs′​ As​
The modification factors most relevant for Bending, Shear and Bearing capacities include:
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Bearing Capacity ﻿Nd,p​
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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.
Explore the toggles below to read about two of the SLS design criteria. 
Deflection
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:
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AS1720.1 Appendix B
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 2.1.3.2 is shown below:
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AS1684.1 Table 2.1.5
Vibration
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.
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Timber Design Standards - AS1720 and AS1684

Guidelines for the manufacture of timber materials

Timber Properties

☀️ Seasoning

🌳 Types

🎓 Stress Grade

Ultimate Strength Limit State (ULS)

Design Capacity

$R_{d}$
and Design Action

$R^{*}$

Capacity Factor

$ϕ$

Modification Factors

$k_{m o d}$

Characteristic Capacity

$f_{o}^{'}$

Geometric Properties

$X$

Compression & Tension Capacity

$N_{d, c}$

Bending & Shear Capacity

$M_{d}$

Bearing Capacity

$N_{d, p}$

Serviceability Limit State (SLS)

Deflection

Vibration

CalcTree

Timber Design Standards - AS1720 and AS1684

Guidelines for the manufacture of timber materials

Timber Properties

☀️ Seasoning

🌳 Types

🎓 Stress Grade

Ultimate Strength Limit State (ULS)

Design Capacity ﻿Rd​ and Design Action ﻿R∗﻿

Capacity Factor ﻿ϕ﻿

Modification Factors ﻿kmod​﻿

Characteristic Capacity ﻿fo′​﻿

Geometric Properties ﻿X﻿

Compression & Tension Capacity ﻿Nd,c​﻿

Bending & Shear Capacity ﻿Md​﻿

Bearing Capacity ﻿Nd,p​﻿

Serviceability Limit State (SLS)

Deflection

Vibration

CalcTree

Design Capacity

$R_{d}$
and Design Action

$R^{*}$

Capacity Factor

$ϕ$

Modification Factors

$k_{m o d}$

Characteristic Capacity

$f_{o}^{'}$

Geometric Properties

$X$

Compression & Tension Capacity

$N_{d, c}$

Bending & Shear Capacity

$M_{d}$

Bearing Capacity

$N_{d, p}$