Timber Design Standards - AS1720 and AS1684

IntroductionA 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
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∗
﻿
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)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
Vibration
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Timber Design Standards - AS1720 and AS1684

Introduction

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

Introduction

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

$N_{d, p}$