The AS 1720.1:2010 accounts for factors that may vary the strength of timber in different environments and beam configurations through the use of modification factors.

In general the design capacity of a structural timber member is as follows:

R_d = Φ * k_mod * f_o’ * X

Where:

• Φ = capacity factors (defined in Clause 2.3)
• k_mod = product of modification factors (k₁ * k... * k_n)
• f_o’ = characteristic strength of material for type of design (e.g. characteristic tension strength)
• X =  geometrical section property related to calculation (e.g. cross-sectional area)

Different modification factors are used for the strength of a timber beam. Several of these properties are used for multiple different designs with the most common modification factor being the factor for load duration.

• k₁ – Factor for load duration
  • k₁ factor ranges from 0.57 to 1.00 for strength of timber
• k₄ – Factor for in-service absorption or desorption of moisture by timber
  • k₄ factor ranges from 0.7 to 1.0 for seasoned timber which may absorb moisture
  • k₄ factor ranges from 1.0 to 1.15 for unseasoned timber which may lose moisture
• k₆ – Factor for temperature / humidity effect
  • Ranges from 0.9 to 1.0 depending on location within Australia.
  • In Victoria, South Australia, New South Wales and Tasmania the k₆ factor is always 1.0
• k₇ – Factor for bearing length
  • Ranges from 1.00 to 1.75
• k₉ – Factor for load sharing in grid systems (is 1 for LVL and glulam)
  • Ranges from 1.0 to 1.33
• k₁₂ – Factor for stability to account for buckling of members in compression or bending
  • Ranges from 0 to 0.5 for slender timber members
  • Ranges from 0.5 to 1.0 for transition timber members
  • Is 1.0 for stocky timber members

The Australian Standard for timber gives the bending strength based on the following equation for unnotched beams:

M_d = Φ * k₁ * k₄ * k₆ * k₉ * k₁₂ * f_b' * Z

where:

• M_d = design capacity in bending
• f_b' = characteristic bending strength
• Z = elastic section modulus (Z = b d² / 6 for rectangular sections)

For an LVL joist with a characteristic bending strength of 50 MPa and a section size of 90 x 63 the design capacity would be:

M_d = Φ * k₁ * k₄ * k₆ * k₉ * k₁₂ * f_b' * Z * 50 MPa * (63 * 90 * 90 / 6)

= Φ * k_mod * 4.25 kN.m

Since Φand k_mod factors in this case are all less than 1 this gives us an upper bound solution to the timber beam bending strength as 4.25 kN.m .

By default the timber member type allows for combined bending and axial forces, however the inputs can be simplified by changing the "Loading" dropdown to change the member type to either a beam (only under bending and shear) or a column (only resisting axial forces).

The user can then input other values to quickly get the strength and utilisation of the timber beam or timber column. This can help with quick residential timber design for member.

The AS 1720.1 sets out the design for the following different types of wood and engineered timber:

• F-Grade sawn timber
• characteristic values for design specified in Table H2.1
• Machine Graded Pine (MGP)
• characteristic values for design specified in Table H3.1
• Laminated Veneer Lumber (LVL)
• characteristic values for design specified by manufacture
• Glulam
• characteristic values for design specified in Table 7.1
• Plywood
• characteristic values for design specified in Table 5.1
• Poles
• characteristic values for design specified in Table 6.1 and Table H2.1

The SkyCiv Timber Beam Calculator automatically pulls data from the tables in the AS 1720.1 saving engineers valuable time in the design of timber structures.

The SkyCiv timber beam calculator can be used to design any type of timber beam since it is a general tool. Common types of beams that get design using this calculator are:

• rafters
• noggins
• studs in light framing
• joists
• ridge beams
• timber bearers
• timber fences posts
• timber lintels
• timber floor beams

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SkyCiv offers a wide range of Cloud Structural Analysis and Design Software for engineers. As a constantly evolving tech company, we're committed to innovating and challenging existing workflows to save engineers time in their work processes and designs.