Introduction
This wood beam calculator allows the user to assess the structural integrity of timber beams to ensure compliance with the Australian Standard AS 1720.1:2010. The calculation will identify the design capacities of timber beams to meet flexural, shear and deflection design requirements to Ultimate Limit State (ULS) and Serviceability Limit State (SLS) methods.
❗This calculation has been written in accordance with AS 1720.
Calculation
⬇️ Inputs
Timber Properties
Timber species
:Ash, alpine
Seasoned or unseasoned?
:Seasoned
Stress grade
:F17
Hardwood or softwood
:Softwood
Moisture content:
EMC
:>15%
Specify EMC here
:20
Geometry
Loads
⬆️ Outputs
Timber strength group
:S4
Timber joint group
:J3Characteristic Values
Modification factors
Design Capacities
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Explanation
Timber Properties
Australian 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 understood and carefully considered when undertaking timber design calculations.
Seasoning
Harvested trees typically contain a lot of water. Once trees are cut and sawn, even more moisture is lost at a highly variable rate. Seasoning is the process of drying timber and removing moisture from the material at a uniform rate by exposing it to circulating air and controlled heat
Seasoned Timber: refers to timbers whose moisture has been dried out through a seasoning process. The moisture content of seasoned timbers typically varies between 10-15%. The process of seasoning can enhance the basic characteristic properties and dimensional stability of timber, increasing stiffness, bending and compression strength. Limiting moisture content to no more than 15%, as per seasoning recommendations in Australian Standards, allows timbers to have satisfactory performance with respect to appearance and stability.
Unseasoned Timber: refers to timbers not far enough along the drying process to be considered seasoned, and its moisture content varies anywhere between 25-100%. Moisture content should be an essential consideration in design as it can affect the outcome of a project. Given that unseasoned timber has a lot more moisture, a frame or building of unseasoned timber will be able to expand, contract and settle onto its weight, adapting to environmental conditions around it better than seasoned timber. If environmental conditions are an important factor in your project, consider unseasoned timbers, as they can settle into different environmental conditions without compromising structural integrity.
Types
Wood from trees is typically classified as either Hardwood or Softwood. You can be forgiven for thinking that Hardwood indicates the relative hardness of the wood, but this is often misleading as many hardwoods are relatively soft and vice versa. These are the main characteristic differences between Hardwoods and Softwoods:
Stress Grade
Structural timber is typically sold and distributed as a stress-graded product. Timbers used in structural applications are assigned a stress grade classification. These grades can be obtained from visual or machine grading. The grading specifies structural timbers' characteristic values of strength and stiffness and stress limits. Stress grades are generally known by either:
- F-Grades: F4 - F34 (i.e. F22 indicates that the bending stress timber is approximately 22 MPa, and it can withstand that force without excessive deflection). Most Hardwoods are ‘F’ graded, with the higher the grade, the stronger the timber. F-Grades for sawn timbers are given in Table H2.1 of Appendix H in AS1720.1, and F-grades for plywood as given in Table 5.1.
- Machine-graded pine (MGP): MGP10 - MGP15 (Structural pine is a plantation species that are generally graded by a machine that bends a wooden member with a set amount of force, measuring deflection. This sets the grading scale for MGP timbers). MGP grades and properties for sawn timber as given in Table H3.1 of Appendix H.
- Australian Ash (A17-grade): A-grades apply to a visually graded mixture of seasoned alpine ash and mountain ash. A17 grades and properties for sawn timber are given in Table H3.1 of Appendix H in AS1720.1.
- Glue-Laminated Timber (GL): GL timbers are engineered timbers whose stress grades and properties are as given in Table 7.1 in AS1720.1 or as detailed in a manufacturer's product specification
It is useful to know what a specific stress grade of timber means for your design. For example, if you require a beam that needs to span a longer distance with no intermediate supports, it would be logical that an F27 timber beam would be able to span further than an F17 beam of the same size without deflecting too much or breaking. In another scenario, you may choose a smaller piece of an F27-graded timber to perform as adequately as a smaller, weaker F17 beam.
Design Properties of Structural Timber Elements
Design capacities for all structural timbers are obtained by modifying characteristic capacities with factors appropriate to service conditions and material property type. This process applies to all types of timber.
Member Design Capacity (Rd)
Characteristic values (f'o)
Capacity Factor (ϕ)
Geometric Properties (X)
Modification Factors (Kmod)
kmod 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.
The determination of allowable permissible stresses in timber beams depends on modification factors. Once calculated, modification factors are multiplied by relevant characteristic properties and geometric properties to obtain the final values for limit states. The modification factors most relevant for Bending, Shear and Bearing capacities include:
k1
k4
k6
k7
k9
k12
Checking Limit States
AS1720.1 provides detailed guidelines for Ultimate Limit State (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. 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: