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How are timber members designed to resist deflection?

Timber has been used as a construction material for centuries, and it continues to be popular today due to its natural beauty, versatility, and sustainability. One of the key design considerations for timber members is their ability to resist bending. In this article, we will discuss how timber members are designed to resist bending.

Understanding bending in timber members


Bending is a common form of stress that occurs in timber members. When a load is applied to a timber member, the member will bend or deflect, causing stresses in the fibers of the wood. The stresses are highest at the outermost fibers of the member, and they decrease towards the center of the member.


Designing for bending stresses

To design timber members to resist bending stresses, several factors must be considered. Some of the key factors include:


  1. Timber species: Different species of timber have different mechanical properties, such as stiffness and strength, which affect their ability to resist bending stresses. The designer must choose an appropriate timber species based on the design loads and the required structural performance.

  2. Cross-sectional shape: The cross-sectional shape of the timber member affects its ability to resist bending stresses. A rectangular cross-section is the most common shape, but other shapes, such as circular or elliptical, can also be used.

  3. Cross-sectional dimensions: The dimensions of the cross-section, such as the height and width of a rectangular section, determine the moment of inertia of the member, which affects its stiffness and ability to resist bending stresses.

  4. Load type and magnitude: The type and magnitude of the loads that the member will be subjected to must be considered in the design. Dead loads, such as the weight of the member itself, and live loads, such as the weight of people or furniture, must be considered.

  5. Support conditions: The support conditions of the member, such as whether it is supported on one or both ends, affect its ability to resist bending stresses.

  6. Design codes and standards: Design codes and standards provide guidelines for designing timber members to resist bending stresses. Some of the key standards that are used for designing timber members include the National Design Specification for Wood Construction (NDS) and the Timber Design Code.

In addition to designing timber members to resist bending stresses, it is also essential to design them for durability. Timber is susceptible to decay and insect attack, which can reduce its strength and affect its ability to resist bending stresses. The designer must consider factors such as the moisture content of the timber, the treatment of the timber, and the environmental conditions that the member will be exposed to.


Designing timber members to resist bending stresses is a critical consideration in timber construction. The designer must consider factors such as timber species, cross-sectional shape and dimensions, load type and magnitude, support conditions, and design codes and standards. By designing timber members to resist bending stresses and ensuring their durability, engineers can create long-lasting, sustainable structures that are both functional and beautiful.

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