Mechanical Properties of Wood

Mechanical Properties of Wood

Introduction

• This lecture provides an introduction to the mechanical properties of wood, essential for structural design.

Orthotropic Nature of Wood

• Wood is idealized as an orthotropic material, meaning it has different mechanical properties in three mutually perpendicular directions.
  • Longitudinal: Along the grain (fiber direction).
  • Radial: Perpendicular to the grain, outward from the center of the tree.
  • Tangential: Perpendicular to the grain, tangent to the growth rings.

Importance of Loading Direction

• The direction of loading significantly affects the strength of wood.
• Wood exhibits different strengths when loaded parallel to the grain versus perpendicular to the grain.

Strength Determination

• The strength of a wood piece is determined based on the relationship between deformation and strength or force and strength, which has been previously established.

Factors Affecting Strength

• Several factors influence the mechanical properties of wood, including:
  • Species
  • Grade
  • Moisture condition

Data on Wood Properties

• The following table presents typical values for various wood species and grades under different moisture conditions:

Dry Condition (m/c = 16%)

Green Condition (m/c = 25%)

Notes:

• No.1 Framing is not verified for strength and stiffness properties but graded to NZS 3631.
• Green condition values are used when the moisture condition is 25% or over, meeting NZS 3602:2003 durability requirements.
• Shear strength for dry Radiata pine: $f_s = 3.8$ MPa.
• Shear strength for dry Douglas fir: $f_s = 3.0$ MPa.
• Compression perpendicular to grain for dry Radiata pine and Douglas fir: $f_p = 8.9$ MPa.
• Modulus of rigidity: $G = E/15$.
• Shear strength for green Radiata pine: $f_s = 2.4$ MPa.
• Compression perpendicular to grain for green Radiata pine: $f_p = 5.3$ MPa.
• VSG10 and VSG8 are visual grades verified in dry conditions; G8 is verified in green conditions.

Additional Factors Affecting Strength

• Load duration
• Service conditions (wet or dry)
• Treatment

Load Duration Effect

• As load duration increases, the strength of wood decreases.

Design Considerations

• The design strength ($f_{design}$) is a function of various factors including load duration.

Load Duration Factor

• The design formula is: $f<em>{design} = f(k</em>1 …)$, where $k_1$ is the load duration factor.

Table 2.4: Duration of Load Factor, $k_1$

Moisture Condition Effect

• $f<em>{design} = f (k</em>1 k_2 …..)$
• $k_2$ accounts for moisture condition and deflection during load duration.
• Some standards provide $f<em>{dry}$ and $f</em>{wet}$ as two different values.

Design of Members in Sawn Timber (NZS 3603)

Basis of Design

• $S* ≤ φ R_n$
  • $S*$: Imposed design action
  • $φ$: Strength reduction factor
  • $R_n$: Nominal resistance

Nominal and Actual Properties of Sawn Timber

• The following table provides nominal and actual dimensions, area, weight, section modulus, and second moment of area for various sawn timber sizes:

Strength Reduction Factors - Φ

• The strength reduction factor, $φ$, has the following values:
  • For timber, poles, and glulam: $φ = 0.8$
  • For nails in lateral loading: $φ = 0.8$
  • For toothed metal plate connectors: $φ = 0.8$
  • For other types of fasteners: $φ = 0.7$
  • For plywood: $φ = 0.9$
  • For actions derived from the strength of ductile elements under large displacements: $φ = 1.0$
  • Design for fire resistance: $φ = 1.0$