cut sizes of timber

IS 1331: Scope - Key Formulas, Tables & Specifications

1. Dimensional Change due to Moisture (Clause 3.2)

• For timber with specific gravity ≥ 0.6 or dimensional stability < 60% of teak:

  [ \text{Dimensional change} = 0.3% \times (\text{Moisture content} - 20%) ]

• Here, 20% is the standard moisture content.

2. Permissible Defects for Structural Timber (Clause 5.2.2, Table 2)

3. Sizes of Cut Timber for Stocking (Clause 3.2, Table 1)

• Thickness ranges from 1.0 cm to 20 cm.
• Widths available from 4 cm up to 30 cm.
• Preferred widths marked with "x" in the table (e.g., 10 cm thickness with 12 cm width preferred).

Summary

• Use 0.3% per 1% moisture difference for dimensional change.
• Follow defect limits per grade for structural use.
• Use standard cut sizes as per Table 1 for stocking.

flowchart TD
    A[Moisture Content] --> B[Calculate Dimensional Change]
    B --> C{Specific Gravity ≥ 0.6?}
    C -- Yes --> D[Use 0

IS 1331 - Key Definitions & Specifications

• Reference for Definitions:
  Clause 2.1 states that definitions are as per IS 707-1968 (Timber - Classification and Terminology).

• Permissible Defects (Clause 5.2.2 & Table 2):
  Defects allowed in structural timber are limited by grade (1, 2, 3) and include:

  • Wane:
    • Grade 1: max 1/8 of surface width
    • Grade 2: max 1/6 of surface width
    • Grade 3: max 1/4 of surface width
  • Worm holes: Permissible except post beetle damage.
  • Slope of Grain:
    • Grade 1: max 1 in 20
    • Grade 2: max 1 in 15
    • Grade 3: max 1 in 12
  • Live Knots: Size limits vary by timber width and grade (see table below).
  • Checks and Shakes: Permissible widths and depths vary by timber size and grade.

• Dimensional Stability (Clause 3.2):
  For species with specific gravity ≥ 0.6, dimensional change is 0.3% per 1% moisture content change from 20%.

Summary Diagram of Permissible Defects by Grade

graph TD
A[Timber Defects] --> B[Wane]
A --> C[Worm Holes]
A --> D[Slope of Grain]
A --> E[Live Knots]
A --> F[Checks & Shakes]

B --> B1[Grade 1: 1/8 width]
B -->

IS 1331: Dimensions and Tolerances for Structural Timber

1. Permissible Tolerances on Cut Sizes (Clause 3.5.1)

• Width & Thickness tolerances:
  • Up to 100 mm: ±1.8 mm
  • Above 100 mm: (Not specified here; refer to IS 707 for details)

2. Preferred Stock Sizes (Clause 3.2 & Table 3.5)

• Sizes are in cm.
• Thickness ranges from 1.0 cm to 20 cm.
• Widths vary from 4 cm to 30 cm.
• Preferred widths are marked with x in the table.

3. Dimensional Changes Due to Moisture (Clause A-3.1)

• For timber with specific gravity < 0.6:

  [ \Delta d = 0.2% \times \Delta M ]

  Where:

  • (\Delta d) = % change in dimension
  • (\Delta M) = difference in moisture content from 20%

Summary Table: Tolerances on Cut Sizes

flowchart LR
    A[Start: Timber Cutting] --> B{Width/Thickness ≤ 100 mm?}
    B -- Yes --> C[Apply ±1.8 mm tolerance]
    B -- No --> D[Refer IS 707 for tolerance]
    C --> E[Check Preferred Sizes from Table 3.5]
    D --> E
    E --> F[Adjust for Moisture Content]
    F --> G[Calculate Dimensional Change: 0.2% per

IS 1331: Grading for Non-Structural Use (Clause 4.3 & 5.3)

Timber for non-structural use is classified into:

• Grade 1
• Grade 2

Key Specifications:

Live Knots & Checks Table (Width in mm):

Notes:

• Permissible defects reduce timber strength but are allowed within limits for non-structural use.
• Refer to Clause 5.3 for detailed defect definitions and limits.
• Structural grades (3 grades) have stricter limits than non-structural (2 grades).

flowchart LR
    A[Timber Cut Sizes] --> B{Use Type}
    B -->|Non-Structural| C[Grade 1 & Grade 2]
    B -->|Structural| D[Grade 1, 2 & 3]
   

IS 1331: Permissible Defects & Grading Criteria for Timber

1. Grading for Structural Use (Clause 4.2)

• Timber is classified into Grade 1, Grade 2, and Grade 3 based on permissible defects.
• Approximate strength reduction correlates with defect severity per grade.

2. Permissible Defects for Structural Use (Table 2, Clause 5.2.2)

Live Knots & Checks (Max size in mm) Example for 100 mm wide timber:

3. Permissible Defects for Non-Structural Use (Table 3, Clause 5.3.2)

IS 1331: Marking and Sizes of Cut Timber

Key Specifications:

• Standard sizes of cut timber for stocking are given in Table 3.5 (Clause 3.2), with thickness and width in cm.
• Sizes are specified at 20% moisture content (Clause 3.2.1).
• Other sizes may be produced for specific demands or to optimize log utilization.

Marking Requirements (Clause 6.1 & 6.1.1):

• Each timber piece must be marked at one end with:
  • Size
  • End use
  • Manufacturer's name or trade-mark
• Optional: ISI Certification Mark can be added, indicating compliance with IS standards and quality control.

Moisture Content Adjustment (Appendix A):

• Dimensions are standard at 20% moisture content.
• No dimensional adjustments along the grain for moisture content variations.

Example Table Extract (Thickness × Width in cm):

This ensures uniformity in timber sizes and traceability for quality assurance.

IS 1331: Method for Adjustment of Timber Dimensions at Different Moisture Content

• Standard Reference Moisture Content: 20% (Clause 3.2.1, A-1)

• Adjustment for Dimensions Along Grain:
  No adjustment needed regardless of moisture content change (Clause A-2).

• Adjustment for Cross-Sectional Dimensions:
  Allowances apply irrespective of radial or tangential directions (Clause A-3).

• Formulas for Dimensional Change:

• Where:
  • D_20 = dimension at 20% moisture content
  • MC = actual moisture content (%)
  • ± = plus if MC > 20%, minus if MC < 20%

Summary:

• No change in length (along grain).
• Cross-sectional dimensions vary by 0.2% or 0.3% per 1% MC change depending on species SG.

flowchart TD
    A[Dimension at 20% MC (D_20)] --> B{Species SG}
    B -->|< 0.6| C[Use 0.2% per 1% MC difference]
    B -->|≥ 0.6| D[Use 0.3% per 1% MC difference]
    C --> E[Calculate D_adj = D_20 × (1 ± 0.002 × (MC - 20))]
    D --> F[Calculate D_adj = D_20 × (1 ± 0.003 × (MC - 20))]

This method ensures accurate sizing for timber at varying moisture contents, crucial for design and