preferred cut sizes of structural timber - Specification

IS 4891: Introduction - Key Specifications & Tables

IS 4891 covers preferred sizes of structural timber for various applications like roof trusses, partition framing, and covering.

1. Preferred Sizes of Structural Timber

2. Notes:

• For truss spans slightly above 20 m, preferred sizes may be allowed.
• Moisture content affects timber dimensions; Appendix A provides adjustment methods.

3. Moisture Content Adjustment (Clause 2.3.4)

• Dimensional changes due to moisture are accounted for by multiplying nominal dimensions by a correction factor (see Appendix A).

Summary Table Example (Roof Trusses):

For detailed design, refer to IS 4891 Appendix A for moisture adjustment and full tables for all sizes.

flowchart LR
    A[Timber Size Selection] --> B{Application}
    B -->|Roof Trusses| C[Use Table 1 Sizes]
    B -->|Partition Framing| D[Use Table 3 Sizes]
    B -->|General Structural| E[Use Table 2 Sizes]
    C --> F[Adjust for Moisture Content (Appendix A)]
    D --> F
    E --> F

**Use these tables

IS 4891: Scope - Key Specifications & Preferred Sizes

Scope: Covers structural timber sizes for roof trusses and partition framing.

Preferred Cut Sizes for Structural Timbers

1. Roof Trusses (Span 3 to 20 m)

Note: Sizes for spans marginally above 20 m may be allowed.

2. Partition Framing and Covering

Additional Notes:

• Timber dimensions include 1.63 mm oversize to account for moisture content variations as per IS 287-1973.
• Timber moisture zones in India affect sizing and storage.

Summary Diagram of Timber Selection for Roof Trusses

flowchart TD
    A[Timber Thickness] --> B{Thickness Range}
    B -->|20 mm| C[Widths: 40-100 mm]
    B -->|25-30 mm| D[Widths: 40-160 mm]
    B -->|35-50 mm| E[Widths: 60-160 mm]
    B -->|60-80 mm| F[Widths: 

IS 4891: Preferred Sizes of Structural Timber

This standard specifies preferred cut sizes for structural timber used in:

• Roof trusses (spans 3 to 20 m)
• Roof purlins, rafters, floor beams
• Partition framing and covering
• Centering
• Doors, windows, ventilators

1. Preferred Sizes for Roof Trusses (Clause 2.1, Table 1)

Note: For trusses >20 m span, preferred sizes may still be allowed.

2. Preferred Sizes for Partition Framing and Covering (Clauses 2.1 & 2.2, Table 3)

IS 4891: Preferred Cut Sizes for Roof Trusses

Key Specifications (Clause 2.1, Table 1)

• Note: For spans slightly above 20 m, these sizes may still be allowed.

Additional Info

• These sizes ensure standardization for easier fabrication and structural efficiency.
• Thickness and width combinations are chosen based on typical load and span requirements.
• For other members like purlins, rafters, floor beams, refer to Clause 2.5 (similar tables exist).

Practical Use

• Choose timber sizes from the table based on span and load.
• Ensure timber quality and grading per IS 4891.
• Use standard sizes to optimize connections and reduce wastage.

flowchart LR
    A[Select Span (3-20 m)] --> B{Choose Thickness}
    B -->|20 mm| C[Widths: 40,50,60,80,100 mm]
    B -->|25-30 mm| D[Widths: 40 to 160 mm]
    B -->|35-50 mm| E[Widths: 60 to 160 mm]
    B -->|60-80 mm| F[Widths: 80 to 160 mm]
    C --> G[Use for Roof Trusses]
    D --> G
   

IS 4891: Preferred Sizes for Centering

• Clause 2.2: Preferred sizes for centering are the same as for partition framing (Table 3).

Table 3: Preferred Cut Sizes for Partition Framing & Covering (Thickness × Width in mm)

Additional Notes:

• These sizes ensure availability, ease of construction, and structural adequacy.
• For roof trusses (span 3-20 m), refer to Table 1 (Clause 2.1) with similar thickness-width combinations.
• Marginally larger sizes may be permitted for trusses slightly over 20 m span.

This standardization simplifies procurement and construction, ensuring timber centering members are structurally sound and readily available.

IS 4891: Tolerances and Dimensional Adjustments

1. Tolerances for Door/Window Components (Clause 2.3.2)

2. Adjustment for Moisture Content (Clause 2.3.4)

• All sizes are standardized at 20% moisture content.
• No adjustment for dimensions along the grain for moisture variations.
• For cross-sectional dimensions, allowances must be applied regardless of radial or tangential directions.
• Adjustment formula (general form):

[ D_{adjusted} = D_{standard} \times (1 + \alpha \times (MC_{actual} - 20%)) ]

Where:

• (D_{adjusted}) = dimension at actual moisture content
• (D_{standard}) = dimension at 20% moisture content
• (\alpha) = shrinkage/swelling coefficient (depends on wood species and direction)
• (MC_{actual}) = actual moisture content in %

3. Reference for Moisture Zones (Clause 1.63)

• India divided into 4 moisture zones (IS 287-1973).
• Initial oversize allowance (e.g., 1.63 mm) applied during sawing/storing to accommodate moisture variation.

Summary Diagram: Moisture Adjustment

flowchart LR
    A[Standard Dimension @ 20% MC] --> B{Moisture Content}
    B -->|MC > 20%| C[Swelling: Increase dimension]
    B -->|MC < 20%| D[Shrinkage: Decrease dimension]
    C --> E[Adjusted Dimension]
    D --> E

Note: For exact shrinkage coefficients and detailed examples, refer to **Append

IS 4891: Measurement Tolerances for Timber Doors/Windows

Key Tolerances (Clause 2.3.2)

Additional Measurement Tolerances (Clause 2.3.1)

• For measurements up to 100 mm: 0 to +3 mm
• For measurements above 100 mm: -3 to +6 mm
• Length for all sizes: 0 to +10 mm

Thickness & Width Preferred Sizes (Clause 2.1)

Moisture Content Adjustment (Clause 2.3.4)

• Sizes standardized at 20% moisture content
• No allowance along grain for moisture variation
• Allowances apply only to cross-sectional dimensions

flowchart TD
    A[Measurement] --> B{Dimension}
    B -->|Up to 100 mm| C[0 to +3 mm]
    B -->|Above 100 mm| D[-3 to +6 mm]
    B -->|Length| E[0 to +10 mm]
    F[Component] --> G{Frames/Shutters}
    G -->|Frames| H[±3 mm]
    G -->|Shutters Width ≤ 40 mm| I[±1 mm]
    G -->

IS 4891: Tolerances & Preferred Sizes for Door/Window/Ventilator Components

1. Tolerances (Clause 2.3.2)

2. Preferred Cut Sizes of Timber (Clause 2.1)

• Thickness (mm): 15, 20, 25, 30, 35, 40, 50, 60, 80
• Width (mm): 40, 50, 60, 80, 100, 120, 160, 180, 200, 220, 240

Examples from Table 3 & 4:

3. Notes:

• Beading thickness: 15 mm (width 25 & 40 mm), 20 mm (width 25, 30, 40 mm) allowed.
• Lock & bottom rail: Thickness 30-50 mm; width 150, 200, 240 mm allowed.

Summary Diagram:

graph TD

IS 4891: Moisture Content Effects on Timber Dimensions

Key Points from Clauses 2.3.4 and 3.1/3.2

• Standard dimension basis: All sizes are at 20% moisture content.
• Along grain: No dimensional adjustment needed for moisture variation.
• Cross-sectional dimensions: Adjustments required for moisture content different from 20%.

Adjustment Formula for Cross-Sectional Dimensions

[ \Delta d = d \times \alpha \times |M - 20| ]

Where:

• ( \Delta d ) = change in dimension
• ( d ) = original dimension at 20% moisture
• ( \alpha ) = shrinkage coefficient (% per 1% moisture change)
• ( M ) = actual moisture content (%)

Shrinkage Coefficients ((\alpha))

Notes:

• Use 0.2% for timbers with SG < 0.6.
• Use 0.3% for timbers with SG ≥ 0.6.
• Applies only to cross-sectional dimensions (radial/tangential).

Example (Appendix A reference)

If a timber with SG = 0.55 and dimension 100 mm at 20% moisture changes to 15% moisture:

[ \Delta d = 100 \times 0.002 \times |15 - 20| = 100 \times 0.002 \times 5 = 1 \text{ mm decrease} ]

flowchart TD
    A[Standard Dimension at 20% Moisture] --> B{Moisture Content Change?}
    B -- No --> C[No Dimension Change]
    B -- Yes --> D[Calculate \Delta d]
    D --> E{Specific Gravity}
    E -- < 0.6 --> F[Use 0.

IS 4891: Adjustment of Timber Dimensions at Different Moisture Contents

Key Points from Clause 2.3.4 & Related Clauses:

• Standard size basis: All tabulated timber sizes are at 20% moisture content.
• Along the grain: No dimensional adjustment needed for moisture variation.
• Cross-sectional dimensions: Allowances must be made for moisture content differing from 20%.
• Dimensional change rate: For timber with specific gravity < 0.6, change in cross-section = 0.2% per 1% moisture content difference from 20%.

Formula for Cross-Sectional Dimension Adjustment

[ D_{new} = D_{20} \times \left[ 1 - 0.002 \times (M - 20) \right] ]

Where:

• (D_{new}) = dimension at moisture content (M) (%)
• (D_{20}) = dimension at 20% moisture content
• (M) = actual moisture content (%)

Summary Table for Cross-Sectional Adjustment

Notes:

• Adjustment applies only to cross-sectional dimensions (width/thickness).
• No adjustment for length (along grain).
• For timbers with higher specific gravity (>0.6), dimensional stability is better; adjust accordingly.
• Refer to Appendix A of IS 4891 for detailed method and example.

flowchart LR
    A[Start: Dimension at 20% moisture] --> B{Moisture Content M?}
    B -->|M > 20%| C[Calculate shrinkage: D_new = D_20 * (1 - 0.002*(M-20))]
    B -->|M < 20%| D[Calculate

IS 4891: Other Sizes for Specific Demands - Key Points

Preferred Sizes (Clause 2.1)

• Timber sizes vary by application: roof trusses, partitions, and general structural use.
• Thickness and width combinations are standardized for economy and availability.

Clause 2.3.5 - Other Sizes

• Custom sizes allowed to meet specific demands or optimize log utilization.
• Enables flexibility beyond preferred sizes for special structural needs.

Moisture Content

• Maximum permissible moisture content varies by timber use (refer to IS 4891 for detailed values).
• Ensures durability and structural performance.

Summary Table Example: Structural Timber Thickness vs Width (mm)

Practical Note:

• When designing or specifying timber, check availability of preferred sizes first.
• Use custom sizes only when necessary, considering cost and supply chain impact.

flowchart TD
    A[Timber Demand] --> B{Preferred Sizes Available?}
    B -- Yes --> C[Select from Standard Sizes]
    B -- No --> D[Produce/Stock Other Sizes]
    D --> E[Optimize Log Utilization]
    C & E --> F[Use in Structural Application]

This approach balances economy and structural requirements effectively.

IS 4891: Dimension Adjustment Due to Moisture Content

Key Points from Clause 2.3.4 & Appendix A:

• Standard dimension basis: All sizes are at 20% moisture content.
• Along grain: No dimension adjustment needed for moisture variation.
• Cross-sectional dimensions: Adjustments required based on moisture content difference from 20%.

Adjustment Formula for Cross-Sectional Dimensions:

Let

• ( \Delta M = ) difference in moisture content from 20% (in %)
• ( D_{20} = ) dimension at 20% moisture content
• ( D_{adj} = ) adjusted dimension

Notes:

• Use + if moisture content increases (swelling).
• Use - if moisture content decreases (shrinkage).
• Applies only to cross-sectional dimensions (radial and tangential directions).

flowchart LR
    A[Dimension at 20% Moisture] --> B{Moisture Content Change?}
    B -- No --> C[No Adjustment]
    B -- Yes --> D{Specific Gravity}
    D -- < 0.6 --> E[Adjust by 0.2% per % moisture change]
    D -- ≥ 0.6 --> F[Adjust by 0.3% per % moisture change]
    E & F --> G[Calculate Adjusted Dimension]

This method ensures accurate sizing of timber elements accounting for moisture-induced dimensional changes.