preferred cut sizes of structural timber - Specification
1988 Edition

IS 4891: Overview - Principal Specifications & Dimension Tables

This standard outlines optimal cut dimensions for structural timber applied in various uses including roof trusses, partition frameworks, and timber coverings.

1. Optimal Structural Timber Sizes

2. Important Notes:

• Sizes slightly exceeding 20 m spans for trusses may be permitted.
• Moisture content influences timber dimensions; refer to Appendix A for adjustment procedures.

3. Moisture Content Dimension Adjustment (Clause 2.3.4)

• Dimension modifications due to moisture changes are calculated by applying correction factors to nominal sizes (details in Appendix A).

Example Table for Roof Trusses:

For comprehensive design details, consult IS 4891 Appendix A for moisture-related adjustments and complete size tables.

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

Refer to these tables for selection guidance.

IS 4891: Scope - Essential Specifications & Preferred Dimensions

Scope: Defines structural timber sizes for applications including roof trusses and partition framing.

Preferred Timber Cut Sizes

1. Roof Trusses (Span 3 to 20 m)

Note: Sizes may be accepted for spans slightly more than 20 m.

2. Partition Framing and Covering

Additional Remarks:

• Timber sizes account for an initial oversize of 1.63 mm to accommodate moisture-related dimension changes per IS 287-1973.
• Timber moisture zones in India influence sizing and storage practices.

Visual Summary: 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: 80–160 mm]

IS 4891: Recommended Sizes for Structural Timber

This code specifies preferred cut dimensions for timber used in:

• Roof trusses with spans from 3 to 20 meters
• Roof purlins, rafters, floor beams
• Partition framing and coverings
• Centering operations
• Doors, windows, and ventilators

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

Note: Sizes may be extended for trusses exceeding 20 m in span.

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

(Complete sizes available in the standard document.)

IS 4891: Recommended Cut Dimensions for Roof Trusses

Specifications (Clause 2.1, Table 1)

• Note: Sizes may be acceptable for spans slightly beyond 20 m.

Additional Details

• These sizes promote standardization for simplified fabrication and structural efficiency.
• Thickness and width combinations are selected based on typical load-bearing and span considerations.
• For other members such as purlins, rafters, and floor beams, refer to Clause 2.5.

Practical Application

• Select timber dimensions from the table according to span and load requirements.
• Confirm timber quality and grading comply with IS 4891.
• Utilizing standard sizes facilitates better connections and reduces material waste.

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[Application: Roof Trusses]
    D --> G
    E --> G
    F --> G

IS 4891: Recommended Dimensions for Centering Timber

• Clause 2.2: The preferred sizes for centering timber correspond with those specified for partition framing (see Table 3).

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

Additional Information:

• These size selections ensure availability, ease in construction, and adequate structural performance.
• Refer to Table 1 (Clause 2.1) for roof truss sizes (span 3–20 m), which have comparable thickness-width combinations.
• Sizes slightly larger than those listed may be permitted for trusses with spans exceeding 20 m.

Standardizing sizes simplifies procurement and construction, guaranteeing centering timber members are structurally reliable and readily sourced.

IS 4891: Allowable Tolerances and Dimension Modifications

1. Tolerances for Door and Window Timber Components (Clause 2.3.2)

2. Moisture Content Dimension Adjustment (Clause 2.3.4)

• All timber sizes are standardized at 20% moisture content.
• No size adjustment is needed along the grain when moisture varies.
• Cross-sectional dimensions must be adjusted when moisture content differs from 20%, regardless of radial or tangential direction.
• General adjustment formula:

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

Where:

• (D_{adjusted}) = size at actual moisture content
• (D_{standard}) = size at 20% moisture content
• (\alpha) = shrinkage/swelling coefficient (species and direction dependent)
• (MC_{actual}) = actual moisture content (%)

3. Moisture Zones in India (Clause 1.63)

• The country is divided into four moisture zones as per IS 287-1973.
• Initial oversize allowance (e.g., 1.63 mm) is applied during sawing and storage to offset moisture variation effects.

Visual Summary: Moisture Adjustment

flowchart LR
    A[Standard Dimension @ 20% MC] --> B{Is MC Different?}
    B -->|MC > 20%| C[Swelling occurs: Increase size]
    B -->|MC < 20%| D[Shrinkage occurs: Decrease size]
    C --> E[Adjusted Dimension]
    D --> E

Refer to Appendix A for specific shrinkage coefficients and detailed examples.

IS 4891: Dimensional Tolerances for Timber Doors and Windows

Primary Tolerances (Clause 2.3.2)

Additional Measurement Tolerances (Clause 2.3.1)

• For dimensions up to 100 mm: 0 to +3 mm
• For dimensions exceeding 100 mm: -3 to +6 mm
• Length tolerance for all sizes: 0 to +10 mm

Thickness and Width Preferred Sizes (Clause 2.1)

Moisture Content Adjustment (Clause 2.3.4)

• Sizes are standardized at 20% moisture content.
• No adjustment is required for length (along grain).
• Adjustments only apply to cross-sectional dimensions.

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

IS 4891: Permissible Tolerances and Preferred Sizes for Door, Window, and Ventilator Timber Parts

1. Tolerance Limits (Clause 2.3.2)

2. Standard Cut Sizes (Clause 2.1)

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

Sample Dimensions from Tables 3 & 4:

3. Additional Notes:

• Beading thickness: 15 mm (with widths 25 & 40 mm), 20 mm (with widths 25, 30, 40 mm) allowed.
• Lock and bottom rails have thicknesses between 30-50 mm and widths of 150, 200, 240 mm.

Summary Diagram:

graph TD
    A[Timber Component] --> B{Type}
    B --> C[Frames: ±3 mm tolerance]
    B --> D[Shutters: Width and Thickness tolerances]

IS 4891: Effects of Moisture Content Variations on Timber Size

Highlights from Clauses 2.3.4 and 3.1/3.2

• The standard sizes are based on timber at 20% moisture content.
• No dimensional changes along the grain are required when moisture varies.
• Cross-sectional dimensions (width and thickness) must be adjusted for moisture contents differing from 20%.

Dimension Adjustment Formula for Cross-Section

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

Where:

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

Shrinkage Coefficients Based on Specific Gravity

Notes:

• Use 0.2% for timbers with SG below 0.6.
• Use 0.3% for timbers with SG 0.6 and above.
• Adjustments apply only to cross-sectional dimensions (radial and tangential directions).

Example (from Appendix A):

For a timber with SG = 0.55, dimension 100 mm at 20% moisture, drying to 15% moisture:

[ \Delta d = 100 \times 0.002 \times 5 = 1 \text{ mm reduction} ]

flowchart TD
    A[Dimension at 20% MC] --> B{Moisture Change?}
    B -- No --> C[No size change]
    B -- Yes --> D[Calculate dimensional change]
    D --> E{Specific Gravity}
    E -- < 0.6 --> F[Use 0.2% shrinkage per 1% MC change]
    E -- ≥ 0.6 --> G[Use 0.3% shrinkage per 1% MC change]
    F & G --> H[Determine adjusted dimension]

IS 4891: Modifying Timber Dimensions According to Moisture Content

Key Points from Clause 2.3.4 and Related Sections:

• All preferred timber sizes are specified at a standard moisture content of 20%.
• No dimension alterations are necessary along the grain when moisture content changes.
• Cross-sectional dimensions must be adjusted when moisture differs from 20%.
• For timber with specific gravity less than 0.6, cross-section changes occur at approximately 0.2% per 1% moisture deviation from 20%.

Adjustment Formula for Cross-Sectional Dimensions

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

Where:

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

Cross-Section Adjustment Factors

Notes:

• Adjustments apply only to width and thickness, not length.
• For species with higher specific gravity (>0.6), dimensional changes are less pronounced and should be adjusted accordingly.
• Detailed methodology and examples are provided in Appendix A of IS 4891.

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

IS 4891: Additional Timber Sizes for Specific Applications

Preferred Sizes (Clause 2.1)

• Timber dimensions vary according to their application such as roof trusses, partitions, and general structural use.
• Thickness and width combinations are standardized to optimize economy and availability.

Clause 2.3.5 - Other Sizes

• Custom or non-standard sizes are permissible to meet unique project demands or to maximize log utilization.
• This flexibility allows for optimization beyond the preferred sizes when necessary.

Moisture Content Considerations

• Maximum permitted moisture content varies based on timber application; refer to IS 4891 for specifics.
• Correct moisture levels ensure durability and structural integrity.

Summary Table: Structural Timber Thickness vs Width

Practical Advice:

• Prioritize preferred sizes during design and procurement.
• Resort to custom sizes only when essential, considering cost implications and supply constraints.

flowchart TD
    A[Timber Requirement] --> B{Preferred Size Available?}
    B -- Yes --> C[Select from Preferred Sizes]
    B -- No --> D[Procure or Fabricate Other Sizes]
    D --> E[Optimize Log Usage]
    C & E --> F[Apply in Structural Components]

IS 4891: Procedure for Adjusting Timber Dimensions Based on Moisture Content

Key Elements from Clause 2.3.4 and Appendix A:

• All timber sizes are referenced at a moisture content of 20%.
• No adjustments for length dimensions along the grain are needed when moisture varies.
• Cross-sectional dimensions require corrections depending on moisture content deviation from 20%.

Adjustment Formula for Cross-Sectional Dimensions:

Define:

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

Additional Notes:

• Use '+' sign when moisture content increases (swelling).
• Use '–' sign when moisture content decreases (shrinkage).
• Applies exclusively to cross-sectional dimensions (radial and tangential).

flowchart LR
    A[Dimension at 20% Moisture] --> B{Is Moisture Content Changed?}
    B -- No --> C[No Adjustment Needed]
    B -- Yes --> D{Specific Gravity}
    D -- < 0.6 --> E[Adjust by 0.2% per 1% moisture change]
    D -- ≥ 0.6 --> F[Adjust by 0.3% per 1% moisture change]
    E & F --> G[Calculate Adjusted Dimension]

This method ensures precise sizing of timber components, accounting for moisture-induced dimensional variations.