structural timber in the building - Specification

IS 3629: Scope - Key Formulas, Tables, and Specifications

1. Scope Overview (Clause 2.0)

• Applies to timber grading and permissible stresses.
• Definitions per IS 707-1976, IS 883-1970 apply.

2. Permissible Stresses (Clause 5.1 & 5.3)

• Basic stresses from IS 1708-1969 are divided by Factors of Safety (Table 2) to get permissible stresses.

3. Adjustment Factors for Grades (Clause 5.3)

• Select Grade Timber: Multiply permissible stresses by 1.16
• Grade II Timber: Multiply permissible stresses by 0.84

4. Effect of Slope of Grain (Clause 5.3.2, Table 3)

*Permissible stress

IS 3629 - Definitions & Key Specifications Summary

Definitions (Clause 2.0)

• Definitions refer to:
  • IS 707-1976 (Timber technology glossary)
  • IS 883-1970 (Structural timber design)
  • Related codes for timber classification, preservation, and identification.
• Treatability: See Clause 3.3.3 for timber treatment-related definitions.

Permissible Stresses (Clause 5.1 & 5.3)

Basic stresses are from small clear specimens (IS 1708-1969) and divided by safety factors (Table 2) to get permissible stresses.

Adjustment Factors (Clause 5.3)

Summary

• Use basic stresses from IS 1708, divide by safety factors (Table 2).
• Adjust permissible stresses by grade factors (Clause 5.3).
• Refer to IS 707 & IS 883 for detailed definitions and timber classification.

flowchart TD
    A[Basic Stress (IS 1708)] --> B[Divide by Safety Factor (Table 2)]
    B --> C[Permissible Stress]
    C --> D{Timber Grade?}
    D -->|Select Grade| E[Multiply by 1.16]
    D

Classification of Timber (IS 3629)

1. Species Classification (Clause 3.1.2)

Timber species are classified based on:

• Modulus of Elasticity (E)
• Extreme fibre stress in bending and tension (ft)

2. Grouping of Timber (Clause 4.2)

Timber species recommended for structural use are grouped by strength properties:

• Group A: Specific gravity > 0.65 (formerly "super")
• Group B: Specific gravity 0.50 to 0.65 (formerly "standard")
• Group C: Specific gravity < 0.50 (formerly "ordinary")

3. Permissible Stresses (Clause 5.1 & Table 2)

Permissible stresses = Basic stress / Factor of Safety (FoS)

Summary:

• Use Group A timber for highest strength needs.
• Apply appropriate FoS to basic stresses to get safe permissible stresses.
• Refer IS 883 for detailed safe working stresses and IS 1708 for test methods.

IS 3629: Suitability and Grouping of Timber for Structural Use

1. Grouping Based on Strength Properties (Clause 4.2)

Timber species are classified into 3 groups by Modulus of Elasticity (E) and Extreme Fibre Stress in Bending/Tension (ft) for Grade 1 structural material:

• Specific Gravity Guide:
  • Group A: > 0.65
  • Group B: 0.50 to 0.65
  • Group C: < 0.50

2. Suitability for Durability and Treatability (Clause 4.1.1 & 3.3.3)

• Durability: High/moderate durability timbers recommended; low durability allowed indoors with treatment.
• Treatability Classes:

3. Permissible Stresses (Clause 5.2)

• Refer IS 883:1970 Tables 2 & 4 for permissible stresses by group and location.
• Conditions:
  • Use high/moderate durability or treated timber.
  • Loads must be continuous/permanent, no impact loads.

Summary Diagram: Timber Grouping by Strength

graph LR
A[Group A] -->|E > 12600 N/mm²| High_Strength[High Strength]
B[Group B] -->|9800 < E ≤ 12600| Medium_Strength[Medium Strength]
C[Group C] -->|5600 < E ≤ 9800| Low_Strength[Low Strength]

IS 3629: Permissible Stresses for Timber

1. Basic Concept

• Basic stresses from small clear specimens (IS 1708-1969) are divided by Factors of Safety (FoS) (Table 2) to get permissible stresses.

2. Factors of Safety (Table 2)

3. Grade Adjustment Factors (Clause 5.3)

4. Slope of Grain Reduction (Table 3)

5. Knot Size Allowance (Clause 8.2.2.3)

• If knot size ≤ 0.5 × max allowed (Table 4

IS 3629: Dimensions and Tolerances for Structural Timber

1. Dimensional Tolerances (Clause 6.1, IS:4891-1968)

2. Maximum Allowable Defects - Knots (Table 4, Clause 8.2.2.3)

• Note: Grain slope angle measured as deviation of fibers from timber edge; max 1 in 15 for Grade I timber.

3. Impact Factor on Strength (Clause 5.4)

• Timber under impact load can resist twice the static load strength.

flowchart TD
    A[Timber Dimensions] --> B[Tolerances]
    B -->|Width ≤100 mm| C[-0 to +3 mm]

Factors Affecting Strength of Timber (IS 3629)

1. Basic Stress Values & Safety Factors (Clause 5.1, Table 2)

Basic stresses from small clear specimens (IS 1708-1969) are reduced by safety factors to get permissible stresses:

Permissible Stress = Basic Stress ÷ Factor of Safety

2. Grade Adjustment Factors (Clause 5.3)

Permissible stresses for other grades:

• Select Grade: Multiply by 1.16
• Grade II: Multiply by 0.84

3. Tolerance on Dimensions (Clause 6.1)

• Width/Thickness ≤ 100 mm: ±3 mm
• Width/Thickness > 100 mm: +6 mm / -3 mm
• Length: +10 mm / 0 mm

4. Defects and Strength Reduction (Clause 8.1)

• Timber defects reduce strength; refer IS 3364 (Part 2)-1976 for defect identification and measurement.
• Design must account for defects by applying reduction factors or selecting higher grades.

Summary Formula:

[ \text{Permissible Stress} = \frac{\text{Basic Stress (IS 1708)}}{\text{Factor of Safety (Table 2)}} \times \text{Grade Factor (Clause 5.3)} ]

flowchart TD
    A[Basic Stress (IS 1708)] --> B[Divide by Factor of Safety (Table 2)]
    B --> C[Multiply by Grade Factor

Influence of Defects on Structural Timber (IS 3629 - Key Points)

• Defects: Knots, checks, shakes reduce timber strength. Their influence depends on size, location, and type.
• Critical Zones in Beams (Fig. 3):
  • Tension (T) zone
  • Compression (C) zone
  • Shear (S) zone
• Allowable Knot Sizes (Fig. 4):
  • ( d_e ): Max knot size on center line of wider face
  • ( d_e ): Max knot size on edges of wider face
  • ( t ): Max knot size on middle half length of narrow face
• Differential Shrinkage (Fig. 7):
  • Radial (R) and Tangential (T) directions affect defect severity and structural use.
• Defect Reduction Factors: Strength reduction depends on defect size/location and is incorporated in grading rules (see IS 3364 Part 2 for measurement).
• Preservative Treatment: Generally does not reduce strength unless high-temperature processes are involved.

Typical Strength Reduction Concept (not exact formula, refer IS 3364 for specifics):

[ f_{allowable} = f_{base} \times (1 - k_d) ]

Where:

• ( f_{allowable} ) = reduced strength
• ( f_{base} ) = base strength of defect-free timber
• ( k_d ) = reduction factor due to defect (depends on size/location)

flowchart LR
    A[Timber Member] --> B{Defects Present?}
    B -- Yes --> C[Identify Defect Type: Knot, Check, Shake]
    C --> D[Locate Defect in Critical Zone (T, C, S)]
    D --> E[Measure Defect Size (de, t)]
    E --> F[Apply Reduction Factor \( k_d \)]
    F --> G[Calculate Reduced Strength]
    B -- No --> H[Use Base Strength]

Refer IS 3364 (Part 2) for detailed grading and measurement procedures.

IS 3629: Storing of Timber - Key Points

Clause 9: Storing of Timber

• Objective: Prevent decay and defects before fabrication/erection.
• Recommended Practices (9.1.1 & 9.1.2):
  • Store timber in a dry, well-ventilated place.
  • Keep timber off the ground on raised platforms to avoid moisture absorption.
  • Protect from direct sunlight and rain using covers.
  • Stack timber with spacers/stickers between layers for air circulation.
  • Avoid contact with soil or wet surfaces to prevent fungal attack.

Factors of Safety for Permissible Stresses (Table 2)

Summary Formula for Permissible Stress:

[ \text{Permissible Stress} = \frac{\text{Basic Stress (IS 1708)}}{\text{Factor of Safety (Table 2)}} ]

flowchart TD
    A[Timber Harvested] --> B[Selection]
    B --> C[Storage]
    C -->|Dry, Elevated, Covered| D[Fabrication/Erection]
    C -->|Avoid moisture & decay| E[Maintain quality]

Ensure proper storage to maintain timber quality and structural integrity.

IS 3629: Conversion of Logs into Structural Timber — Key Points

1. Log Conversion Guidelines (Clause 8.3.1)

• Shortening Long Logs: Long logs can be cut into shorter lengths to avoid severe slope of grain (8.3.1.3).
• Member Size & Log Portion: Sizes and log portions must be adjusted to minimize effects of knots, checks, and shakes (8.3.1.2).
• Differential Shrinkage: Structural timber from different log zones is selected based on shrinkage behavior (Fig. 7).

2. Allowable Knot Sizes (Fig. 4)

3. Grain Orientation & Structural Use (Fig. 6)

• R (Radial/Quarter Sawn): Best for dowels, planks (structural).
• B (Best for beams/joists)
• T (Tangential/Flat Sawn): Not good for structural planks.
• D: Discarded for beams/joists.

4. Measurement Tolerances (Clause 6.1, IS 4891)

Diagram: Critical Zones in Beam (Fig. 3)

graph LR
A[Load Applied on Wider Face]
T[Tension Zone (T)]
C[Compression Zone (C)]
S[Shear Zone (S)]
A --> T
A --> C
A --> S

Summary:
Select timber portions and sizes carefully to minimize defects. Follow knot size limits per face zones. Use radial/quarter sawn timber for best structural performance. Adhere to dimensional tolerances from IS 4891 for accuracy.

IS 3629 - Preservative Treatment of Timber: Key Points

Applicability (Clause 3.3.2)

• Timber requiring preservative treatment:

  • Heartwood of moderate & low durability species.
  • Heartwood of high durability species with >15% sapwood.
  • All sapwood regardless of durability.

• Treatment protects against fungi, termites, borers, marine organisms per IS 401-1982.

Timber Selection (Clause 4.1.1.1)

• Use:
  • Untreated heartwood of high durability (Table 1 species).
  • Treated heartwood of moderate/low durability and Class 'a'/'b' treatability.
  • Heartwood of moderate durability with Class 'c' treatability after pressure impregnation.
  • Sapwood of all classes after thorough treatment.

Treatment Effects on Strength (Clause 7.9)

• Preservatives/fire retardants do not impair strength except when applied at high temperatures.
• Design must allow for strength reduction if high-temperature treatment is used.
• Normal exposure temperatures cause no serious permanent strength loss.

Typical Preservative Retention (IS 401-1982 reference)

Summary Diagram: Timber Treatment Decision Flow

flowchart TD
    A[Select Timber] --> B{Durability Class}
    B -->|High Durability| C[Check Sapwood %]
    C -->|>15% Sapwood| D[Treat Heartwood + Sapwood]
    C -->|≤15% Sapwood| E[Use Untreated Heartwood]
    B -->|Moderate/Low| F[Treat Heartwood + Sapwood]
    F --> G[Pressure Impregnation for Class c Treatability]
    A --> H[Sapwood (Any Class)]
    H --> I[Always Treat Thoroughly]

References:

• IS 3629:

IS 3629 - Erection Stage Considerations (Clause 8.3.4)

• General Arrangement Check (8.3.4.1):

  • Engineer must verify correct orientation and placement of members (beams, joists, trusses) before erection.
  • Incorrect positioning (e.g., beam upside down) can shift knots or defects into tension zones, reducing strength.
  • Requires thorough supervision to avoid erection errors affecting structural integrity.

• Stress Development During Erection (8.3.4.2):

  • Minimize unintended stresses by careful handling of timber units.
  • Avoid impacts, bending, or twisting that can cause damage not accounted for in design.

Key Points Summary:

Practical Tips:

• Mark timber members clearly for orientation.
• Use proper lifting devices to avoid bending stresses.
• Inspect for damage or defects before and after erection.

flowchart TD
    A[Pre-Erection Check] --> B{Member Orientation Correct?}
    B -- Yes --> C[Proceed with Erection]
    B -- No --> D[Reorient Member]
    C --> E[Careful Handling]
    E --> F[Minimize Stress Development]
    D --> C

This ensures structural performance per IS 3629 during erection.