Code of practice for design and construction of bolt-jointed timber construction
1984 Edition

Scope Overview (Clause 1.1)

Defines the limits and applications for designing, fabricating, finishing, and maintaining timber joints secured with bolts for load-bearing purposes.

Principal Load Formulas for Individual Bolts (Appendix A, Clause 4.4.4)

Definitions:

• ( f_{o1}, f_{o2} ): allowable compression stresses parallel and perpendicular to grain (N/mm²)
• ( a = t \times d ): bolt projected area on timber surface (mm²)
• ( t ): thickness of timber member (mm)
• ( d ): bolt diameter (mm)
• ( d_a, d_e ): modification factors for perpendicular load
• ( M_1 ): factor based on thickness-to-diameter ratio parallel to grain
• ( \theta ): angle between load direction and grain

Maintenance Requirements (Clause 6.3)

• Tightening of bolts should occur one year post-construction and subsequently every 2 to 3 years.

SI Units:

• Force: Newton (N)
• Stress: Pascal (Pa)
• Length: millimeter (mm), meter (m)

flowchart LR
    Load[Load on Bolt] -->|Parallel to Grain| Parallel[\( P = f_{o1} \times a \times M_1 \)]
    Load -->|Perpendicular to Grain| Perpendicular[\( Q = f_{o2} \times a \times d_a \times d_e \)]
    Load -->|At Angle \( \theta \)| Angle[\( F = P \sin^{2} \theta + Q \cos^{2} \theta \)]

Materials and Component Details with Essential Formulations

1. Permissible Load on a Single Bolt in Wooden Splice Plates (Appendix A, Clause 4.4.4)

• Load parallel to grain (P): ( P = f_{on} \times a \times A_1 )
• Load perpendicular to grain (Q): ( Q = f_{oi} \times a \times A_2 )
• Load at angle ( \theta ) (Hankinson's formula): ( F = P \sin^{2} \theta + Q \cos^{2} \theta )

Where:

• ( f_{on} ): permissible compression stress parallel to grain (N/mm²)
• ( f_{oi} ): permissible compression stress perpendicular to grain (N/mm²)
• ( a = t \times d ): projected bolt area (mm²)
• ( t ): thickness of timber (mm)
• ( d ): bolt diameter (mm)
• ( A_1, A_2 ): correction factors related to thickness-to-diameter ratio

2. Stress Adjustment Factors for Thickness-to-Diameter Ratio (Clause 4.4.2, Table 4.4.3.1)

3. Bolt Tightening Schedule (Clause 6.3)

• Bolts must be tightened once after one year of installation and thereafter at intervals of 2 to 3 years.

Essential Data and Design Parameters for Bolt-Connected Timber Joints

1. Data Collection (Clause 3.1)

Before initiating the design process, gather:

• Timber species and grading information
• Relevant design parameters including loads and stresses
• Moisture content of timber materials
• Details of any preservative or fire-retardant treatments applied

2. Calculating Allowable Bolt Load (Appendix A, Clause 4.4.4)

• ( t ): timber thickness (mm)
• ( d ): bolt diameter (mm)
• ( M_1, d_a, d_e ): correction factors based on geometry and material

3. Bolt Spacing and Stress Percentage (Clause 4.4.2, Table 4.4.3.1)

Note: Stress capacity decreases as the t/d ratio increases.

4. Maintenance (Clause 6.3)

• Tighten bolts one year following project completion, then every 2-3 years thereafter.

Design Guidelines for Timber Joints Using Bolts

Essential Inputs (Clause 3.1):

• Identification of timber species and grading
• Design load data in accordance with IS 875
• Timber moisture content
• Treatment details including preservatives

Allowable Bolt Loads (Appendix A, Clause 4.4.4)

Variables:

• ( t ): timber thickness (mm)
• ( d ): bolt diameter (mm)
• ( n ): number of bolts

Maintenance Schedule (Clause 6.3)

• Bolts must be tightened after one year and subsequently every 2 to 3 years.

Load Calculation Notes

• Follow IS 875 for load combinations and worst-case scenarios.
• Wind and seismic loads are not considered simultaneously.

SI Units Summary

graph TD
    A[Collect Data: Timber Species, Grading, Moisture, Treatment] --> B[Calculate Design Loads (IS 875)]
    B --> C[Determine Bolt Load Components]
    C --> D{Direction of Load}
    D -->|Parallel| E[Calculate: P = f_on * a * M1]
    D -->|Perpendicular| F[Calculate: Q = f_oi * a * d_a * d_e]
    D -->|Angle \(\theta\)| G[Apply Hankinson's Formula]

Fabrication and Finishing Guidelines

1. Pre-camber Specification (Clause 4.4.4.5)

• A minimum pre-camber of 100 mm must be incorporated at the mid-span of the bottom chord in bolt-connected timber trusses.

2. Painting and Surface Treatment (Clause 5.3)

• Post-fabrication, timber components should be coated in compliance with IS 2338 (Parts 1 & 2) - 1967, which governs wood finishing practices.

3. Bolt Maintenance (Clause 6.3)

• Bolts require tightening one year after installation and then on a 2-3 year cycle.

4. Allowable Bolt Loads (Appendix A)

• Load capacity of bolts is influenced by the load direction relative to the grain:

Key terms:

• ( a = t \times d ): projected bolt area
• ( t ): timber thickness
• ( d ): bolt diameter
• ( M_1, d_a, d_e ): correction factors based on geometry and grain direction

Summary Table: Allowable Bolt Loads

Maintenance Guidelines for Bolted Timber Connections

Bolt Tightening Schedule (Clause 6.3)

• Initial bolt tightening should be performed one year after structure completion.
• Subsequent tightening is recommended every 2 to 3 years to maintain joint integrity.

Design Load Considerations for Bolted Joints (Appendix A, Clause 4.4.4)

Bolt Spacing Influence on Stress (Clause 4.4.2)

Stress percentages decline as t/d ratio grows.

Symbols Legend

Load Calculation Procedures for Bolted Timber Joints

1. Load Determination Basis:

• Design loads should be computed in accordance with IS 875-1964.
• The most severe load combinations and positions must be considered.
• Wind and earthquake loads are not to be combined simultaneously.

2. Allowable Load on a Single Bolt (Clause 4.4.4):

3. Bolt Spacing and Stress Adjustments (Clause 4.4.2, Table 4.4.3.1):

Note: Stress capacity reduces with increasing thickness-to-diameter ratio.

4. Wet Service Condition (Clause 4.4.4.2):

• For continuously wet environments, allowable bolt loads should be reduced to one-third of dry condition values.

SI Unit Conventions

graph TD
    Load[Load on Bolt] -->|Parallel| ParallelCalc[\( P = f_{on} \times a \times M_1 \)]
    Load -->|Perpendicular| PerpendicularCalc[\( Q = f_{oi} \times a \times M_2 \)]
    Load -->|Angle \(\theta\)| AngleCalc[\( F = P \sin^{2} \theta + Q \cos^{2} \theta \)]

Essential Guidelines for Bolt Spacing and Arrangement

1. Bolt Spacing Requirements (Clause 4.4.3 & Table 4.4.2)

• Minimum spacing between bolts in a row is calculated as:

  [ \text{Minimum spacing} = \max[(r - 4) \times d, 2.5d] ]

  where:

  • ( d ) = bolt diameter
  • ( r ) = number of bolts in the row

• Minimum distance between bolt rows:

  [ \text{Minimum spacing} = \max[1.5d, \frac{\text{distance between rows}}{2}] ]

• Minimum edge and end distances:

  [ \text{Minimum edge/end distance} = \max[(n - 4) \times d, 2.5d] ]

  where ( n ) is the total number of bolts in the joint.

2. Influence of Thickness-to-Diameter Ratio (( t/d )) on Stress Distribution

See Clause 4.4.2 for intermediate values.

3. Typical Bolt Spacing for Different Joint Types (Figure 1)

• Lengthening joints require spacing of 2.5d for t/d = 2 and 5d for t/d = 6.
• Node joints loaded perpendicular to grain follow spacing recommendations as per figure with minimum edge and end distances.

Summary

• Maintain minimum bolt spacing based on the above formulas.
• Adjust bolt arrangement according to thickness-to-diameter ratio to prevent timber overstressing.
• Ensure sufficient edge and end distances to avoid splitting.

Allowable Stress and Strength Data for Bolted Timber Connections

1. Reference for Allowable Stresses

• Timber allowable stresses are aligned with IS 883:1970 (Clause 4.2).

2. Allowable Load on a Bolt in Timber Joints (Appendix A, Clause 4.4.4)

3. Effect of Thickness-to-Diameter (t/d) Ratio on Stress (Table 2, Clause 4.4.2.1)

(Full table available in Clause 4.4.2)

4. Bolt Maintenance (Clause 6.3)

• Bolts should be tightened one year after construction and then every 2 to 3 years.

Summary

• Calculate bolt load capacities using permissible compression stresses parallel and perpendicular to grain, adjusted for geometry and load direction.

Shear Load Considerations for Bolted Timber Joints

1. Single and Multiple Shear Strength (Clause 4.4.4.4)

• Single shear load capacity is half the double shear load capacity:

  [ P_{single} = \frac{1}{2} P_{double} ]

  where ( P_{double} ) refers to the allowable load for joints with three members where the main member is twice the thickness of the thinner member.

• For multiple shear planes:

  [ P_{multiple} = n \times P_{single} ]

  where ( n ) is the number of shear planes.

2. Bolt Bearing Capacity (Clause 4.4.2 and Table 4.4.3.1)

• Allowable load in single shear equals half of that in double shear for the same t/d ratio.
• Safe working compressive stress percentages vary with t/d ratio and orientation:

3. Design Equations (Appendix A)

• Load parallel to grain:

  [ P = f_{on} \times a ]

• Load perpendicular to grain:

  [ Q = f_{oi} \times a ]

• Load at angle ( \theta ) to grain (Hankinson’s formula):

  [ F = P \sin^{2} \theta + Q \cos^{2} \theta ]

Where ( f_{on} ), ( f_{oi} ) are allowable compression stresses and ( a = t \times d ) is the projected bolt area.

Protective Coating Requirements for Timber Structures

Key Points:

• After fabrication, timber units must be coated following IS 2338 (Parts 1 & 2) - 1967, which provides guidelines for finishing wood and related materials.

• Maintenance painting intervals:

  • Structures not exposed to weather should be repainted every 5 years.
  • Weather-exposed structures require enamel repainting every 2 years.

• Bolts should be retightened one year after completion, then every 2 to 3 years to maintain joint security.

Painting Maintenance Schedule

Reference

• IS 2338 (Parts 1 & 2) - 1967 outlines workmanship and procedures for wood finishing.

flowchart TD
    Fabrication[Fabrication Completed] --> Painting[Painting per IS 2338]
    Painting --> Exposure{Is structure weather-exposed?}
    Exposure -->|Yes| Enamel[Apply Enamel Paint Every 2 Years]
    Exposure -->|No| Paint[Apply Paint Every 5 Years]
    Bolts[Bolts] --> Tighten1yr[Tighten after 1 Year]
    Tighten1yr --> TightenCycle[Tighten every 2-3 Years]

Guidelines for Pre-cambering in Timber Trusses

• As per Clause 4.4.4.5, a minimum pre-camber of 100 mm must be introduced at the center of the bottom chord in bolt-jointed timber trusses. This compensates for deflections under load, maintaining truss levelness.

Specifications Summary:

Design Notes:

• Select timber with known compressive strengths (e.g., Hardwickia Binala).
• Account for moisture and treatment factors to ensure durability.
• Pre-camber offsets dead and live load deflections.

Pre-camber Formula:

[ \text{Pre-camber} \geq 100 \text{ mm (minimum)} ]

graph LR
    Start[Begin Timber Truss Fabrication] --> Precamber[Apply Pre-camber ≥ 100 mm at Bottom Chord Center]
    Precamber --> Assembly[Assemble Bolt-Jointed Timber Units]
    Assembly --> Verify[Verify Timber Species & Grading per IS 3629]
    Verify --> MoistureCheck[Confirm Moisture Content per IS 287]
    MoistureCheck --> Finalize[Final Inspection and Installation]