Code of practice for design, fabrication and maintenance of nail-jointed timber posts from small dimensional timber for overhead electric distribution lines for low voltage
1975 Edition

Scope Overview for Nail-Jointed Timber Posts in Overhead Electric Distribution

• Applicability:
  Covers use of small-sized timber posts for overhead low voltage electric distribution lines (refer to IS 707-1968 for timber definitions).

• Essential Design Data (Clause 3.1):
  Includes:

  • Selection and classification of timber species
  • Moisture content considerations impacting strength
  • Details on preservative and protective treatments

• Value Rounding (Clause 0.6):
  All computed and tested values must be rounded following IS 2-1960, preserving significant figure consistency.

• Global Coordination (Clause 0.4):
  The standard is harmonized with international norms while catering to local environmental conditions.

Design Input Summary Table

For formulas or detailed design tables (e.g., allowable stresses, dimensions), consult IS 7683 annexes or IS 707 for grading specifics.

Summary of Definitions in the Standard

• Reference Standard:
  All terminology in this code refers to definitions established in IS 707-1968 (Clause 2.1).

• Rounding Practices:
  Calculated or measured values must be rounded according to IS 2-1960, matching the precision of values specified in this standard (Clause 0.6).

Additional Points:

• Alignment with international standards and Indian conditions (Clause 0.4).
• For precise definitions such as "bearing capacity," "load," and "foundation types," refer directly to IS 707-1968.

Quick Reference Table:

For exact definitions or formulae, consult IS 707-1968 as needed.

Critical Information for Designing Overhead Timber Posts

1. Required Data (Clause 3.1)

• Identification of timber species and grading to establish strength properties.
• Specification or measurement of moisture content impacting timber performance.
• Details of chemical or preservative pre-treatment enhancing durability.

2. Wind Load Considerations (Clause 5.3)

• Wind pressures must comply with IS 802 (Part I)-1973 or Indian Electricity Rules, considering location and elevation.

3. Design Guidelines

• Reference IS 7683 alongside IS 707-1968 for definitions and IS 802 for wind load parameters.
• Account for bending, compression, and shear stresses in timber post design.
• Apply safety factors consistent with timber grade and treatment.

Representative Wind Pressure Values (From IS 802)

flowchart TD
    Start[Start Design Process] --> CollectData[Gather Timber Data]
    CollectData --> SpeciesGrade{Identify Species & Grade}
    CollectData --> Moisture{Determine Moisture Content}
    CollectData --> Treatment{Document Pre-treatment}
    SpeciesGrade --> DetermineStrength[Calculate Strength]
    Moisture --> DetermineStrength
    Treatment --> DetermineStrength
    DetermineStrength --> WindLoads[Calculate Wind Loads per IS 802]
    WindLoads --> DesignPost[Design Timber Post]
    DesignPost --> VerifySafety[Check Stress and Safety Factors]
    VerifySafety --> Finalize[Complete Design]

This process ensures a reliable and regulation-compliant timber post design.

Summary of Timber Requirements and Selection Criteria

Core Requirements (Clauses 3.1 and 4.1)

• Identification and grading of timber species.
• Control of moisture levels.
• Documentation of preservative treatment methods.
• Minimum thickness of timber planks: 45 mm.

Timber Species Classification

Example Species Table

Additional Information

• Sapwood of durable species requires preservative treatment.
• Species with high seasoning resistance better withstand defects.
• Refer to Appendix A of IS 7683 for full species listings.
• Anti-termite treatments must comply with IS 6313 series.

flowchart TD
    TimberSelection[Start Timber Selection] --> SpeciesChoice{Choose Species}
    SpeciesChoice -->|First Choice| DurableHeartwood[Naturally Durable Heartwood]
    SpeciesChoice -->|Third Choice| ModerateDurability[Moderately Durable, Treatable]
    DurableHeartwood --> MinimalTreatment[Minimal Chemical Treatment]
    ModerateDurability --> PressureTreatment[Pressure Treatment for ≤ 60 mm]
    TimberSelection --> MoistureControl[Control Moisture Content]
    TimberSelection --> PreTreatment[Document Pre-treatment]
    TimberSelection --> ThicknessCheck[Ensure Thickness ≥ 45 mm]

This ensures appropriate timber quality and durability for post fabrication.

Overview of Load Factors Affecting Timber Posts

• Wind Loads (Clause 5.3):

  • Must comply with IS 802 (Part I)-1973 or Indian Electricity Rules.
  • Wind load (p) is calculated as: [ p = k_z \times k_d \times p_0 \times C_f ] where:
  • (k_z): height factor
  • (k_d): wind direction factor
  • (p_0): basic wind pressure
  • (C_f): force coefficient based on shape

• Load Types on Posts (Clause 5.1):

  1. Wind load acting on the post above ground.
  2. Wind load on conductors and earth wires spanning horizontally.
  3. Unbalanced longitudinal pull caused by conductor breakage.

• Joint and Post Design (Clause 6.2.2): Joints must be capable of resisting combined forces as detailed in Clause 5.

Load Calculation Summary

Design Recommendations:

• Follow IS 802 (Part I)-1973 for wind pressure parameters.
• Address combined bending and tension effects on posts and joints.
• Use timber properties and definitions from IS 707-1968.

flowchart LR
    WindPost[Wind Load on Post] --> DesignMoment[Calculate Bending Moment]
    WindConductors[Wind Load on Conductors] --> DesignMoment
    UnbalancedPull[Unbalanced Wire Pull] --> DesignMoment
    DesignMoment --> JointDesign[Design Joints per IS 7683]

This methodology guarantees secure and efficient timber post design under varying load conditions.

Fundamental Design Concepts per IS 7683

• Design Approach: Timber posts are treated as simple cantilever beams subjected to bending moments.

• Bending Moment Calculation: [ M = f \times Z ] where:

  • (M) = design bending moment (kg·mm)
  • (f) = allowable maximum bending stress (kg/mm²)
  • (Z) = section modulus of the timber cross-section (mm³)

• Wind Load Compliance: Wind forces on posts, conductors, and wires must align with IS 802 (Part I)-1973 or Indian Electricity Rules.

• Joint Design Considerations: Joints should be designed to withstand combined load effects specified in Clause 5.

Additional Details:

• Section Modulus for Rectangular Sections: [ Z = \frac{b \times h^2}{6} ] where (b) = width and (h) = height (depth) of the timber element.

• Timber properties and allowable stresses are referenced from IS 707-1968.

flowchart TD
    Start[Begin Post Design] --> LoadDetermination[Identify Loads (Wind, Conductors)]
    LoadDetermination --> MomentCalc[Compute Bending Moment (M)]
    MomentCalc --> SectionSelect[Select Timber Cross-section]
    SectionSelect --> SectionModulus[Calculate Section Modulus (Z)]
    SectionModulus --> StressCheck[Verify Bending Stress (M ≤ f × Z)]
    StressCheck --> JointDesign[Design Joints per Load Requirements]

This framework ensures timber posts are structurally sound and compliant.

Essential Details for Overhead Timber Posts

1. Dimensional Marking (Clause 10.2.2)

• Mark post dimensions, including flanges and web, on a level surface.
• Indicate splice plate locations and thicknesses prior to assembly.

2. Protective Measures (Clause 14.3)

• Install galvanized iron (G.I.) sheet caps atop posts to prevent moisture ingress.

3. Timber Characteristics (Appendix A)

• Employ Indian structural timbers that:
  • Resist distortion such as warping, twisting, and splitting.
  • Contain naturally durable heartwood.
  • Are amenable to chemical preservative treatment.
  • Provide adequate sapwood for effective treatment.

4. Load Aspects (Clause 5.2)

• Posts located at angle points with guy wire support are subject to vertical downward forces.

Typical Dimensional Examples

Moisture Protection Illustration

flowchart TB
    PostTop[Timber Post Top]
    GICap[Galvanized Iron Sheet Cap]
    Rain[Rain/Moisture]

    Rain -->|Blocked by| GICap
    GICap --> PostTop

Summary:

• Ensure precise dimensional marking before fabrication.
• Use durable, chemically treatable Indian timbers.
• Protect post tops with G.I. caps against moisture.
• Account for vertical loads at guy-wire supported posts.

Overview of Preservative Treatment Requirements

1. Recommended Preservatives (Clause 8.1, Table 1)

• Treatments include pressure or hot-and-cold application methods.
• Preservative agents:
  • Creosote or a 50:50 Creosote-Fuel Oil mixture.
  • Copper-Chrome-Arsenic (CCA) formulations.
• Applicable for sawn timber exposed to outdoor conditions.

2. Timber Species and Treatability (Appendix A and Table 4)

• Third choice species are moderately durable and require pressure treatment for thicknesses up to 60 mm.
• Examples:
  • Anogeissus acuminata (high refractoriness)
  • Altingia excelsa (high refractoriness)
  • Artocarpus chaplasha (moderate refractoriness)
  • Elacocarpus tuberculatus (limited treatability but preferred)
• Effectiveness of treatment depends on species’ permeability.

3. Design Integration

• Foundations must be designed per IS 4091-1967.
• Moisture content and pre-treatment status are critical for durability (Clause 3.1).

Summary Table of Treatments

Additional Information:

• Follow IS 401-1970 for preservative formulation guidelines.
• Ensure timber moisture is appropriate prior to treatment.
• Pressure treatment improves preservative penetration in less durable species.

flowchart TD
    Timber[Select Timber Species] --> Durability{Assess Durability & Treatability}
    Durability -->|Highly Durable| MinimalTreatment[Minimal or No Treatment]
    Durability -->|Moderately Durable| PressureTreatment[Pressure Treatment]
    Durability -->|Poorly Durable| SpecialConsideration[Enhanced Treatment or Alternative Timber]

This process enhances the longevity and performance of timber posts.

Foundation Methods for Timber Posts

Key Foundation Approaches (Clause 9.1)

Two primary foundation techniques are recognized:

• Method A:

  • Posts are elevated on cement concrete dwarf pillars.
  • Stability is ensured by fixing with mild steel (MS) channels and bolts.

• Method B:

  • Posts are embedded directly into the ground.
  • Surrounding concrete dwarf foundation provides protection and load transfer.

Supporting Standards and Practices

• Anti-termite soil treatment must comply with IS 6313 (Part II)-1971.
• Foundation design principles are established in IS 4091-1967.

Design Considerations

• Typical dwarf foundation dimensions:
  • Width and length: 300 to 500 mm.
  • Depth: Minimum 450 mm below ground or per soil bearing capacity.
• Concrete mix usually 1:2:4 (cement:sand:aggregate).

Summary Table of Foundation Types

flowchart LR
    TimberPost[Timber Post] --> FoundationChoice{Select Foundation Type}
    FoundationChoice --> Elevated[Elevated on Dwarf Pillar]
    FoundationChoice --> Buried[Buried and Concrete Encased]
    Elevated --> ConcretePillar[Concrete Pillar]
    Elevated --> MSFixing[MS Channels and Bolts]
    Buried --> ConcreteEncasement[Concrete Encasement]
    Buried --> SoilContact[Direct Soil Support]

Ensure:

• Anti-termite soil treatment per IS 6313.
• Proper concrete curing and workmanship.
• Compliance with IS 4091 structural requirements.

Key Fabrication Guidelines for Timber Posts

1. Timber Preparation

• Utilize structural timbers (coniferous or hardwood) with durable heartwood and treatable sapwood (per Appendix A).
• Maintain plank thickness not less than 45 mm (Clause 4.1).
• Confirm moisture content and pre-treatment status (Clause 3.1).

2. Moisture Protection

• Install one galvanized iron (G.I.) sheet cap at the post top to prevent moisture ingress (Clause 14.3).

3. Design Calculation for Bending Stress (Clause 6.1.1)

Timber posts act as cantilever beams under lateral loads:

[ M = f \times Z ] Where:

• (M): design bending moment (kg·mm)
• (f): allowable bending stress (kg/mm²)
• (Z): section modulus (mm³)

4. Section Modulus for Rectangular Cross-Section

[ Z = \frac{b \times d^2}{6} ] Where:

• (b): plank width (mm)
• (d): plank thickness (mm)

Timber Post Specification Summary

flowchart TD
    SelectTimber[Choose Timber Species] --> CheckMoisture[Verify Moisture Content]
    CheckMoisture --> Pretreatment[Apply Pre-treatment]
    Pretreatment --> FabricatePlanks[Fabricate Planks ≥ 45mm Thick]
    FabricatePlanks --> AssemblePost[Assemble Timber Post]
    AssemblePost --> AttachCap[Fit G.I. Sheet Cap on Top]
    AttachCap --> DesignPost[Design as Cantilever Beam (M = f × Z)]

This ensures posts meet durability and structural performance criteria.

Summary of Fabrication and Erection Tolerances and Procedures

1. Fabrication Tolerances (Clause 10.3 & Fig. 2)

• Maximum permissible deviations on a level platform:
  • ±15 mm at post top (from the straight line joining end points)
  • ±15 mm at post midpoint (from the straight line joining end points)
  • ±10 mm at bottom end (from the line connecting bottom and one-third length point)

2. Foundation Setting (Clause 11.1)

• Follow Clauses 11.1.1 to 11.1.4 for foundation preparation, including:
  • Precise marking of post dimensions on level ground.
  • Marking flange, web, and splice plate locations before erection.
  • Laying a dwarf concrete base with specified cement concrete mix.

3. Additional Recommendations

• Fabricate posts on a leveled platform to ensure straightness.
• Properly mark and store timber posts before installation (Clause 12).

Allowed Fabrication Deviations

flowchart TD
    Start[Fabricate Post on Level Platform] --> MarkDimensions[Mark Dimensions on Ground]
    MarkDimensions --> MarkComponents[Mark Flanges, Web, Splice Plates]
    MarkComponents --> CheckDeviation[Verify Deviations ±15/±10 mm]
    CheckDeviation --> SetFoundation[Set Foundation per Clauses 11.1.x]
    SetFoundation --> LayConcrete[Lay Dwarf Concrete Base]
    LayConcrete --> StorePosts[Store Posts Properly (Clause 12)]
    StorePosts --> ErectPost[Erect Timber Post]

Note: Confirm concrete mix, dimensions, and workmanship meet IS code requirements.

Marking Requirements for Timber Posts

• Marking Location: At 2.5 meters from the post's bottom end (Clause 12.1).
• Marking Details:
  • Clearly denote top and bottom ends to prevent incorrect installation.
  • Specify timber species.
  • Indicate the year of preservative treatment.
• Additional Marking on Level Platform (Clause 10.2.2):
  • Mark overall post dimensions including flanges and web.
  • Indicate splice plate positions and thickness.
• Top Protection (Clause 14.3):
  • Fit a galvanized iron sheet cap to prevent moisture entry.
• Timber Selection (Appendix A):
  • Use species with durable heartwood and suitable sapwood for preservative treatment, resistant to warping and splitting.

Marking Summary Table

flowchart LR
    TimberPost[Timber Post] --> MarkEnds[Mark Top & Bottom Ends]
    MarkEnds --> MarkSpeciesYear[Mark Species & Treatment Year @ 2.5 m]
    TimberPost --> MarkDimensions[Mark Dimensions & Splice Plates on Ground]
    TimberPost --> AttachCap[Fit G.I. Sheet Cap on Top]

This system ensures precise identification and protection before installation.

Erection Practices for Nail-Jointed Timber Posts

Specifications (Clauses 10.3 & 13.1)

• Posts must be fabricated straight on leveled surfaces.
• Permissible deviations:
  • ±15 mm at top and center from the straight line joining end points.
  • ±10 mm at bottom end from line joining bottom and one-fifth length point.

Erection Steps (Clause 13.1 & Fig. 3)

• Manually lift posts using two small timbers secured at the base.
• Position posts vertically near the dwarf concrete base.
• Place posts between steel channels.
• Pre-drill holes for bolting posts to MS channels.
• Embed posts in the ground encased with cement concrete (without steel channels).

Fabrication Notes (Clause 10.2.3.2)

• Fabricate flanges and web separately.
• Use splice plates at joints to extend length.
• Nail the central row in flanges during web-to-flange assembly to create a rigid I-beam section.

Deviation Limits Table

Erection Flow

flowchart LR
    Fabrication[Fabricate Post on Level Platform] --> Marking[Mark Flanges, Web & Splice Plates]
    Marking --> ManualLift[Manual Lifting with Timbers]
    ManualLift --> Position[Position Vertically Near Concrete Base]
    Position --> PlaceChannels[Place Between Steel Channels]
    PlaceChannels --> PreboreBolt[Prebore Holes and Bolt]
    PreboreBolt --> Embed[Embed in Ground with Concrete]

These procedures assure post straightness, stability, and longevity.

Guidance on Painting and Upkeep of Timber Posts

Painting Requirements (Clause 14.1)

• A decorative and protective painting system is mandatory.
• Coating details:
  • One priming coat using aluminium wood primer, white lead primer, or authentic pink primer.
  • For timbers containing substances that inhibit oil paint drying, aluminium wood primer is compulsory.

Timber Treatment Considerations

• Durability and treatability vary by species (see Table 4 for third choice species).
• Pressure treatments improve penetration in smaller timber sizes (up to 60 mm thickness).

Maintenance Tips

• Periodically inspect paint layers for deterioration.
• Reapply primers and finishing coats as necessary to prevent moisture damage and decay.
• Ensure proper erection and embedding as per IS 7683 Fig. 3 for durability.

Painting Coats Summary

Erection and Maintenance Flow

flowchart TD
    Erection[Post Erection by Manual Labour] --> VerticalPosition[Keep Post Vertical Near Concrete Base]
    VerticalPosition --> Lifting[Use Two Small Timbers for Lifting]
    Lifting --> ChannelPlacement[Place Post Between Steel Channels]
    ChannelPlacement --> PreboreBolt[Prebore and Bolt with MS Channels]
    PreboreBolt --> ConcreteEmbedding[Embed and Encapsulate Post in Cement Concrete]

References:

• IS 7683 Clause 14.1 for painting requirements
• Table 4 for timber species selection
• Fig. 3 for erection methodology

Indian Structural Timber Species for Nail-Jointed Overhead Posts

Key Specifications:

• Fit galvanized iron sheet caps on post tops as per Clause 14.3 to prevent moisture entry.
• Use Indian coniferous and hardwood species with:
  • Naturally durable heartwood
  • Sapwood suitable for chemical treatment
  • Resistance to warping, twisting, and splitting
  • Treatability for longevity in outdoor conditions

Timber Selection

• Follow relevant IS codes for nail-jointed timber posts used in low voltage distribution.
• Prefer species with demonstrated durability and good preservative retention.

Common Indian Timbers (Examples)

Protection and Maintenance

• Apply preservative treatments to sapwood.
• Use G.I. sheet caps to protect from water ingress.
• Regularly inspect for cracks, splits, and deterioration.

flowchart TD
    SelectTimber[Choose Durable Timber] --> CheckProperties{Assess Properties}
    CheckProperties --> HeartwoodQuality[Evaluate Heartwood Durability]
    CheckProperties --> SapwoodAvailability[Confirm Sapwood for Treatment]
    CheckProperties --> WeatherResistance[Assess Resistance to Warping & Splitting]
    HeartwoodQuality & SapwoodAvailability & WeatherResistance --> FabricatePost[Fabricate Post]
    FabricatePost --> AttachCap[Install G.I. Sheet Cap]
    AttachCap --> InstallMaintain[Install and Maintain]

This ensures long-term performance and structural reliability of overhead timber posts.