Handbook on Timber Engineering

Scope & Key Specifications from IS SP:33 (S&T)-1986 (Timber Engineering)

1. Specimen Sizes & Tests (Clause 12.5)

• Load Test Specimen: 20 × 22 × 125 to 150 mm (for brittleness test)
• Charpy Test Specimen: 12.5 × 12.5 × 125 mm with notch
• Cross Sections:
  • Load test: 22 × 12 mm at 75 mm from one end (tangential side)
  • Charpy test: 10 × 12.5 mm at center on reverse side
• Impact: Pendulum released freely; energy absorbed expressed in cm·kg

2. Preferred Sizes for Structural Timber (Clause 2.5)

• Thickness (cm): 6, 8, 10, 12, 14, 16, 18, 20
• Width (cm): 5, 6, 8, 10, 12, 14, 16, 18, 20, 24
• Example Table for Roof Purlins, Rafters, Floor Beams:

3. Key Formulae for Mechanical Properties (Based on IS:1708-1969)

Key Specifications & Tables for Timber for Structural Purposes (IS SP Part 33):

1. Preferred Cut Sizes (Clause 2.5.2)

• Roof purlins, rafters, floor beams:

• Partition framing and covering:

2. Working Stress - Specific Gravity Relationship (Table 10)

3. Effect of 1% Moisture Content Change on Strength (Table 11)

IS SP 33 (S&T)-1986: Methods of Test & Evaluation of Properties

1. Specimen Sizes & Test Speeds (Clause 2.5 & Tables)

2. Key Formulas (Clause 3.1.1 & IS 170

Fire Performance and Preservation as per IS SP Part 33

Key Points from Clause 6.2.10.3:

• Fire-proofing wood involves chemical fire-retardant treatments (see Clause 4.4).
• Design should consider the rate of charring to calculate safe load-bearing dimensions.
• Fire stops should be provided at potential fire start points to prevent flame spread.
• Joints, especially with metal parts, are fire-vulnerable; protect by:
  • Locating joints away from fire source.
  • Using materials with higher fire resistance.
  • Increasing net section or testing joints under fire conditions.

Fire-retardant Treatment (Clause 4.5 & 4.5.3):

• Treatment method: Full-cell pressure treatment is most effective.
• Penetration depth: ~1.5 cm from surface.
• Heavy salt absorption is necessary for flame control.

Table 16: Salt Absorption for Fire Retardant Treatment

Rate of Charring (Typical values from literature):

• 0.6 mm/min (approximate) for softwoods.

• Design residual section after charring time ( t ):

  [ d_{residual} = d_{original} - \beta \times t ]

  where (\beta) = charring rate (mm/min), (t) = fire exposure time (min).

flowchart LR
    A[Wood Member] --> B[Fire Exposure]
    B --> C[Surface Charred]
    C --> D[Reduced Cross-section]
    D --> E[Check Load Capacity]
    E --> F{Safe?}
    F -->|Yes| G[Continue Use]
    F -->|No| H[Apply Fire-retardant Treatment or Redesign]

Summary: Use full-cell pressure treatment with adequate salt absorption per Table 16, design for charring by reducing effective cross-section, protect joints,

IS SP Part 33: Design Considerations & Safety Factors

1. Factors of Safety & Working Stresses (Clause 3.3)

• Creep Factor (f):
  • Traditionally: 0.6 (based on USA experience)
  • Recent studies suggest: 0.3 (0.45 × 0.6) for long-term loads ≤ 45% elastic limit
• Elastic Limit: ~60% of modulus of rupture

2. Duration of Loading - Modification Factor ( K_2 ) (Table 12)

3. Bearing Stresses & Constants (Clause 12.9)

• Constants:

• Parameters:

4. Table 23: Values of ( a ) and ( \lambda

Key Formulas & Specifications for Timber Structural Elements (IS SP 33)

1. Preferred Cut Sizes (Clause 2.5.2)

• For roof purlins, rafters, floor beams:

• For partition framing and covering (Thickness × Width in cm):

2. Mechanical Properties & Test Formulas (Clause 3.5)

• Modulus of Rupture (MOR):
  [ MOR = \frac{(p + 0.75w) \times L}{b h^2} ] where:

  • (p) = load at failure (kg)
  • (w) = weight of specimen (kg)
  • (L) = span length (cm)
  • (b) = width (cm)
  • (h) = height (cm)

• Compression Parallel to Grain:
  [ CS = \frac{P}{A} ] where:

  • (P) = max load (kg)
  • (A) = cross-sectional area (cm²)

• Deflection Measurement: At mid-span under incremental loads (e.g., 2000 kg intervals).

3. Loading Speed for Compression Tests:

• For compression perpendicular to grain:
  [ N = 0.0291 \times p^{4/9}

IS SP Part 33: Testing & Classification of Timber Species

Key Testing & Evaluation Points (Clause 3.1.1 & 3.1.4)

• Tests on small clear specimens (IS:1708-1969) and structural sizes (IS:2408-1965).
• Properties tested in green and dry conditions; green values are safer for design.
• Suitability indices (Table 8) compare species relative to teak (index = 100) for:
  • Strength as beam
  • Suitability as post/strut
  • Shock resistance
  • Surface hardness
  • Splitting (refractorness)
  • Nail/screw holding

Table 8: Sample Suitability Indices (Teak = 100)

Important Formulae (Appendix D, IS:1708-1969)

Timber Connections & Joint Design - Key Points from IS SP:33 & Related Codes

1. Types of Connectors (6.3.4.1)

• Wooden disc type
• Metal type (embedded partly in adjacent members, used with small diameter bolts)
• Testing & evaluation standardized as per IS:4907-1968.

2. Nail Joint Design (6.2.11.3 & IS:4983-1968)

• Laminated beams: 2-3 cm thick planks, staggered joints (30-60 cm apart).
• Nails: minimum 4 in vertical row, spacing max 7.5 cm.
• Pre-boring depends on wood hardness; pre-bore diameter less than nail diameter by 0.5-1.5 mm.

3. Bolt Joint Design Example

• Safe working stress for dry teak: 120 kg/cm².
• Bolt diameter: 10 mm, length: 35 mm, thickness of member: 3.5 cm.
• Safe load per bolt = 120 × 3.5 = 420 kg.
• Number of bolts for 1200 kg load ≈ 3 bolts on each side.
• Plate thickness = half the member thickness (1.75 cm).
• Plate length = 2 × (7d + 4d + 7d) = 36 cm (d = bolt diameter).

4. Metal Connectors (6.3.4.3 & IS:4907-1968)

• Types: split rings, toothed rings, claw plates, spike grids, shear plates.

• Design based on test results; exact stress distribution complex.

• Allowable load = connector-factor × number of connectors × working load.

• Minimum spacing for connectors at angle θ:

  [ R = \sqrt{A^2 \sin^2 \theta + B^2 \cos^2 \theta} ]

  • (A) = spacing at 0° load direction
  • (B) = spacing at 90° load direction

• Connectors placed symmetrically; max 3 connectors per face with full load, additional connectors counted at 1/3 load.

Nail Spacing Guidelines (from Figures 11-13)

| Joint Type | Minimum End Distance |

Key Points on Surface Finishing and Maintenance (IS SP Part 33)

Types of Wood Finishes (Clause 9.1.2)

• Drying Mechanism:

  • Chemical Reaction (Irreversible): e.g., drying oils, resins.
  • Solvent Evaporation (Reversible): e.g., lacquers.
  • Combination: Initial solvent evaporation followed by chemical set.

• Principal Wood Finishes:

  • Shellac: Quick drying, low strength film.
  • Lacquers: Glossy, decorative finishes.
  • Paints: Pigment + vehicle mixtures; colors per IS:86-1950.
  • Enamels: Varnish/lacquer + opaque pigments for wear resistance.
  • Fillers/Sealers: Base coat to prevent penetration, ease spreading.

Relevant Indian Standards (Clause 9.1.4)

Maintenance Tips

• Conduct field trials and accelerated weathering tests before application.
• Select finishes based on wood species and environmental exposure.
• Use plasticisers and driers as needed to improve finish properties (e.g., hardness, flexibility).

Summary Table: Wood Finish Types and Characteristics

IS SP Part 33: Special Applications - Marine & Chemical Factory Constructions

Marine Constructions (Clause 6.2.16)

• Structures exposed to seawater require materials resistant to corrosion and marine biofouling.
• Use corrosion-resistant metals (stainless steel, coated steel) or treated timber.
• Design for wave, current, and salt spray loads.
• Protective coatings and cathodic protection recommended.

Chemical Factory Constructions (Clause 6.2.15 & 6.2.15.1)

• Timber Selection:
  • Resistant timbers: teak, conifers (acid-resistant up to 10%), hardwoods (acid-resistant up to 5%).
  • Recommended species: white siris, pussar, sissoo, white cedar, cypress, dhaman, jarul, lendi, kail, chir, kindal.
• Treatment:
  • Neutralize acid/alkali effects via chemical treatments or coatings (commercial paints, shellac, asphalted paraffin).
  • Use resin-impregnated/compressed timbers (compregs).
• Joints:
  • Prefer all-wood joints to avoid metal corrosion.
  • Protect joints with coatings.
• Testing:
  • Perform accelerated exposure and strength tests pre- and post-treatment.

Summary Table: Timber Resistance to Chemicals

Key Recommendations:

• Avoid metals at joints in chemical environments.
• Use treated or naturally resistant timber.
• Conduct accelerated chemical exposure and strength tests before design finalization.

flowchart TD
    A[Chemical Exposure] --> B{Material Choice}
    B -->|Resistant Timber| C[Teak, Conifers, Sissoo, etc.]
    B -->|Treated Timber| D[Compregs, Coatings]
    B -->|Avoid Metals| E[All-Wood Joints]
    C & D & E -->

Key Specifications & Formulas for Poles, Posts, Pitprops, and Piles (IS SP Part 33, Clause 6.2.18)

1. Poles, Posts, Pitprops (Clause 6.2.18.1)

• Wood Species Groups (IS:876-1970):

  • Group A: Modulus of rupture > 850 kg/cm² (e.g., Sal, Teak, Chir)
  • Group B: 630 - 850 kg/cm²
  • Group C: 450 - 630 kg/cm²

• Load Classifications (Ultimate breaking load):

  • a) ≥ 1350 kg
  • b) 1100 - 1350 kg
  • c) 850 - 1100 kg
  • d) 700 - 850 kg
  • e) 550 - 700 kg
  • f) 400 - 550 kg
  • g) 300 - 400 kg

• Circumference at Ground Line (cm) for Poles (Table 25):

2. Testing Poles (Clause 6.2.18.4)

• Poles tested as cantilever with load applied 60 cm from top.
• Maximum fibre stress (σ) formula:

[ \sigma = \frac{P \times l}{Z} ]

Where:

• (P) = Load at failure (kg)
• (l) = Distance between load point and break point (cm)
• (Z = \frac{3}{32} \pi r^3) (Section modulus for circular cross-section)
• (r) = radius of pole (cm)

3. **Timber Piles (Clause

Key Indian Standards Relevant to Timber Engineering (IS SP Part 33)

Important IS Codes (Appendix A Highlights)

• IS 190-1974: Specification for coniferous sawn timber baulks and scantlings.
• IS 287-1973: Maximum permissible moisture content of timber.
• IS 883-1970: Code of practice for design of structural timber in buildings.
• IS 401-1982: Code of practice for preservation of timber.
• IS 1150-1976: Trade names and symbols for timber species.
• IS 1326-1976: Specification for non-coniferous sawn timber.
• IS 1708-1969: Methods of testing small clear specimens of timber.
• IS 3629-1966: Specification for structural timber in building.
• IS 4891-1968: Preferred cut-sizes of structural timbers.
• IS 4924-1968: Test methods for nail-jointed timber trusses.

Structural Timber Groups & Properties (Table 1)

Permissible Stresses for Grade 2 Timber (Table 2) (kgf/cm²)