Design of Structural Timber In Building -Code of Practice

IS 883: Scope & Key Formulas for Columns

• Scope: IS 883 covers design and specifications for steel structures, including columns with various end conditions and spacing.

Key Points from Clauses:

• Clause 7.6.1:

  • Formulae for solid columns apply to spaced columns with a restraint factor (k) = 2.5 or 3, based on end connector distances.

• Clause 7.6.1.5:

  • Formulas given are for pin-ended columns.
  • For other end conditions, effective length (L_eff) must be modified accordingly.

• Clause 7.6.3.2:

  • For long spaced columns, a specific formula applies (not detailed here, but involves modification of effective length and slenderness).

• Rounding Off:

  • Follow IS 2:1960 for rounding numerical results to maintain precision consistent with specified values.

Typical Column Buckling Formula (Pin-Ended):

[ P_{cr} = \frac{\pi^2 E I}{(L_{eff})^2} ]

• (P_{cr}): Critical buckling load
• (E): Modulus of elasticity
• (I): Moment of inertia
• (L_{eff}): Effective length (modified for end conditions)

Restraint Factor Application:

flowchart LR
    A[Column Type] --> B{Solid or Spaced?}
    B -->|Solid| C[Use 7.6.1 formula]
    B -->|Spaced| D[Apply restraint factor (2.5 or 3)]
    D --> E[Modify effective length for end conditions]
    E --> F[Calculate buckling load]

Summary: IS 883 provides formulas for columns considering spacing and end conditions, with effective length modification and restraint factors critical for design accuracy.

IS 883 Key References, Formulas & Tables Summary

1. Column Design Formulas (Clause 7.6.1.5, 7.6.3.2)

• Pin-ended columns: Use formula as per 7.6.1.5; modify length factor for other end conditions.
• Long spaced columns: Use formula in 7.6.3.2 (specific formula not provided in excerpt).
• Spaced columns: Apply solid column formula (7.6.1) with restraint factor = 2.5 or 3, depending on end connector spacing.

2. Combined Bending and Axial Tension (Clause 7.7.2)

Design must satisfy:

[ \frac{f_{at}}{f_{a}} + \frac{b}{s} \leq 1 ]

Where:

• ( f_{at} ) = axial tension stress
• ( f_a ) = permissible axial tension stress
• ( b ) = bending stress
• ( s ) = permissible bending stress

3. Important Tables (Clause 7.7.2)

• Table 7.6.1: Solid column properties (refer IS 883 for detailed values).

4. Referenced Indian Standards (Annex A)

Summary Diagram: Column End Conditions & Restraint Factor

graph TD
    A[Column Design] --> B[Pin End Condition]
    A --> C[Other End Conditions]
    B --> D[Use formula 7.6.1.5]
    C --> E[Modify length factor accordingly]
   

IS 883: General Design Requirements - Key Points

1. Design Approach (Clause 7.3):

   • Use mechanics-based calculations or prototype testing for design validation.

2. Column Design (Clause 7.6.3.2 & 7.6.1.5):

   • For long spaced columns, use the formula:
     [ f = \frac{\pi^2 E}{(L/k)^2} ]
     where (L) = effective length, (k) = radius of gyration, modified for end conditions beyond pin-ended.
   • Modify length (L) for different end conditions (fixed, free, etc.).

3. Permissible Stress Adjustment (Clause 6.4.2):

   • Multiply permissible stresses from Table I by the modification factor (K_2) from Table 5 based on load duration.

Summary Table: Permissible Stress Adjustment

End Condition Length Modifiers (Typical)

flowchart TD
    A[Design Requirements] --> B[Mechanics or Prototype Testing]
    A --> C[Column Design]
    C --> D[Use formula with L/k]
    D --> E[Modify L for End Conditions]
    A --> F[Permissible Stress]
    F --> G[Table I values]
    G --> H[Multiply by K2 from Table 5]

This concise framework ensures compliance with IS 883 general design principles.

IS 883: Material Requirements and Grading - Key Formulas & Tables

1. Recommended Moisture Content (Clause 5.3, Table 2)

• Zones based on average annual relative humidity:
  • I: <40%
  • II: 40-50%
  • III: 50-67%
  • IV: >67%

2. Minimum Permissible Stress Limits (N/mm²) for Grade I Timber (Clause 6.2, Table 3)

• For outside/wet conditions, multiply stresses by:
  • 5/6 for inside stresses
  • 2/3 for outside/wet

3. Permissible Stress Multiplication Factors (Clause 6.3)

4. **Modification

IS 883: Properties of Structural Timber

1. Grading & Species (Clause 5.1 & 5.6)

• Timber species recommended are listed in Table 1 (not provided here).
• Other species allowed if strength properties are verified.
• Grading ensures timber quality for structural use.

2. Permissible Stress Limits (Table 6.4.2, Clause 6.2 & 6.3)

3. Modifiers for Location

• For outside/wet conditions, multiply stresses by:
  • 5/6 for outside dry
  • 2/3 for wet locations

4. Key Notes

• Horizontal shear values apply only to beams.
• Use shear along grain values for other cases.
• Check sections for dead loads with modification factor K2 = 1.00.

Summary Formula for Design Stress:

[ f_{design} = f_{perm} \times k_{location} \times k_{duration} ]

• (f_{perm}): Permissible stress from table
• (k_{location}): 1 (inside dry), 5/6 (outside dry), 2/3 (wet)
• (k_{duration}):

IS 883: Modification Factors for Design (Clause 6.4)

1. Modification Factors for Slope of Grain (K1) — Table 4

Use K1 to reduce permissible stress based on grain slope.

2. Modification Factors for Duration of Load (K2) — Table 5

Permissible stress = Base stress × K2

Usage:

[ \text{Permissible Stress} = \text{Base Stress} \times K_1 \times K_2 ]

• K1 adjusts for grain slope.
• K2 adjusts for load duration.

flowchart LR
    A[Base Permissible Stress] --> B[Apply K1 (Slope of Grain)]
    B --> C[Apply K2 (Duration of Load)]
    C --> D[Final Permissible Stress]

This ensures safe design considering wood anisotropy and loading conditions.

IS 883: Design of Flexural Members (Clause 7.5 Highlights)

1. Basic Flexural Strength Formula

• Use the usual bending stress formula:
  [ f_b = \frac{M}{Z} ]
  where:
  • ( f_b ) = bending stress
  • ( M ) = bending moment
  • ( Z ) = section modulus

2. Form Factors for Flexural Members (Clause 7.5.4)

These factors adjust bending stress for different cross-section shapes:

3. Stiffening Requirements (Clause 7.5.6.1)

• If depth ( > 3 \times ) width or span ( > 50 \times ) width, provide lateral restraints against twisting/buckling.
• Max spacing between restraints = 50 × width.

Summary Diagram

flowchart TD
    A[Flexural Member Design] --> B[Calculate bending stress \(f_b = M/Z\)]
    B --> C{Cross-section type}
    C -->|Rectangular| D[Apply \(K_g = 0.81 D^2 + 55,000\) if \(D > 300\) mm]
    C -->|Box/I-beam| E[Apply complex \(K_4\) formula]
    C -->|Solid Circular| F[Use \(K_5 = 1.18\)]
   

IS 883: Design of Compression Members (Timber Structures)

Key Formulas

1. Combined Bending & Axial Compression (Clause 7.7.1):

[ \frac{f_{ac}}{f_c} + \frac{f_{ab}}{f_b} \leq 1 ]

• (f_{ac}) = actual compressive stress
• (f_c) = permissible compressive stress
• (f_{ab}) = actual bending stress
• (f_b) = permissible bending stress

2. Long Columns Permissible Compressive Stress (Clause 7.6.2.4):

[ f_c = \frac{f_{c0}}{1 + \beta \left(\frac{l}{r}\right)^2} ]

• (f_{c0}) = permissible compressive stress for short column
• (\beta) = factor depending on timber species and conditions
• (l) = effective length
• (r) = radius of gyration

3. Combined Bending & Axial Tension (Clause 7.7.2):

[ \frac{f_{at}}{f_t} + \alpha_b \leq 1 ]

• (f_{at}) = actual axial tensile stress
• (f_t) = permissible tensile stress
• (\alpha_b) = bending stress ratio

Important Tables (Refer Clause 7.7.2)

• Table 7.6.1: Permissible stresses for solid timber columns based on species, moisture content, and loading conditions.

Design Notes

• Use effective length (l) considering end conditions (pinned, fixed).
• Calculate radius of gyration (r = \sqrt{\frac{I}{A}}) for cross-section.
• Check combined stresses using interaction formulas above.
• Refer to IS 883 Annexes for timber species properties and correction factors.

flowchart TD
    A[Axial Load] --> B{Compression or Tension?}
    B -->|Compression| C[Calculate \(f_{ac}\)]
    B -->|Tension| D[Calculate \(f_{at}\)]
    C --> E[Calculate bending stress \(f_{ab}\)]
    D -->

IS 883: Design of Box and Built-up Columns

Key Definitions:

• Box Column (Clause 3.1.1): Formed by four members joined to form a hollow box with solid blocks at ends/intermediate points.
• Built-up Columns: Assembled from rolled sections or plates, connected by welding/bolting.

Design Considerations (Clause 7.6.2 & 7.6.1.5):

• Effective Length (L): Use pin-ended length; modify for other end conditions using effective length factors (K).
• Slenderness Ratio (λ):
  [ \lambda = \frac{K L}{r} ] where (r) = radius of gyration of the column cross-section.

Axial Load Capacity:

• Use Euler's buckling formula for long columns: [ P_{cr} = \frac{\pi^2 E I}{(K L)^2} ]

• For built-up columns, calculate (I) considering the composite section and intermediate stiffeners.

Intermediate Stiffeners (Clause 7.6.3.2):

• For long spaced columns, spacing (s) of stiffeners must satisfy: [ s \leq \text{(value from code tables depending on thickness and slenderness)} ]

Typical Table Snippet (from IS 883):

Summary:

• Use pin-ended length or modify with effective length factors.
• Calculate buckling load with Euler's formula.
• Provide solid blocks and intermediate stiffeners to prevent local buckling.
• Follow spacing limits for stiffeners from IS 883 tables.

flowchart TD
    A[Box/Built-up Column] --> B[Calculate Effective Length (K*L)]
    B --> C[Determine Slenderness Ratio (λ)]
    C --> D[Check Buckling Load (Euler's Formula)]
    D --> E{Is λ within limits?}
   

IS 883: Load Considerations & Load Combinations

1. Duration of Loads (Clause 6.4.2, 6.4.2.2)

• Modification factor K2 accounts for load duration.
• Use the shortest duration load in combinations for K2 (increases permissible stress).
• Example: For dead + snow + wind, use K2 = 1.33 (wind is shortest duration).

2. Load Combinations (Clause 7.2)

• Consider worst load combinations and locations.
• Wind and seismic loads NOT simultaneous.
• Typical combinations include:
  • Dead + Live
  • Dead + Snow
  • Dead + Wind
  • Dead + Earthquake

3. Columns (Clause 7.6.1.5)

• Formulae for pin-ended columns.
• Modify effective length for other end conditions:

[ L_{eff} = K \times L ]

Where (K) depends on end fixity (e.g., 0.5 for fixed-fixed).

Summary Mermaid Diagram

graph TD
A[Load Types] --> B[Dead (Continuous) K2=1.0]
A --> C[Snow K2~1.15]
A --> D[Wind/Earthquake K2=1.33]

E[Load Combinations] --> F[Dead + Live]
E --> G[Dead + Snow]
E --> H[Dead + Wind]
E --> I[Dead + Earthquake]

J[Columns] --> K[Pin-ended formula]
J --> L[Modify length for other end conditions]

Key Takeaway: Use K2 from shortest duration load in combinations; never combine wind and seismic simultaneously; adjust column length factor for end conditions.

IS 883: Defects and Limitations in Timber

1. Prohibited Defects (Clause 5.6.2.1)

Timber with these defects must not be used structurally:

• Loose grain, splits, compression wood (conifers)
• Heartwood rot, sap rot, crookedness
• Worm holes by powder post beetles, pitch pockets

2. Permissible Defects (Clause 5.6.2.2)

Allowed if strength reduction ≤ max allowable knots effect:

• Wanes (if not combined with knots and suitable for bearing/nailing)
• Worm holes (except powder post beetles)
• Other defects unlikely to affect strength

3. Modification Factor for Slope of Grain (Clause 6.4.1)

When slope of grain exists, multiply permissible stresses by K1 from Table 4.

(Refer IS 883 Table 4 for detailed values)

Summary Diagram

flowchart TD
    A[Timber Defects] --> B{Prohibited Defects?}
    B -- Yes --> C[Reject for Structural Use]
    B -- No --> D{Permissible Defects?}
    D -- Yes --> E[Use with Strength Consideration]
    D -- No --> C
    E --> F{Slope of Grain Present?}
    F -- Yes --> G[Apply K1 factor to stresses]
    F -- No --> H[Use standard permissible stresses]

Note: Always refer to IS 3629:1986 for grading and detailed defect evaluation.

IS 883: Bearing and Support Conditions - Key Formulas & Tables

1. Bearing Stress (Clause 7.5.8.3)

• Permissible bearing stress perpendicular to grain, ( f_{en} ), depends on length and position of bearing.
• For bearings ≥ 150 mm or at member ends, use values from Table 1 (compression perpendicular to grain).
• For bearings < 150 mm and located ≥ 75 mm from member end, multiply permissible stress by modification factor ( K_7 ).

2. Modification Factor ( K_7 ) (Table 7, Clause 7.5.8.3.1)

3. Bearing Stress at an Angle ( \theta ) to Grain (Clause 7.5.8.3.1 g)

[ f_{eb} = f_{ep} \sin^2 \theta + f_{on} \cos^2 \theta ]

• ( f_{ep} ) = permissible stress perpendicular to grain
• ( f_{on} ) = permissible stress parallel to grain

4. Deflection Factors ( K ) (Clause 7.5.9.2)

• Cantilever, load at free end: K = 1/3
• Cantilever, uniformly distributed load: K = 1
• Beam both ends, point load center: K = 1/48
• Beam both ends, uniformly distributed load: K = 1/5

5. Column Length Modification (Clause 7.6.1.5)

• Formulas given for pin-ended columns.
• Modify effective length factor ( K ) for other end conditions (fixed, free, etc.).

This summary aids quick design checks for bearing stresses

IS 883 - Notching and Boring in Structural Timber

Key Specifications (Clause 7.5.7.4)

• Notch depth: ≤ 1/5 of beam depth (d)
• Notch location: ≤ 1/6 of span length from support edge
• Hole diameter: ≤ 1/4 of beam depth (d)
• Hole location: Middle third of beam depth and length
• Bending strength: Use net remaining depth if notch/hole is > 3d from nearest support edge

Important Tables from IS 883

Modification Factor for Slope of Grain (K1)

Practical Notes:

• Always check local stresses if notches/holes exceed limits.
• Use net section properties for bending strength when holes/notches are far from supports.
• Apply modification factors for grain slope to adjust permissible stresses.

flowchart TD
    A[Beam] --> B[Notch Depth ≤ d/5]
    A --> C[Notch Location ≤ span/6 from support]
    A --> D

IS 883: Key Formulas for Calculation Methods (Clauses 7.5 & 7.6)

1. Flexural Strength (Clause 7.5.3)

• Use the usual flexural strength formula:

  [ M = f_y \times Z ]

  Where:

  • ( M ) = Moment capacity
  • ( f_y ) = Yield strength of steel
  • ( Z ) = Section modulus

2. Column Buckling (Clause 7.6.1 & 7.6.3.2)

• For solid columns (7.6.1), use Euler’s buckling formula modified by effective length factor ( K ):

  [ P_{cr} = \frac{\pi^2 E I}{(K L)^2} ]

  Where:

  • ( P_{cr} ) = Critical load
  • ( E ) = Modulus of elasticity
  • ( I ) = Moment of inertia
  • ( L ) = Actual length
  • ( K ) = Effective length factor (depends on end conditions)

• For long spaced columns (7.6.3.2), the formula adjusts for spacing and restraint:

  [ P_{cr} = \frac{\pi^2 E I}{(K L)^2} \times \text{Restraint Factor} ]

• Restraint factor per 7.6.1:

  • Use 2.5 or 3 depending on the distance of end connectors.

3. End Conditions (Clause 7.6.1.5)

• Formulae given assume pin-ended columns.
• Modify ( K ) for other end conditions:

Summary Table:

IS 883: Appendices and Annexures - Key Points

1. Annex A: List of Referred Indian Standards (Clause 2.1)

Annex A provides essential complementary IS codes necessary for timber structures:

2. Key Formula: Combined Bending and Axial Tension (Clause 7.7.2)

For members under bending and axial tension:

[ f_{at} + \alpha b \leq 1 ]

• ( f_{at} ) = axial tension stress ratio
• ( \alpha b ) = bending stress ratio

3. Column Design Notes (Clause 7.6.1.5)

• Formulas given assume pin-ended columns.
• Modify effective length factor for other end conditions.

4. Tables

• Table 7.6.1: Solid column properties (refer IS 883 for dimensions and design values).

Summary Diagram: Design Checks for Timber Columns

flowchart TD
    A[Start: Structural Member] --> B{Load Type}
    B -->|Axial Compression| C[Use Column Formula]
    B -->|Bending + Axial Tension| D[Check: fat + αb ≤ 1]
    C --> E[Modify length factor if not pin-ended]
    D --> F[Design OK if ≤ 1]
    F --> G[Proceed with Detailing]

Use Annex A IS codes for detailed timber properties, testing, and preservation.
For combined stresses, ensure the sum of normalized axial and bending stresses does not exceed unity. Adjust column formulas for end conditions.