Code of Practice for Use of Steel Tubes In General Building Construction

Scope of IS 806: Structural Steel Tubes

IS 806 covers the use of steel tubes for structural purposes including design, permissible stresses, and fabrication.

Key Specifications & Formulas:

1. Permissible Axial Stress in Compression (Clause 5.2, Table 2)

• Note: Use linear interpolation for intermediate values.
• Purpose: Limits direct compression stress on tube cross-section.

2. Length of Curve of Intersection of Tubes (Appendix B, Clause 5.7.2)

For a branch tube intersecting a main tube or flat plate:

[ P = a + b + \frac{3}{\sqrt{a^2 + b^2}} ]

Where:

• (a = d \cdot \csc \theta \cdot \sqrt{1 - (d/D)^2})

• (b = d \cdot \sqrt{1 - (d/D)^2}) (for tube intersection)
  or
  (b = d \cdot \theta) (for flat plate, (\theta) in radians)

• (d) = outside diameter of branch tube

• (D) = outside diameter of main tube

• (\theta) = angle between branch and main tube

Summary

• IS 806 provides design limits and formulas for structural steel tubes.
• Permissible stresses depend on tube grade and slenderness ratio (l/r).
• Intersection curves between tubes use geometric relations for fabrication.

flowchart LR
    A[Main Tube (D)] -->|Intersect at angle θ| B

IS 806: Terminology & General Considerations - Key Formulas & Tables

1. Length of Curve of Intersection of Tubes (Appendix B, Clause 5.7.2)

For intersection of a tube with another tube or flat plate:

[ P = a + b + \frac{3}{\sqrt{a^2 + b^2}} ]

Where:

• ( a = d \cdot \csc \theta \cdot \sqrt{1 - \left(\frac{d}{D}\right)^2} )
• ( b = d \cdot \sqrt{1 - \left(\frac{d}{D}\right)^2} ) (for tube intersection)
• ( b = d ) (for flat plate intersection)
• ( d ) = outside diameter of branch tube
• ( D ) = outside diameter of main tube
• ( \theta ) = angle between branch and main tube

2. Permissible Axial Stress in Compression (Clause 5.2, Table 2)

Intermediate values obtained by linear interpolation.

3. Formula for Permissible Axial Stress (Appendix A)

For slenderness ratio ( 60 \leq l/r \leq 150 ):

[ F_e = \frac{1}{1 + 0.15 \sec \left(\frac{l/r}{m} \sqrt{\frac{f_y}{E}}\right)} ]

Where:

• ( F_e ) = permissible axial compressive stress
• ( f_y ) = yield stress (kgf/cm

IS 806: Materials - Key Formulas, Tables & Specifications

1. Permissible Axial Stress in Compression (Clause 5.2)

• Direct compression stress (Fe) on steel tube cross-section must not exceed values from Table 2, based on slenderness ratio ( \frac{l}{r} ) (effective length/radius of gyration).
• Grades: YSt 22, YSt 25, YSt 32 (in kgf/cm²)
• Intermediate values can be linearly interpolated.

(Full table in code Appendix)

2. Minimum Thickness of Metals (Clause 6.3)

• Specifies minimum thickness for steel tubes to ensure structural safety (refer IS 806 for exact values).

3. Curve of Intersection Length (Appendix B, Clause 5.7.2)

• For tube-to-tube or tube-to-plate intersections:

[ P = a + b + \frac{3}{\sqrt{a^2 + b^2}} ]

Where:

• ( a = d \cdot \csc \theta \cdot \sqrt{1 - \left(\frac{d}{D}\right)^2} )
• ( b = d \cdot \sqrt{1 - \left(\frac{d}{D}\right)^2} ) (for tube intersection)
• ( d ) = branch tube diameter, ( D ) = main tube diameter, ( \theta ) = angle between tubes.

Notes:

• Steel grades correspond to yield strengths in kgf/cm².
• Use linear interpolation for intermediate slenderness ratios.
• Welding electrodes and other materials must comply with IS specifications (e.g., covered electrodes IS 814).

Design Principles for Tubular Structures (IS 806)

Context:
IS 806 refers to steel tubular structures, recommending design principles aligned with IS 800-1962 Section IV and British Standard B.S. 449:1959.

Key Design Principles (from IS 800-1962 & IS 806):

• Material: Use structural steel tubes as per specified grades.
• Load Considerations: Include dead, live, wind, and seismic loads.
• Limit State Design: Check for strength, serviceability, and stability.
• Cross-Section: Typically circular, square, or rectangular hollow sections.
• Buckling: Consider local, flexural, and torsional buckling.
• Connections: Welded or bolted, designed for moment and shear transfer.

Important Formulas (from IS 800-1962 Section IV):

1. Axial Compression Capacity: [ P_u = A \times f_{cd} ] Where:

   • (A) = Cross-sectional area
   • (f_{cd}) = Design compressive stress (factored)

2. Bending Moment Capacity: [ M_u = Z \times f_{yd} ] Where:

   • (Z) = Section modulus
   • (f_{yd}) = Design yield stress

3. Combined Axial & Bending: [ \frac{P_u}{P_{allow}} + \frac{M_u}{M_{allow}} \leq 1.0 ]

Typical Section Properties (Example for Circular Hollow Section):

(Refer IS 806 Annexures or IS 800 tables for detailed values)

Summary:

• Follow IS 800 Section IV for design methods.
• Use **limit state

IS 806: Permissible Stresses for Steel Tubes

1. Axial Stress in Tension (Clause 5.1)

2. Axial Stress in Compression (Clause 5.2)

• Permissible compressive stress, Fe, depends on the slenderness ratio ( l/r ) (effective length / radius of gyration).
• Values from Table 2 (partial extract):

Use linear interpolation for intermediate values.

3. Bending Stress (Clause 5.3)

4. Shear Stress (Clause 5.4)

• Maximum shear stress calculated on half the net cross-sectional area.

Summary Formula for Axial Compression:

[ \sigma_c \leq F_e(l/r) ]

Where:

• (\sigma_c) = actual compressive stress
• (F_e) = permissible compressive stress from Table 2

IS 806: Key Points on Connections (Clause 6.7 & 6.4.3)

1. Welded Connections (6.7.3)

• Welds must be designed to transfer forces without undue eccentricity.
• Ensure weld size and length satisfy strength requirements per load.

2. Eccentricity of Connections (6.7.2.1)

• The centre of resistance of the connection should align with the line of action of the load.
• This reduces or eliminates eccentric moments, preventing unwanted stresses.

3. Eccentricity of Beam Reactions on Columns (6.4.3, Table 8)

Practical Design Tip:

• When designing connections, always check eccentricities using the above assumptions.
• Use these eccentricities to calculate moments:
  [ M = P \times e ] where
  (P) = load,
  (e) = eccentricity from Table 8.

flowchart LR
    Load -->|Acts on| Connection
    Connection -->|Centre of Resistance| AlignedLoadLine
    AlignedLoadLine -.->|If eccentricity exists| Moment(M = P × e)
    Moment -->|Influences| ConnectionDesign

This ensures safe, efficient connection design per IS 806.

IS 806: Fabrication and Protective Coatings Key Points

1. Protective Coating Requirements (Clause 7.8)

• Tubes not galvanized must be painted, oiled, or otherwise coated before weather exposure.
• Special painting requirements are to be agreed upon by purchaser and manufacturer.

2. Minimum Wall Thickness Based on Coating & Exposure

3. Painting Maintenance

• Painting system (zinc primer + paints) renew every 2 years for weather-exposed tubes.
• Aluminium or other metallic coatings may allow longer renewal intervals.

Summary of Protective Coating System (Clause 6.3.2)

1. Zinc primer coat (IS:104-1962)
2. One coat paint (IS:2074-1962)
3. Two coats paint (IS:123-1962)

flowchart TD
    A[Tubes Fabrication] --> B{Galvanized?}
    B -- Yes --> C[No additional coating needed]
    B -- No --> D[Apply protective coating]
    D --> E{Exposure?}
    E -- Exposed to weather --> F[Min thickness 4 mm (red oxide-zinc chromate) or 3.2 mm (zinc primer system)]
    E -- Not exposed --> G[Min thickness 3.2 mm (red oxide-zinc chromate) or 2.6 mm

IS 806 - Inspection and Testing: Key Points

• Clause 7.9: Marking, shop erection, and packing follow IS 800-1962 provisions.

• Clause 8 (Inspection and Testing): Requires agreement between purchaser and manufacturer on inspection/testing procedures (Clause 7.8).

• Rounding off values: As per IS 2-1962, final test/analysis results should be rounded off with the same significant digits as specified.

Important Formula (Appendix B, Clause 5.7.2)

Length of curve of intersection (P) between tubes or tube and flat plate:

[ P = a + b + \frac{3}{\sqrt{a^2 + b^2}} ]

Where:

• (a = \frac{d}{\sin \theta} \sqrt{1 - \left(\frac{d}{D}\right)^2})
• (b = d \sqrt{1 - \left(\frac{d}{D}\right)^2}) (for tube intersection)
• (b = d) (for flat plate intersection)

(\theta) = angle between branch and main tube;
(d) = outside diameter of branch;
(D) = outside diameter of main tube.

Summary

• Inspection/testing details are to be mutually agreed.
• Use IS 800 for marking and packing.
• Use the above formula for tube intersection curve length calculation.
• Round off test values per IS 2-1962.

flowchart LR
    A[Start Inspection] --> B{Agreement on Procedure?}
    B -- Yes --> C[Perform Inspection & Testing]
    B -- No --> D[Negotiate Procedure]
    C --> E[Record Results]
    E --> F[Round off per IS 2-1962]
    F --> G[Accept/Reject]
    G --> H[Mark, Pack as per IS 800]
    H --> I[Dispatch]

For detailed procedures, refer to IS 806 and IS 800 standards.

IS 806: Determination of Length of Curve of Intersection of Tubes

From Clause 5.7.2 (Appendix B):

The length of the curve of intersection ( P ) of a branch tube with a main tube or flat plate is approximated by:

[ P = a + b + \frac{3}{\sqrt{a^2 + b^2}} ]

Where:

• ( a = d \cdot \csc \theta \cdot \sqrt{1 - \left(\frac{d}{D}\right)^2} )
• ( b = d \cdot \sqrt{1 - \left(\frac{d}{D}\right)^2} ) (for tube intersection)
• ( d ) = outside diameter of branch tube
• ( D ) = outside diameter of main tube
• ( \theta ) = angle between the axes of the tubes

Note: For intersection with a flat plate, ( b = d ).

Summary Table:

Additional Notes:

• Use radians for trigonometric functions.
• For non-intersecting axes (Clause 6.7.3.4), ensure the curve lies within permissible limits.
• Welding details depend on angle and diameter ratios (Clauses 6.7.3.2 & 6.7.3.3).

graph LR
A[Branch Tube] -- Intersects