Design of Purlin to BS 5950

The purlin design principles specifically according to BS 5950 (British Standard for Structural Use of Steelwork in Building):

1. Load Combinations (BS 5950-1):

• Ultimate Limit State (ULS) combinations used here: 1.4Gk + 1.6Qk
• The standard requires consideration of all relevant load combinations including:
  • Dead + Imposed loads
  • Dead + Wind loads
  • Dead + Imposed + Wind loads

2. Section Classification (BS 5950-5):

• Cold-formed sections (like the purlin shown) are covered in Part 5
• The standard requires checking local buckling of thin elements
• Section properties (Ixx, Iyy, Zxx, Zyy) must be calculated considering effective sections where elements might be subject to local buckling

3. Member Design Checks (BS 5950-5):

• Bending Capacity:
  • Moment capacity = py × Zxx (where py = 440 N/mm² for high tensile steel)
  • Must consider lateral-torsional buckling for unsupported lengths
  • For purlins, the roof sheeting typically provides lateral restraint to the top flange

4. Serviceability Limits (BS 5950-1):

• Deflection limits:
  • L/300 for roof members (as used in your calculation)
  • Must be checked under unfactored imposed loads
  • Uses elastic modulus E = 200 GPa as specified in the standard

5. Additional BS 5950-5 Considerations:

• Web crippling at supports
• Combined bending and web crippling
• Interaction of local and overall buckling
• Effects of holes and cut-outs if present
• Shear capacity of the section

The calculation shown focuses primarily on the bending and deflection checks, which are often the controlling factors for purlin design. However, a complete design to BS 5950 would typically include these additional checks depending on the specific application.