The question of whether the bottom chord of a roof truss needs lateral restraint depends on several factors, including the stress conditions and the slenderness ratio of the chord. Here’s a detailed analysis:

1. Theoretical Considerations

• No Compression Stress:
  • If the bottom chord does not experience compressive stress (i.e., the wind uplift force does not exceed the self-weight of the truss), lateral restraint is not necessary.
  • In such cases, the bottom chord primarily acts in tension, and lateral buckling is not a concern.

2. Practical Considerations for Long-Span Trusses

• Slenderness Ratio (L/r):
  • For long-span trusses, the slenderness ratio (L/r) of the bottom chord must be checked to ensure it does not exceed 300.
  • This limit applies regardless of the stress condition and ensures that the chord does not buckle out-of-plane.
• Connection to Diagonal Members:
  • The bottom chord is connected to the compression diagonals of the truss.
  • Since the buckling length of the diagonals is calculated assuming hinged connections at both ends, the bottom chord must provide sufficient strength and stiffness to support these assumptions.

3. Design Requirements

• Strength and Stiffness:
  • The bottom chord must be designed to resist tensile forces and provide adequate stiffness to prevent excessive deformation.
  • If lateral restraint is not provided, the chord’s slenderness ratio must be carefully controlled to avoid buckling.
• Lateral Restraint:
  • In cases where the bottom chord is subjected to compressive stresses (e.g., due to wind uplift), lateral restraint is essential to prevent buckling.
  • Even in tension-dominated scenarios, lateral restraint may be required to ensure the stability of the truss system.

4. Key Takeaways

• No Compression: Lateral restraint is not required if the bottom chord is in pure tension.
• Long-Span Trusses: The slenderness ratio (L/r) must be checked and kept below 300 to prevent out-of-plane buckling.
• Diagonal Connections: The bottom chord must provide sufficient strength and stiffness to support the compression diagonals.
• Wind Uplift: If compressive stresses are possible, lateral restraint is necessary.

Conclusion:

The need for lateral restraint in the bottom chord of a roof truss depends on the stress conditions and the slenderness ratio. While lateral restraint may not be required in tension-dominated scenarios, it is essential for long-span trusses or when compressive stresses are present. Engineers must carefully evaluate these factors to ensure the stability and performance of the truss system.