In seismic design, tension-only bracing systems are commonly used in steel frames with limited ductility. These systems are designed to resist tensile forces while buckling under compressive forces, ensuring that the braces do not contribute to the structure’s stiffness during compression. Here’s a detailed explanation of their design and behavior:
1. Code Requirements for Tension-Only Bracing
- Maximum Slenderness Ratio:
- According to seismic design codes, the maximum slenderness ratio (L/r) for tension-only braces is 300.
- This ensures that the brace is slender enough to buckle elastically under compression without sustaining damage.
- Minimum Slenderness Ratio:
- To ensure reliable performance under cyclic loading (e.g., earthquakes), a minimum slenderness ratio of 200 is recommended.
- This prevents inelastic buckling, which can reduce the brace’s tensile capacity and lead to premature failure.
2. Behavior Under Cyclic Loading
- Tension Phase:
- The brace resists tensile forces, contributing to the structure’s lateral stiffness and strength.
- Compression Phase:
- The brace buckles elastically under compressive forces, effectively becoming inactive and not contributing to the structure’s stiffness.
- This behavior ensures that the brace does not undergo inelastic deformation, preserving its tensile capacity for subsequent loading cycles.
3. Design Considerations
- Elastic Buckling:
- The brace must be designed to buckle elastically under compression without sustaining damage.
- This requires careful control of the slenderness ratio and cross-sectional properties.
- Connection Details:
- The connections between the brace and the frame must accommodate the large deformations associated with buckling.
- Pin-ended connections are typically used to allow free rotation and prevent moment transfer.
- Material Selection:
- High-strength steel is often used to maximize the brace’s tensile capacity while maintaining a slender cross-section.
4. Advantages of Tension-Only Bracing
- Simplified Analysis:
- The brace’s behavior is predictable, simplifying the structural analysis and design process.
- Energy Dissipation:
- The cyclic buckling and straightening of the brace dissipate energy, enhancing the structure’s seismic performance.
- Cost-Effectiveness:
- Tension-only braces are often lighter and less expensive than compression-resistant braces.
5. Limitations
- Reduced Stiffness:
- The structure’s lateral stiffness is reduced during the compression phase, which may lead to larger displacements.
- Design Complexity:
- Ensuring elastic buckling requires careful design and detailing, particularly for the connections.
Conclusion:
Tension-only bracing systems are an effective solution for seismic-resistant design, particularly in steel frames with limited ductility. By ensuring that the braces buckle elastically under compression and resist tensile forces effectively, engineers can enhance the structure’s performance during earthquakes. However, careful attention must be paid to the slenderness ratio, connection details, and material selection to ensure reliable and efficient performance
