The fundamental difference between TB (Tensioned Bolted) and TF (Tensioned Friction) bolts lies in how they resist loads and manage movement within a connection. TB connections permit a degree of slip, while TF connections are designed to prevent any slip whatsoever.
Understanding TB (Tensioned Bolted) Connections
TB connections are considered the norm in many structural applications. Their design principle is to allow for some initial movement or slip under load before the bolts fully engage and bear against the edges of their holes.
- Mechanism: When a load is applied, the connected elements are allowed to slip. The bolts then bear against the sides of the bolt holes, and the load is transferred through this bearing action.
- Hole Size: A key characteristic of TB connections is the use of bolt holes that are typically 2 mm larger than the bolt's diameter. This oversized hole facilitates the intended initial slip.
- Application: These connections are suitable for situations where a slight amount of deflection or movement is acceptable in the structure without compromising its integrity.
Understanding TF (Tensioned Friction) Connections
In contrast, TF connections are specifically employed in applications where even minimal building movement or slip is unacceptable. They achieve this no-slip condition by relying heavily on friction.
- Mechanism: TF bolts are tightened to a very high tension, creating a significant clamping force between the connected members. This high clamping force generates enough friction between the contact surfaces to prevent any slip, ensuring the connection remains rigid and tight against applied loads. The bolt uses this friction to prevent it from slipping to the edge of the hole.
- Movement: The primary goal of TF connections is to prevent any relative movement or slip between the joined components.
- Application: These are critical for structures requiring high rigidity, precision, or where vibrations and fatigue are major concerns.
Key Differences at a Glance
The following table summarizes the main distinctions between TB and TF bolts:
| Feature | TB Bolts (Tensioned Bolted) | TF Bolts (Tensioned Friction) |
|---|---|---|
| Design Principle | Allows for initial slip, then load transfer through bearing. | Prevents all slip by relying on friction from high clamping force. |
| Movement | Permits minor slip before bolts bear against holes. | Designed for no detectable slip. |
| Hole Clearance | Uses 2 mm oversized holes to allow for slip. | Focuses on maintaining high friction to prevent movement to the hole edge. |
| Load Transfer | Primarily through bolt bearing against the hole. | Primarily through friction between connected surfaces. |
| Typical Use | Standard structural connections where some movement is permissible. | Critical applications requiring no movement, such as high-rise buildings or bridges. |
Practical Considerations
- Pre-load Requirements: TF connections demand precise and typically higher pre-load (tension) in the bolts to generate the necessary friction. TB connections also require proper tensioning, but the exact pre-load is less critical for preventing initial slip since slip is inherent to their design.
- Performance: TF connections offer superior rigidity and are preferred for structures subject to dynamic loads, fatigue, or where precise alignment must be maintained. TB connections are more forgiving regarding minor misalignments due to the allowed slip.
- Inspection: Verifying the proper pre-load is crucial for TF connections to ensure their slip-critical performance. For TB connections, ensuring proper tightening and visual inspection for bearing is common practice.
Examples of Application
- TB Bolts: Commonly found in general structural steel framing in buildings where typical deflections and movements are within acceptable limits, and the cost-effectiveness of an easier installation process is preferred.
- TF Bolts: Used in critical infrastructure like long-span bridges, structures in high-seismic zones, connections for heavy machinery, or any building where minimizing sway and ensuring absolute rigidity is paramount for safety and performance.
