A slip-critical bolt connection is a type of structural steel connection designed to prevent any relative movement (slip) between the connected members under typical service loads by relying on friction.
Understanding Slip-Critical Connections
In structural engineering, connections between steel members are crucial for transferring loads. Most standard bolted connections, known as bearing-type connections, transfer load by the bolt shaft bearing directly against the sides of the holes in the connected plates once slight slip occurs.
However, some applications require connections where even minimal slip is unacceptable. This is where slip-critical connections are used.
As stated in the provided reference:
- Slip-critical connections do not rely on bearing between the plate and the bolt to transfer the load between joint members.
- Instead, pretensioning of the bolt creates a clamping force between the connected members and the joint relies on friction to transfer the load between joint members.
This means that high-strength bolts in a slip-critical connection are significantly tightened (pretensioned) to create a large clamping force between the steel plates. This clamping force generates friction resistance between the contact surfaces of the connected members. It is this friction that resists the applied load and prevents slip, not the bolt shaft pressing against the hole.
Key Characteristics
- Load Transfer Mechanism: Primarily through friction between faying (contact) surfaces.
- Mechanism for Friction: High pretension in the bolts creating a clamping force.
- Behavior under Load: Designed to prevent noticeable slip at service loads.
- Bolts Used: Typically high-strength bolts (e.g., ASTM A325, A490).
- Surface Preparation: Often requires specific preparation of the contact surfaces to achieve the desired friction coefficient.
Slip-Critical vs. Bearing Connections
Here's a simple comparison:
| Feature | Slip-Critical Connection | Bearing Connection |
|---|---|---|
| Primary Load Transfer | Friction between plates | Bolt bearing against plate holes |
| Bolt State | High pretension (tightened) | Snug-tight or fully tensioned (depending on type) |
| Required Slip | None (at service loads) | Small amount of slip occurs for load transfer |
| Surface Prep | Critical (to control friction coefficient) | Less critical |
| Cost | Generally higher (materials, installation) | Generally lower |
When are Slip-Critical Connections Used?
Slip-critical connections are specified in situations where limiting joint deformation and preventing slip are critical for the structure's performance, such as:
- Connections subject to fatigue loading, where repeated load cycles could cause wear or failure in bearing connections.
- Connections where minimal deflection or movement is essential for the proper function of the structure or attached elements (e.g., supporting sensitive equipment, preventing cracking in finishes).
- Connections subject to significant vibration.
- Connections in structures sensitive to dynamic effects, such as bridge connections or connections in crane girders.
- Connections using oversized or slotted holes, where a bearing connection would have excessive initial slip.
In summary, a slip-critical bolt connection is a premium type of connection designed to avoid slip under service loads by utilizing bolt pretension to generate friction between the connected components, ensuring a rigid and predictable joint behavior.
