When designing a slip-critical bolted connection, what is the primary mechanism by which the connection transfers shear forces, and why is this mechanism different from a bearing-type connection?

When designing a slip-critical bolted connection, the primary mechanism by which the connection transfers shear forces is friction. Shear forces are forces that act parallel to the surface of a material, tending to cause one part of the material to slide past another. In a slip-critical connection, high-strength bolts are installed and tightened to a specified tension, known as pretension. This pretension creates a significant clamping force that presses the connected elements, or plates, tightly together. The surfaces in contact, called the faying surfaces, are designed to have a high coefficient of friction, often achieved through surface preparation like grit blasting. This high clamping force, combined with the friction between the faying surfaces, generates a substantial friction resistance that prevents any relative movement, or 'slip', between the connected parts when subjected to shear forces. The connection is designed to resist the applied shear load entirely through this friction without any slip occurring, thus maintaining the original alignment and stiffness of the structure. This mechanism is crucial for connections where even a small amount of slip could be detrimental, such as in fatigue-sensitive structures or those requiring precise alignment. The bolts themselves are primarily stressed in tension due to the pretension, not in shear or bearing during normal service. The pretension also keeps the bolts from loosening and enhances the fatigue life of the connection. This prevents the bolt shank from contacting the hole wall. The bolt only functions in shear if the friction capacity is overcome, at which point the connection behaves like a bearing-type connection. This state is considered the ultimate limit state for a slip-critical connection, not its primary service mechanism.

This mechanism is fundamentally different from a bearing-type connection, where the primary mechanism for transferring shear forces involves the direct contact and deformation, or 'bearing', of the bolt shank against the sides of the holes in the connected plates, along with the shearing resistance of the bolt itself. In a bearing-type connection, the bolts are tightened only to a snug-tight condition, meaning there is little to no significant pretension and therefore negligible clamping force to create friction resistance between the faying surfaces. When shear forces are applied, the connected plates are permitted to move relative to each other, or 'slip', until the bolt shanks make firm contact with the walls of the holes. The bolt shank is the unthreaded portion of the bolt's body. Once contact is made, the shear forces are transferred through direct bearing stress on the contact area between the bolt shank and the hole wall, and the bolts themselves are subjected to shear stress, meaning they are being cut or sheared across their cross-section. The design strength of a bearing-type connection is governed by the bearing strength of the connected material (resistance to deformation around the hole) and the shear strength of the bolts. Unlike slip-critical connections, some initial slip is an expected and acceptable part of the load transfer mechanism in a bearing-type connection, meaning they do not maintain precise alignment under all service loads and can experience local deformations around the holes.