Slip-critical and bearing-type bolted joints

Bolted steel joints can look similar but rely on different load-transfer behavior. The key term is the faying surface: the contact surface between the connected steel plies. In a slip-critical connection, friction at that interface is part of the design. In a bearing-type connection, the joint can experience some slip until the bolt and connected material bear against one another.

How a slip-critical joint carries load

High-strength bolts are installed with the required pretension to clamp the connected plies. When force acts parallel to the faying surfaces, friction resists relative movement. Design resistance depends on factors such as bolt pretension, the number of slip planes, the specified slip coefficient, and the condition or coating of the faying surfaces. Surface preparation matters because grease, paint, galvanizing or other treatments can change the available slip resistance; only approved conditions and the governing specification should be used.

How a bearing-type joint carries load

A bearing-type connection is designed for load transfer through bearing of the connected material against the bolt and shear through the bolt, subject to the applicable design checks. Depending on the detailing and loading, small movement can occur before bearing develops. This joint may also use pretensioned bolts for reasons such as fatigue or serviceability, but that alone does not make the connection slip-critical.

When the distinction matters

Slip resistance is important where movement at the joint would impair performance, including certain fatigue-sensitive or load-reversal conditions and connections designated by the design requirements. It can require additional surface preparation, inspection and installation control. A bearing-type connection may be appropriate where the design permits bearing behavior. The engineer's drawings, specifications and applicable building code determine which joint is required; field personnel should not change the classification or surface treatment on their own.

Compare the load path

• Slip-critical: friction between faying surfaces resists slip; pretension and surface condition are essential design inputs.
• Bearing-type: connected steel bears on the bolt and the bolt resists shear after movement brings the parts into contact.
• Pretensioned: describes bolt installation; it does not by itself identify the joint as slip-critical.
• Faying surfaces: the contact faces between plies, not the surfaces beneath bolt heads or nuts.
• Inspection: follows the governing specification and the connection's specified installation method.

Exam approach

Look for the intended service behavior in the question. If the requirement is to prevent slip, identify a slip-critical joint and its faying-surface controls. If the question describes load transfer by bearing of connected material against bolts, it is a bearing-type joint. Then distinguish the joint's design function from the method used to tighten the bolts.

Key takeaway

Slip-critical connections use friction at prepared faying surfaces to resist movement; bearing-type joints transfer load through bearing and bolt shear. The design designation—not simply the fact that bolts were tightened—sets the requirements.