Bearing-Type vs. Slip-Critical Bolted Connections: Key Differences in Design and Application

Bolted steel connections play a central role in how structures behave under load—from everyday gravity forces to wind, vibration, and seismic activity. In Ontario’s industrial and commercial sectors, the choice between Bearing-Type and Slip-Critical connections is not just a detailing preference; it directly impacts structural performance, installation costs, and long-term durability.

While both systems are accepted under CSA S16 (Design of Steel Structures), they function differently in service. Understanding where "slip" is acceptable—and where even a few millimetres of movement can compromise safety or operations—is essential for efficient design.

The Core Difference

To the untrained eye, the bolts look the same. The difference lies in the mechanism of load transfer:

• Bearing-Type Connections: The bolts are tightened to a "snug-tight" condition. The connection allows for a tiny amount of slip until the bolt shank physically contacts (bears against) the side of the hole.

  
  

• Slip-Critical Connections: The bolts are fully pretensioned to create a massive clamping force. The load is transferred strictly through friction between the steel surfaces (faying surfaces), preventing any slip under service loads.

  
  

Why the Distinction Matters

1. Serviceability & Alignment: In bearing connections, minor settling is expected. However, for long-span trusses, crane runways, or precise equipment supports, this movement can throw systems out of alignment. Slip-critical connections lock the geometry in place.

   
   

2. Fatigue & Vibration: If a structure is subject to load reversals (e.g., wind on a tall structure) or vibration (e.g., machinery), a bearing connection may loosen over time or damage the bolt hole (ovalization). Slip-critical connections protect against this fatigue.

   
   

3. Execution & Cost: This is the biggest factor for owners and contractors.

   
   

   • Bearing-Type: Fast to install (snug-tight) and requires minimal surface prep.

   • Slip-Critical: Significantly more expensive. It requires specific surface preparation (Class A or B friction surfaces), strictly controlled bolt pretensioning (turn-of-nut or calibrated wrench), and rigorous third-party inspection to verify tension.

Typical Applications in Ontario

Bearing-Type (The Standard Choice)

• Standard beam-to-column shear tabs.

• Secondary steel: platforms, stairs, and rooftop dunnage (equipment supports).

• Light-industrial framing where minor settling is imperceptible.

Slip-Critical (The High-Performance Choice)

• Crane Runways: Where impact and fatigue are constant.

• Bracing Members: To ensure the building generally remains rigid under wind/seismic loads.

• Oversized/Slotted Holes: If a connection uses oversized holes for installation tolerance, it is often required to be slip-critical to prevent the bolt from sliding around in the slot.

• Splices: In long girders where slip would cause visible sagging.

Key Design & Execution Considerations

Successful steel projects require more than just picking a bolt type; they require execution.

• Construction Reality (Faying Surfaces): Slip-critical connections rely on friction. This means the steel surfaces (faying surfaces) must be rated as Class A (clean mill scale) or Class B (blast-cleaned or zinc-rich paint). If a contractor accidentally paints these surfaces with standard primer, the connection loses its friction rating and may fail inspection.

  
  

• The Risk of Overdesign: We often see slip-critical bolts specified "just to be safe." This increases erection costs (due to tensioning labor) and inspection costs without adding structural benefit. We avoid this by isolating slip-critical requirements only to where they are strictly needed.

  
  

• Edge Distance & Tear-Out: For bearing-type connections, the bolt pushes against the steel plate. If the bolt is too close to the edge of the plate, it can cause "tear-out" or plate deformation. We carefully check edge distances to prevent this failure mode.

  
  

• Inspection Protocols: Bearing connections generally only require verification that bolts are "snug-tight." Slip-critical connections, however, require third-party verification of bolt tension, which adds time to the schedule.

  
  

Decision Matrix: Bearing vs. Slip-Critical

Design Factor	Bearing-Type	Slip-Critical
Load Transfer	Shear/Bearing on bolt shank	Friction between steel plates
Installation	Snug-Tight	Fully Pretensioned
Surface Prep	Minimal (Standard Shop Primer)	Strict (Masked/Clean or Zinc Silicate)
Vibration/Fatigue	Not Recommended	Strongly Recommended
Oversized Holes	Permitted (restrictions apply)	Preferred for adjustability
Cost	Lower	Higher (Labor + Inspection)
Typical Applications	Shear tabs, stairs, platforms, light framing	Bracing, crane runways, long splices, vibrating equipment

Conclusion: Engineering for Value and Safety

There is no "better" connection—only the correct connection for the application. Over-specifying slip-critical connections wastes budget on unnecessary prep and inspection. Under-specifying them risks maintenance issues and alignment failure.

At Parsways Inc., we ensure our designs meet CSA S16 requirements while respecting the construction budget. We focus on detailing that is efficient, buildable, and reliable for the lifecycle of the structure.