In structural steel design, slip-critical joints play a vital role in ensuring the safety and performance of buildings and infrastructure. At Tnemec, we recognize the importance of these connections and the role protective coatings play in enhancing their durability and functionality. Let’s explore the details of slip-critical joints, mean slip coefficients, and how Tnemec coatings meet the necessary standards to support structural integrity.

What is a Slip-Critical Joint?

A slip-critical joint is a connection that transmits shear loads—or shear loads combined with tensile loads—through pre-tensioned bolts. These bolts provide a clamping force on the faying surfaces, which are the planes of contact between the connected components. These joints are designed to resist any movement or “slip” at the faying surfaces to maintain the structure's performance.

Slip-critical joints are essential in specific applications, including:

• Joints that are subject to fatigue loads with a reversal of the loading direction.
• Joints utilizing oversized holes.
• Joints using slotted holes, except where the load direction is approximately normal (80 to 100 degrees) to the slot.
• Joints where slip occurs at the faying surfaces that could compromise structural performance.

The Role of Protective Coatings in Slip-Critical Joints

Corrosion protection is a critical consideration for structural steel joints. However, not all coatings are suitable for faying surfaces in slip-critical connections. To ensure structural performance, coatings must meet specific slip-coefficient and creep resistance standards established by the American Institute of Steel Construction (AISC).

Coatings are tested according to the Research Council on Structural Connections (RCSC) Specification for Structural Joints Using High-Strength Bolts (Appendix A). These tests assess:

1. Slip Coefficient: The frictional resistance of the coating under compression.
2. Creep Deformation: The coating's mechanical response to sustained loads.

Understanding Creep Deformation Testing

Creep deformation testing ensures the coating will not deform significantly under sustained loading conditions. A key parameter is the maximum allowable creep deformation of 0.005 inches. If a coating exceeds this limit during testing, it fails to meet the slip coefficient requirements.

Mean Slip Coefficient Classes

The RCSC defines three main classes of slip coefficient for coatings:

• Class A: For faying surfaces with a mean slip coefficient of 0.33 or greater and creep deformation of less than 0.005 inches. These include uncoated, clean-mill-scale surfaces or blast-cleaned surfaces with Class A coatings.
• Class B: For faying surfaces with a mean slip coefficient of 0.50 or greater and creep deformation of less than 0.005 inches. These include blast-cleaned steel surfaces or surfaces with Class B coatings.
• Class C: For roughened hot-dip galvanized surfaces with a mean slip coefficient of 0.35 or greater and creep deformation of less than 0.005 inches.

Tnemec’s Coatings for Slip-Critical Applications

Tnemec offers a range of coatings that meet Class B requirements when corrosion protection is needed for slip-critical joints.

The following coatings have been rigorously tested to ensure compliance with the RCSC specifications:

• Series 90-75 Tneme-Zinc: Class B Surfaces
• Series 90-97 Tneme-Zinc: Class B Surfaces
• Series 90-98 Tneme-Zinc: Class B Surfaces
• Series 394 PerimePrime: Class B Surfaces

Note: Coatings are subject to recertification. For the latest information, please contact your local Tnemec Representative.

At Tnemec, we’re committed to providing high-performance solutions that enhance the safety and longevity of structural steel projects. Whether you’re designing a bridge, a building, or any other steel structure, our coatings are here to help you meet the highest performance standards.