Direct Answer
A slip-critical (SC) connection relies on friction between the contact surfaces of connected plies — the pretensioned bolts clamp the plates together so that the shear load is transferred by friction, not bolt bearing. A bearing-type connection allows the plates to slip until bolts bear against the hole walls. SC connections are used where slip would be unacceptable: connections subject to load reversal, fatigue, or precision-alignment requirements.
Full Explanation
SC connections require: (1) Full pretension (same as snug-tight plus additional torque — typically 70–100% of bolt proof load). (2) Faying surface preparation — surfaces in contact must meet a minimum Class A or B surface condition (Class A: clean mill scale or hot-dip galvanized; Class B: blast-cleaned to SSPC-SP6). No oil, paint, or coatings on faying surfaces unless specifically tested and rated. (3) Inspection — slip-critical connections require pre-installation verification, calibrated wrench torque, or turn-of-nut method for tensioning. High-strength bolt pretensioning methods are defined in AISC Specification Section J3.
What This Means for Your Shop
SC connections have strict surface preparation requirements: faying surfaces must NOT be painted, primed, or coated (unless using a tested friction-rated coating). If you apply primer to a faying surface in the shop, you've created a Class A surface degradation that may require removal and re-blasting.
Common Mistakes
Painting all surfaces of a connection — including faying surfaces — without checking whether slip-critical connections are specified. A painted SC faying surface requires grinding before field installation.
Recommended Process
Mark all slip-critical faying surfaces on the shop drawing with a clear notation: 'NO PAINT — SC FAYING SURFACE.' Include this notation in your coating procedure. Inspect all SC surfaces before shipping to confirm they are uncoated and clean.