Direct Answer
A moment connection transfers both shear and bending moment between connected members, making the joint rigid. Simple connections (like shear tabs) transfer shear only and allow rotation. Moment connections are used in moment frames — the primary lateral force-resisting system in many buildings — and are significantly more complex and expensive to fabricate than simple connections.
Full Explanation
Common moment connection types: bolted flange plates (BFP), welded unreinforced flanges (WUF), extended end plates (EEP), and reduced beam section (RBS or 'dogbone'). Each has specific geometry, weld, and bolt requirements per AISC 358 (Prequalified Connections for Special and Intermediate Steel Moment Frames) for seismic applications. Moment connections require CJP flange welds — the most critical and most expensive welds in structural steel. Seismic moment connections (SMF, IMF) require Demand Critical weld qualifications and additional UT inspection.
What This Means for Your Shop
Moment connection fabrication is a specialty. Not all shops have the welding qualifications, equipment, and QC infrastructure for seismic moment connection welding. If your project has Special Moment Frames, verify your shop's D1.8 capabilities and your welder's Demand Critical qualifications before bidding.
Common Mistakes
Treating seismic moment connection welds the same as standard structural welds. D1.8 has additional requirements for preheat, interpass temperature, heat input, and UT inspection that go beyond D1.1.
Recommended Process
For seismic moment frame projects, review AISC 358 for the specific prequalified connection type specified. Confirm your WPS meets D1.8 requirements. Engage your special inspector before welding begins to coordinate inspection requirements.