Direct Answer
A moment-resisting frame (MRF) resists lateral loads (wind, seismic) through rigid beam-to-column connections that develop moment — the frame acts as a rigid rectangle that deflects into an 'S' shape under lateral load. A braced frame uses diagonal members or a shear wall to resist lateral loads through axial forces, without requiring moment at the beam-column connections. MRFs offer more architectural openness; braced frames are typically stiffer and more economical.
Full Explanation
MRF behavior: columns and beams are rigidly connected (moment connections resist rotation) forming a frame that resists lateral load by bending the columns and beams simultaneously. SMF (Special Moment Frame) and IMF (Intermediate Moment Frame) are seismically detailed versions with specific connection requirements per AISC 341. Braced frames — CBF (Concentrically Braced Frame), EBF (Eccentrically Braced Frame), BRB (Buckling-Restrained Brace) — transfer lateral load to foundations through the brace axial forces, with simple beam-column connections. Most buildings combine both systems: a perimeter moment frame for one direction, a stair or elevator core braced frame for another.
What This Means for Your Shop
Moment frames require moment connections — a significant fabrication premium over simple connections. A project that switches from a braced frame to a moment frame mid-design (not uncommon due to lease plan changes) can increase your fabrication cost by 20–40% for the affected framing lines.
Common Mistakes
Bidding moment frame connections at the same price as simple connections. Even a basic moment connection (welded flange plates) costs 3–5× more in labor than a shear tab. Price them separately.
Recommended Process
At bid review, identify all moment-resisting framing lines. Count moment connections. Price these separately from the simple connections — a moment frame bay with 4 columns and 2 levels has at least 8 moment connections, each priced individually.