Direct Answer
A moment frame is a structural system that resists lateral loads (wind and seismic) through rigid beam-to-column connections that transfer bending moment. Unlike braced frames that use diagonal braces, moment frames have open bays with no diagonal interference — valuable for architectural flexibility. The rigid connections allow the frame to flex under lateral loads while maintaining integrity, with energy dissipated through controlled yielding at the beam-column interface.
Full Explanation
Moment frames are classified by ductility: Ordinary Moment Frame (OMF), Intermediate Moment Frame (IMF), and Special Moment Frame (SMF). OMF is used in low-seismic applications; SMF is required in high-seismic zones (SDC D and above) and uses prequalified connections from AISC 358 (reduced beam section, extended end plate, etc.) that are designed and tested to maintain strength through large inelastic cycles. The detailing complexity and fabrication cost of SMF significantly exceeds OMF.
What This Means for Your Shop
If your project has a Special Moment Frame, your shop must have the welding capabilities, welder qualifications, and inspection infrastructure to comply with AISC 341 and AWS D1.8. The cost premium for SMF fabrication over standard work is significant — typically 30–50% higher labor per connection.
Common Mistakes
Bidding an SMF project without identifying it as SMF during scope review. The structural general notes or drawings will reference AISC 341 and specify the seismic system — reviewing these before bidding is essential.
Recommended Process
On every bid, read the structural general notes completely. If you see 'Special Moment Frame,' 'AISC 341,' 'IMF,' or 'SMF,' treat this as a specialty scope and price accordingly.