What Is a Moment Frame?
A moment frame is a structural system that resists lateral forces through rigid beam-to-column connections. Unlike braced frames that use diagonal members, moment frames achieve stiffness through the bending resistance of the frame itself — leaving open bays for windows, doors, and flexible floor plans.
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How moment frames work
In a moment frame, beams and columns are connected with rigid (moment-resisting) connections that don't rotate freely. When wind or earthquake forces push the building sideways, the rigid joints force the beams and columns to bend, absorbing the energy. This is different from a simple frame where connections act as pins.
Steel vs concrete moment frames
Steel moment frames use welded or bolted connections qualified per AISC 358. Special moment frames (SMF) for high-seismic zones have strict detailing requirements. Concrete moment frames rely on continuous reinforcement through the joints, with special confinement in high-seismic applications. Both systems are designed per ASCE 7 seismic provisions.
When to use them
Moment frames are ideal when you need open floor plans without diagonal bracing obstructing windows or circulation. They're common in office buildings, retail, and parking structures. The tradeoff: moment frames are more expensive than braced frames and result in larger member sizes because bending is less efficient than axial force transfer.
The bottom line
Moment frames resist lateral forces through rigid connections, keeping bays open. More expensive than braced frames but architecturally flexible.
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