What Is a Moment Frame?

Three structural systems resist lateral load in a building:

1. shear walls, solid walls of plywood, drywall, or concrete that resist racking
2. braced frames, diagonal bracing inside the wall plane
3. moment frames, rigid beam-to-column connections that transfer lateral force through bending of the beams and columns themselves. In a moment frame, the beam-to-column connection is welded or bolted with full-strength fasteners so it can transfer rotation, not just shear.

A force pushing sideways on the building tries to bend the connection rather than slide along it. The frame resists by bending, hence "moment" frame, where moment is the engineering term for bending force. Moment frames are nearly always steel in residential construction, though wood moment frames exist in specialty timber buildings.

Moment frames appear in residential construction when the architecture demands large openings or open floor plans that don't leave room for shear walls:

1. ground-floor garage with a full second storey above and only one or two narrow walls between bays, the second-storey load and lateral resistance has to come from somewhere, and a steel moment frame around the garage opening is often the answer
2. glass-wall main living spaces, a wall of windows can't carry shear, so a moment frame at the rear of the home transfers lateral load to the side walls
3. cantilevered upper floors, second-storey overhangs with no walls below need moment frames to transfer load back to the foundation
4. Toronto laneway and garden suite designs, small floor plates with large openings often require moment frames where a bigger building would just use shear walls.

We see moment frames in roughly 20-30% of GTA custom home projects, usually concentrated at the garage opening or rear glass wall.

Moment frames are always engineered (P.Eng-stamped). Cost varies with span and load:

1. typical 2-bay garage opening moment frame: $15,000–$35,000 supplied, fabricated, and installed including foundation enhancements and fireproofing
2. rear-wall glass moment frame for a 30-foot opening: $25,000–$60,000
3. cantilever moment frames for second-storey overhangs: $10,000–$25,000. Fabrication requires a CWB-certified shop. On-site connections must be either pre-engineered bolted detail or certified field-welded, both require third-party inspection in most Ontario municipalities.

Allow 6-10 weeks from engineering approval through shop fabrication to on-site install.

Shear walls are cheaper and faster, typically $1,000–$5,000 per shear wall versus $15,000+ for an equivalent moment frame. Use shear walls whenever the architecture allows, which is most of the time. Moment frames are reserved for the cases where the design specifically prevents shear walls: open garage bays, glass walls, cantilevers, narrow lots with large openings. From a structural engineer's perspective, the conversation usually goes "can we get a 4-foot section of wall here? If yes, shear wall.

If not, moment frame." Architects who specify large openings without consulting an SE early often discover that the moment frame premium changes the project budget meaningfully, worth catching at design stage.