A moment frame is a structural system in which beams and columns are connected by rigid joints capable of transferring bending moments, providing lateral resistance through flexural action. Moment frames are among the most common lateral force-resisting systems in steel and reinforced concrete buildings.
A moment frame is a structural system consisting of beams and columns connected by rigid joints (moment connections) that can transfer bending moments, shear, and axial forces. When subjected to lateral loads, the frame resists them through the flexural action of its members: the columns bend in double curvature, the beams bend in single curvature, and the joints remain essentially rigid. Unlike braced frames or shear wall systems, moment frames have no diagonal or planar bracing elements, which leaves the interior of the building open and flexible β a major architectural advantage that has made moment frames the system of choice for office buildings, commercial spaces, and structures where open floor plans are required.
Moment frames are classified by their ductility and detailing requirements. In steel construction, the primary distinction is between Special Moment Frames (SMF), Intermediate Moment Frames (IMF), and Ordinary Moment Frames (OMF) β with SMF requiring the most stringent detailing and offering the highest ductility (R = 8 in ASCE 7), IMF intermediate (R = 4.5), and OMF the least (R = 3.5). SMF connections must be qualified through testing and must be capable of sustaining large inelastic rotations without fracture β a lesson painfully learned from the 1994 Northridge and 1995 Kobe earthquakes, where welded beam-column connections fractured prematurely. Modern SMF connections use prequalified configurations (as specified in AISC 358) with improved weld details, backing bar removal, and reinforced or reduced beam sections. In reinforced concrete construction, moment frames are similarly classified as Special (SMF), Intermediate (IMF), and Ordinary (OMF), with ductility requirements that focus on confinement, joint detailing, and the strong-column/weak-beam principle.
Moment frames are flexible systems that often govern design by drift rather than strength. Because they lack the stiffness of braced or wall systems, moment frames undergo larger lateral displacements and require careful drift control β particularly for tall buildings. This has led to the widespread use of dual systems, where moment frames are combined with shear walls or braced frames: the wall provides stiffness and the frame provides redundancy and additional energy dissipation. In low- and mid-rise buildings, moment frames alone can be economical and architecturally desirable. In high-rise construction, pure moment frames are rarely used because the drift demands become impractical. A variant called the dual frame system with moment frame is common in high-seismic regions; it uses a moment frame as the primary lateral system but requires that the frame alone be able to resist at least 25% of the design base shear, providing redundancy. In Iran, moment frames (both steel and reinforced concrete) are widely used, and the Iranian Standard 2800 and the applicable design codes (AISC-based for steel, ABA-based for concrete) define the requirements for SMF, IMF, and OMF. The performance of older moment frame buildings in Iranian earthquakes has been mixed β some performed well, while others suffered connection fractures or excessive drift.