A moment frame is a box-shaped frame with special moment connections or joints that help in the resistance of wind and earthquake damage. The frame helps a building to flex as necessary to remain the building's integrity. There are three types of moment frames: ordinary moment frames (OMF), intermediate moment frames (IMF), and special moment frames (SMF).

The word "moment" in the term moment frame, or joint, refers to the moment of inertia placed on a building when in wind or earthquake conditions. A building faces two primary types of inertia. One is outer inertia caused by wind pressure. This is the same pressure applied to a person if they are standing in a strong wind. Inner inertia, like that from an earthquake, comes from the ground up. A person feels similar inertia when standing on a train that takes off quickly and they are shaken from the feet up.

Buildings are intended to bend with moderate inertia and return to their former state. This is called elasticity. Just like a rubber band, the building should have no permanent change from being stretched or bent lightly and temporarily.

In order to survive a strong earthquake, the opposite principle is true. A building constructed properly should have permanent damage following a major earthquake. The frame should bend and absorb energy without falling. A frame built improperly will become brittle and break during difficult conditions. Such was the case in Northridge, California in 1994, when an earthquake decimated buildings and spurred the implementation of new moment frame construction practices.

The Federal Emergency Management Agency (FEMA) has regulations in place to protect structures from significant damage. They provide the standards that buildings in high seismic regions are intended to keep, including the construction of moment frames. The frame systems can be classified into three major types.

An ordinary moment frame (OMF) is used in low-seismic areas and is expected to remain elastic through regular conditions. They do not have the rigid standards in place that a moment frame used in a high seismic region must. They are not intended to handle any interstory drift, which means that the floors should not shift relative to one another.

An intermediate moment frame (IMF) is used in low to midpseismic areas. They are intended to withstand some permanent damage following limited inertial force. They are required to sustain a moderate interstory drift.

A special moment frame (SMF) is used in mid- to high-seismic areas. They are intended to withstand significant permanent damage following high inertial forces. They must sustain a high level of interstory drift.