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 force a building experiences when in wind or earthquake conditions. When force is applied in such a way to a structural element that the element bends, it is called bending moment. A building faces two primary types of force. One is outer, caused by wind pressure. This is the same pressure applied to a person if they are standing in a strong wind. The other is an inner force, like that from an earthquake, comes from the ground up. A person experiences something similar when standing on a train that takes off quickly and they are shaken from the feet up.
Buildings are intended to bend with moderate moment 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.
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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 mid-seismic areas. They are intended to withstand some permanent damage following limited 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 level forces. They must sustain a high level of interstory drift.