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Moment frames are box-shaped frames with special joints that aid in resistance to wind and earthquake damage. There are three types: ordinary, intermediate, and special. Buildings are intended to bend with moderate momentum and return to their previous state, but to survive a major earthquake, the frame should bend and absorb energy without falling over. The Federal Emergency Management Agency (FEMA) has regulations in place to protect structures from significant damage.
A moment frame is a box-shaped frame with special moment connections or joints that aid in resistance to wind and earthquake damage. The frame helps a building flex as much as necessary to maintain 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 frame moment, or joint, refers to the force a building experiences when it is in wind or earthquake conditions. When force is applied to a structural member in such a way that the member bends, this is referred to as a bending moment. A building faces two main types of force. One is external, caused by wind pressure. This is the same pressure applied to a person if they are in a strong wind. The other is an inner force, like that of an earthquake, which comes from below. A person experiences something similar when standing on a fast-moving train and being shaken from the feet up.
Buildings are intended to bend with moderate momentum and return to their previous state. This is called elasticity. Just like a rubber band, the building shouldn’t experience any permanent changes from being stretched or bent slightly and temporarily.
To survive a major earthquake, the opposite principle applies. A properly constructed building should have permanent damage following a major earthquake. The frame should bend and absorb energy without falling over. An improperly constructed frame will become brittle and break under harsh conditions. Such was the case in Northridge, California in 1994 when an earthquake decimated buildings and spurred the implementation of new frame building practices of the moment.
The Federal Emergency Management Agency (FEMA) has regulations in place to protect structures from significant damage. They provide the standards that buildings in high earthquake regions are designed to maintain, including moment frame construction. Frame systems can be classified into three main types.
An ordinary moment frame (OMF) is used in low seismic areas and is expected to remain elastic under regular conditions. They don’t have the strict standards in place that a moment frame needs to be used in a high seismic region. They are not designed to handle any drift between floors, which means the floors should not move relative to each other.
An intermediate moment frame (IMF) is used in low to medium seismic areas. They are intended to resist some permanent damage following limited force. They are required to sustain moderate interstory drift.
A special frame moment (SMF) is used in medium to high seismic areas. They are intended to withstand significant permanent damage as a result of high level forces. They have to sustain a high level of drift between floors.
