Pre-Northridge Steel Moment Frame Buildings
Steel Moment Frame buildings have large, steel frames to withstand shaking during an earthquake. “Moment” is a term used by structural engineers to refer to forces that cause bending in a beam or column. During an earthquake, the beams and columns of a building must resist large “moments”, or bending forces as the building tries to sway sideways. Although steel moment frame buildings have historically performed well during earthquakes, engineers were surprised to discover that a number of “modern” moment-resisting frame structures experienced unexpected brittle damage to their beam-to-column connections in the Los Angeles area during the 1994 Northridge earthquake. Experts discovered that these buildings suffered from a wide range of flaws resulting from shortcomings in established design and construction procedures, including poor basic connection geometry, poor connection welding materials, and inadequate quality control among other factors. Affected steel moment frame buildings were generally constructed throughout the 1960’s to 1990’s.
Although steel is normally quite ductile, the poor connection geometry of the beam-to-column connection in the image above, along with other factors, induced a brittle fracture through the column.