Washington, DC -- To address problems with steel-frame skyscrapers and high-rise buildings once thought to be "earthquake proof," the Federal Emergency Management Agency (FEMA) today released the findings and recommendations of a six-year, $12 million project initiated in the aftermath of the Northridge earthquake.
The project was launched after damage inspectors discovered that cracking had occurred unexpectedly in the welded connections between horizontal beams and vertical columns in modern steel-frame buildings designed to bend but not crack under the stress of earthquakes. A few of the structures sustained significant damage in the earthquake, which has been classified as a strong, but not a major earthquake.
"Damage assessments after Northridge led FEMA to conclude that modern steel-frame buildings designed to sway without fracturing in earthquakes are not as safe as previously thought and may be susceptible to failure in major earthquakes," said FEMA Director Joe M. Allbaugh. "As the new director of FEMA and in light of my recent experience with the Seattle earthquake, I am pleased to present these guidelines that FEMA has developed to help protect the American people from the dangers of earthquakes."
The study found that the damage sustained by steel-frame buildings was the result of a number of factors, including construction defects such as welds that were not bonded well with steel columns, changes in material properties of weld metal and structural steel, and a prescriptive design spelled out in building codes whose connection configurations became problematic and unreliable when used with large beams and columns.
The findings and recommendations that came out of the study are outlined in A Policy Guide to Steel Moment-Frame Construction, released today by FEMA. It explains in layman's terms how the study was conducted and what is being recommended in connection with steel-frame buildings and earthquakes. Four other publications to come out of the study, FEMA 350 - 353: Program to Reduce the Earthquake Hazards of Steel Moment-Frame Structures, are technical manuals for the building codes community and the design and construction industries. They have not previously been released to the public.
Thousands of welded steel moment-frame buildings have been built throughout the United States and the world in the last 30 years, and most high-rise structures built in the U.S. since 1970 use this type of construction. A welded steel moment-frame is an assembly of beams and columns, rigidly joined together to resist both vertical and lateral forces. The buildings are designed to rely on these connections between beams and columns to resist ground movements caused by earthquakes. Until Northridge, this type of construction was considered to an effective seismic-resistant structural system - widely accepted in seismic prone areas as a way of constructing building that are resistant to earthquakes.
At the time of the Northridge earthquake, there were about 2,000 welded steel-frame buildings in Los Angeles. The city required an inspection of nearly 200 buildings in areas that experienced the most intense ground shaking; about 30 of those buildings sustained significant damage. Although no steel-frame buildings collapsed in the Northridge earthquake, these discoveries alarmed the structural engineering community, building codes officials and emergency managers, especially in earthquake prone areas. The findings called into question all building codes developed over the previous 20 years that addressed this type of construction.
The study was a coordinated, problem-focused program of research and investigation. Its goals were to develop and validate reliable a...