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February 27, 2002
Steel buildings and structures have been designed and built for over 200 years. With improvements in steel making and joining technologies, such as welding, it has been possible to build taller buildings with longer spans between supports.
Get the FEMA reports
Order a free copy of FEMA's steel moment-frame reports by calling (800) 480-2520. Find them online at www.fema.gov/library/lib06.htm. A 31-page nontechnical summary for owners and tenants is also available online at www.fema.gov/library/354final.pdf.
The ductile and toughness properties of steel also have made it an ideal material to resist earthquake forces. Ductility is the ability of a material to stretch and deform without breaking or cracking. Brittle materials, like glass or cast iron, would exhibit poor ductility and will crack when overloaded. Toughness, a property related to ductility, is the ability of a material to resist fracture in the presence of a notch or defect.
Since the 1960s structural engineers have used welded steel moment-frame designs in seismic zones such as the Puget Sound. This configuration utilizes steel beams welded to steel columns with complete-penetration welded connections designed to fully transmit stresses from the beam to the column. These structures were thought to be earthquake-proof, or at least incapable of collapse during a seismic event.
The 1994 Northridge California earthquake changed this perception. A large number of steel moment-frame buildings were found to have cracks at or near the welded connections. These cracks were brittle fractures instead of the expected ductile behavior that was expected. These findings alarmed designers, building officials and contractors. While there were no collapses of steel frame buildings at Northridge, the damage compromised structural systems and created significant economic losses as a result of repair and relocation of building occupants.
After the Northridge earthquake, the Federal Emergency Management Agency (FEMA) provided funding to the SAC Joint Venture, which was formed by the Structural Engineers of California, the Applied Technology Council and California Universities for Research in Earthquake Engineering to provide research and testing to solve the welded moment frame fracture problems.
New beam-to-column connections were developed and tested. Procedures for welding, testing and inspection were extensively reviewed. Welding and steel products were analyzed and tested.
The result of this $11 million effort was four publications: FEMA-350, "Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings"; FEMA-351, "Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings"; FEMA-352, "Recommended Postearthquake Evaluation and Repair Criteria for Welded, Steel Moment-Frame Buildings"; FEMA-353: "Recommended Specifications and Quality Assurance Guidelines for Steel Moment-Frame Construction for Seismic Applications"; and FEMA-354, "A Policy Guide to Steel Moment-Frame Construction."
These documents are written as recommendations. Structural engineers may incorporate them into designs and specifications or may provide other justifications, such as connection testing, to support alternative structural systems. While the recommendations are written for moment frames, many engineers have adopted FEMA welding provisions for other earthquake-resisting lateral systems, such as steel brace frames.
The FEMA recommendations, while not mandatory, have become the state of the art for new construction. Efforts are currently under way by the American Welding Society and American Institute of Steel Construction to incorporate the FEMA provisions into codes and standards.
What's new for contractors
Welding procedures: New welding procedures may require qualification testing.
Welder qualifications: Welders welding on steel moment-frame connections need to demonstrate competence on special weld joint mockups.
New connection designs: Structural engineers are incorporating new connection configurations.
Base metal: New steels are specified for seismic applications.
Welding wire: Many welding products are no longer suitable for seismic work.
Welder training: New seismic welding wires will require training and practice for welders.
Weld testing: New requirements for testing and inspection. Contractors need to provide access and allow time for increased inspection.
Specifications: New recommendations are being incorporated into project specifications and structural notes. Contractors need to review these documents to properly qualify subcontractors and suppliers.
FEMA-352 evaluation procedures were used to evaluate several existing steel moment-frame buildings after the Nisqually earthquake. At this time there doesn’t seem to be any of the brittle fracture-type damage detected in Puget Sound structures. Since the design and construction of Puget Sound moment-frame buildings is similar to the damaged California buildings, it is likely that recent earthquakes have not been strong enough to cause brittle fracture.
FEMA-351 provides methods to evaluate probable performance of existing structures in future earthquakes. Owners of existing buildings can use these procedures to evaluate safety and financial factors when considering upgrades to higher seismic performance standards. FEMA-354 provides a nontechnical overview of the of the steel moment-frame issue.
FEMA-350 and FEMA-353 apply to design and construction of new buildings. Recent or current buildings under construction, which incorporate the new FEMA provisions, include the Sea-Tac Airport terminal expansion, Sea-Tac air traffic control tower and the new federal courthouse project.
The new FEMA recommendations have required the entire construction team to adopt new practices and procedures. Structural engineers are incorporating new FEMA details and connections into drawings and specifications.
Most structural engineering curriculums do not include welding metallurgy or weld joint design. Many engineers are attending seminars or are using welding engineers as subconsultants to review welding procedures on their projects.
Steel fabricators and erectors have had to write, and qualify by testing, new welding procedures that produce weld joints with good ductility and toughness. Welding product manufacturers such as Lincoln, Hobart and ESAB are now producing "FEMA" wire with special packaging in controlled lots. The certified lots of welding material are tested by the manufacturer or user using maximum and minimum welding variables. Steel mills are now providing steels with tighter control of chemical and mechanical properties.
The new FEMA recommendations place much more emphasis on tighter quality control, by both contractors and the owner’s inspector. There is increased nondestructive examination such as ultrasonic and magnetic particle testing. Ultrasonic testing technicians must be trained in new evaluation techniques and must demonstrate that they can accurately detect weld flaws.
Fabricators and erectors need to set up quality-control systems which monitor welding variables. Welders must weld special weld joint mockups to demonstrate their ability to weld the configurations that will be used for construction.
Once systems and procedures have been set up, projects are proceeding without delay or significant cost impact. The lessons of past earthquakes are providing better and tougher structures for future earthquakes.