Revisiting Earthquake Lessons - Welded Steel Moment Resisting Connections
Saturday, September 22, 2018
This is one in a series of short articles revisiting lessons learned in past earthquakes. This series is being presented to share past lessons with newer members of our profession who were not there to experience them first hand. Contact Dave McCormick or Kelly Cobeen with your comments and suggestions.
Ronald O. Hamburger, SE
James O. Malley, SE
Structure Type: Welded Steel Moment Frame (WSMF) Structures
Earthquakes: 1994 Northridge
Lessons: Immediately following the 1994 Northridge earthquake, engineers discovered fractures in some welded joints of beam bottom flanges to column flanges in moment-resisting steel frames. The fractures exhibited a variety of patterns and sometimes extended into and across column panel zones. Many structures, including the Getty Museum, then under construction, the Santa Clarita City Hall, the California State Automobile Association, the Trillium Towers and the U.S. Borax corporate headquarters buildings, among others, were heavily damaged. The affected connection type included bolted shear tabs connecting beam webs to columns with complete joint penetration (CJP) groove welded joints of beam flanges to column flanges or column flange continuity plates. The affected connection detail had been in common use since the early 1970s and was prescribed by the building code starting in 1988. These unanticipated failures in structures previously thought to be highly reliable caused great concern. SEAOC, together with the Applied Technology Council, and California Universities for Research in Earthquake Engineering formed the SAC Joint Venture, which received $12 million of funding from FEMA to perform research into the causes of the problem and develop engineering recommendations for design of new structures, investigation and repair of damage in existing structures and assessment of the vulnerability of existing structures to damage. The resulting performance-based design procedures resulted in substantial revision of the AISC Seismic Provisions and also the procedures used to validate structural systems for inclusion in the building code. Further, new standards and procedures for welding and inspection, new material specifications and requirements to qualify connection performance through rigorous testing were introduced into the code.
Starting with the 1906 San Francisco earthquake, and for nearly 90 years following that, engineers observed that buildings with complete vertical load carrying frames of steel performed exceptionally well in earthquakes. By the 1960s, California engineers believed moment-resisting steel frames were highly damage-resistant and the 1961 Uniform Building Code (UBC) required all buildings with 13 or more stories or exceeding 160 feet in height to include a moment-resisting steel frame in their seismic force-resisting system and permitted reduced design forces for structures having these frames. The 1967 UBC extended these conditions to specially detailed reinforced concrete frames, and these basic requirements remain in the code today.
Steel building frames were first used in the 1880s. These early steel frames incorporated built-up members with riveted, partially restrained connections but derived much of their lateral resistance from their unreinforced masonry infill walls. Following WWII, architecture evolved to favor glazed curtain wall systems. Connections evolved from riveted to bolted types using WTs and angles to connect beams to columns. In the 1960s, as code-specified forces began to exceed the practical capacity of bolted connections, engineers turned to a connection with welded beam flanges and bolted webs, later known as the WUF-B connection. Research by Popov and Stephen (1970) confirmed the viability of this connection type. Ominously, however, while performing research on eccentric braced frames, Engelhardt and Popov (1989) observed unanticipated, premature failures in some WUF-B connections. Both researchers reported their findings, but most engineers continued to believe these connections and frames were highly reliable. Apparent good performance of these structures in earthquakes in the 1980s including the ’83 Coalinga, ’84 Morgan Hill, ’87 Whittier and ’89 Loma Prieta events bolstered the common belief in the structure’s outstanding performance capability.
Early evidence that the connections did not have the perceived performance capability was available but ignored. Following the 1971 San Fernando earthquake, engineers discovered connection fractures in the Arco Towers in downtown Los Angeles (SAC Joint Venture, FEMA 355e). These were ascribed to construction quality issues and repaired without further investigation. Similar damage was observed during construction of the 1111 Broadway building in Oakland, CA, following the 1989 Loma Prieta earthquake (SAC Joint Venture, FEMA 355e). Again, damage was ascribed to construction quality issues, repairs instituted, and construction completed.
Perhaps because of the warning issued by Popov and Engelhardt, when damage was discovered in the Getty Museum in 1994, followed by several other structures, engineers took note. Engineers began to require removal of fireproofing in buildings to allow observation and to require ultrasonic testing (UT) of exposed joints. While visibly evident fractures were discovered in some buildings, UT revealed rejectable indications, originally interpreted as cracking initiation, in many structures. Later, these weld indications were determined to be construction flaws, rather than earthquake damage, but still, their wide spread presence lead to alarm. Though no structures collapsed, the International Conference of Building Officials (ICBO), a forerunner to the International Code Council (ICC), adopted an emergency code change, deleting the prescriptive WUF-B connection from the code and substituting a requirement for connection qualification testing in future deigns. Engineers began investigating structures that experienced prior earthquakes, including the 1993 Landers Big Bear and 1989 Loma Prieta events, and found similar damage in some structures.
The FEMA/SAC Recommendations
The SAC Steel project drew participation of more than 100 researchers, institutions, firms and engineers and received support from the State of California, the Federal Emergency Management Agency and the steel industry. It included analytical and laboratory investigations into base and weld metal behavior; joint, connection and frame performance; inspection techniques and construction means and methods, and design and repair procedures. Ultimately, the project concluded that poor connection performance could be attributed to many factors including: the WUF-B’s unfavorable geometry, which produced stress and strain concentrations; inadequate control of base metal strength and toughness; common use of low-toughness weld filler metals; endemic poor-quality welding and inspection technique. SAC (FEMA 352) recommended detailed inspections of all WSMF structures experiencing 0.2g or higher ground motion. Major code changes resulting from the findings and recommendations of the SAC project included:
- Requirement to qualify the performance capability of moment-resisting steel connections. AISC published the AISC 358 standard containing a series of connection prequalifications. Some individuals and firms developed a series of proprietary qualified connection technologies including SidePlate, slotted web, Kaiser Bolted Bracket and others, still available on the market.
- AWS published the D1.8 Seismic Supplement to its D1.1 Structural Welding Code, specifying special toughness and welding requirements for welded joints in seismic applications.
- ASTM introduced the A992 specification improving control on strength and weldability of rolled steel shapes.
- The AISC Seismic Provisions underwent major evolution and expansion, becoming more performance-based; recognizing capacity design principles, introducing the concept of Ordinary, Intermediate and Special Quality systems, not only for moment frames, but other structural systems.
- The profession obtained new awareness and concern for the importance of controlling welding materials and procedures, and assuring construction quality.
- Probabilistic assessment of system performance became an underpinning of the procedures used to introduce and substantiate structural system criteria in the building code.
- ASCE 41 directly addressed the seismic evaluation and retrofit of pre-Northridge steel moment frames.
The ASCE 7, AISC 341, 358 and 360 standards contain the requirements for steel moment frame structures designed for seismic resistance. A few public agencies and private owners have seismically upgraded their WSMF buildings and The City of Santa Monica and other communities in Southern California have adopted ordinances requiring evaluation and upgrade of older WSMF structures. Recent media coverage has encouraged other cities to take similar action. Hundreds of pre-Northridge WSMF buildings remain unretrofitted, including some very tall buildings in Los Angeles, San Francisco, Seattle and other cities Post-earthquake inspections of these buildings is difficult and costly. Damage is often difficult to detect and requires removal of fireproofing, which in some buildings contains asbestos. FEMA 352 recommends visual inspections of large random samples of connections in WSMF structures when they experience ground shaking with peak horizontal acceleration exceeding 0.2g and provides guidance for determining post-earthquake tagging. Some owners have installed instrumentation in WSMF buildings to guide engineers in assessing the likelihood of damage, and its probable locations. Pre-Northridge WSMF buildings remain a problem.
AISC, Specification for Structural Steel Buildings, AISC 360-16, 2016, American Institute of Steel Construction, Chicago, IL
AISC, Seismic Provisions for Structural Steel Buildings, AISC 341-16, 2016, American Institute of Steel Construction, Chicago, IL
AISC, Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, AISC 358-16, 2016, American Institute of Steel Construction, Chicago, IL,
ASTM International, Standard Specification for Structural Steel Shapes, A992, 2015. American Society of Testing and Materials International, West Conshohocken, PA.
ASCE, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE/SEI 7-16, American Society of Civil Engineers, Reston, VA.
AWS, Structural Welding Code, Steel, D1.1, 2015, American Welding Society, Miami, FL.
AWS, Seismic Supplement, AWS D1.8, 2016, American Welding Society, Miami, FL
ASCE, Seismic Evaluation and Retrofit of Existing Buildings, ASCE 41-17, 2017, American Society of Civil Engineers
Engelhardt, M. D., and Popov, E.P., Behavior of Long Links in Eccentrically Braced Frames, UCB/EERC Report -89/01, January 1989, , Earthquake Engineering Research Center, University of California at Berkeley, Berkeley, CA.
Popov, E.P., Stephen, R.M, Cyclic Loading of Full-Scale Steel Connections, Report No. UCB/EERC-70/03, 1970, Earthquake Engineering Research Center, University of California at Berkeley, Berkeley, CA.
SAC Joint Venture: Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings, Report FEMA 350, June 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings, Report FEMA 351, June 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: Recommended Postearthquake Evaluation and Repair Criteria for Welded Steel Moment-Frame Buildings, Report FEMA 352, June 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: Recommended Specifications and Quality Assurance Guidelines for Steel Moment-Frame Construction for Seismic Applications, Report FEMA 353, June 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: A Policy Guide to Steel Moment Frame Construction, Report FEMA 354, June 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: State of Art Report on Base Metals and Fracture, Report FEMA 355A, September 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: State of Art Report on Welding and Inspection, Report FEMA 355B, September 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: State of Art Report on Systems Performance of Steel Moment Frames Subject to Earthquake Ground Shaking, Report FEMA 355C, September 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: State of Art Report on Connection Performance, Report FEMA 355D, September 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: State of Art Report on Past Performance of Steel Moment-Frame Buildings in Earthquakes, Report FEMA 355E, September 2000, Federal Emergency Management Agency, Washington D.C.
SAC Joint Venture: State of Art Report on Performance Prediction and Evaluation of Steel Moment-Frame Buildings, Report FEMA 355F, September 2000, Federal Emergency Management Agency, Washington D.C.