Steel's Performance in the Northridge Earthquakeby John Shipp

Immediately after the Northridge Earthquake, damage surveyors found a surprising number of connection failures insteel moment-framebuildings that had been designed for seismic loads. Further investigation is revealing even more of these steel structures with connection damage that, in many cases, is not readily apparent. A massive research effort will soon be underway to study, retrofit, and design steel moment-frame buildings.

Fig 1. Fracture of a column flange and web at a moment connection.IntroductionThe magnitude 6.7 Northridge Earthquake of January 17, 1994, was the most significant earthquake to occur in the United States since the 1906 San Francisco event. Besides being one of the most costly natural disasters in the nation's history (current loss estimates are as high as $15 billion), this earthquake raises many questions regarding the state of knowledge and practice in earthquake-resistant design.The Northridge Earthquake caused extensive structural damage to more than 100 modern, steel moment- frame buildings. These structures were designed as special moment-resisting-frame buildings and apparently met the stringent detailing requirements of the current building code, which were intended to ensure ductile performance during major earthquakes.Although independent testing agencies had performed special inspection during construction, in some cases it was evident that poor welding practices were partly to blame for the observed damage. Faulty welding was not observed atalllocations, however. Another reason for the extent of observed damage was the intensity of ground accelerations (both horizontal and vertical) at these sites-in some cases, as much as twice those anticipated by the building code. Intense ground shaking and faulty welding in themselves, however, do not adequately explain the observed brittle behavior.The most alarming aspect of the post-earthquake steel building failures is that structural damage is not necessarily obvious. The Structural Engineers Association of California (SEAOC), the Applied Technology Council (ATC), and the California Universities for Research in Earthquake Engineering (CUREe) are joining forces in a multimillion-dollar project to address this and other related issues.Ductile Steel FramesDuctile steel frames have long been considered one of the premier lateral-force-resisting systems. The Northridge Earthquake, however, suggests that there are many areas in which we must improve the performance of these steel-frame systems.In a number of steel-frame buildings affected by the Northridge Earthquake, failures in the bolted-web, welded-flange connections in the seismic moment frames were reported (see the bottom figure at left). Despite the careful (or in some cases, not too careful) detailing and inspection, damage to moment frames included brittle failures of the beams' flange weld connections, and fracture of column flanges including portions of the column web. Permanentdrift(displacement of one floor relative to another) was observed, but in some cases required surveyors to detect.These failures did not result in any building collapses, and in many instances, the accompanying nonstructural damage was slight enough to make the subsequent discovery of extensive connection damage a genuine surprise. Nevertheless, the poor performance of these connections merits careful scrutiny regarding the repair of damaged joints and careful consideration regarding the continued use of bolted-web, welded-flange connections.Because steel-frame connection failures have not been widely reported after other earthquakes and such failures in Southern California were only recently discovered, opinions are currently evolving about joint failures' causes, significance, and solutions.Description of Connection FailuresAlthough other types of steel connection failures have been discovered, this article focuses on those in bolted-web, welded-flange moment connections. These connections are the most prevalent in steel buildings and therefore provide the largest pool of failure data. Based upon a review of available information, joint failures included cracks across the column-flange thickness, weld metal fractures, fractures at weld-metal/column-flange interfaces, cracks in fillet welds at shear connection plates, and cracks in beam shear connection plates along the bolt line or in the plate at the end of the fillet welds.There is a broad range of building characteristics in structures reporting connection failures. Building location varies greatly, up to 25 km from the epicenter. Building height is from one to 22 stories, and it appears that most of the failures occurred in the upper half to two-thirds of tall buildings and at all levels in low-rise buildings (fewer than six stories). Connection failure rates range from less than 10% to as high as almost 100% over a large number of floors or in specific compass directions.Failures occurred in connections with and without column-flange stiffeners as well as connections with and without return welds on the shear connection plates. Both wide-flange columns and built-up box sections appear to have been affected. Failures have been noted in buildings with a relatively small number of frame bays in each direction as well as in buildings where nearly every girder line is part of a moment frame. It is interesting to note that many of the affected buildings appear to be fairly new, with most having been completed in the 1980s. Some of the buildings damaged were under construction when the earthquake struck.From a materials standpoint, it appears that most of these frames were welded according to theStructural Welding Codeof the American Welding Society (AWS). Several of the recently completed buildings and those under construction were field-welded using an AWS-approved arc-welding process; it is not known if this was the case in the older buildings affected. Steel for beams and columns was of acceptable stock. Selected toughness testing of steel and weld metal showed that toughness exceeded the lower limits of acceptability.Possible Causes of Connection FailuresAs more and more cases of ductile-frame connection failures were reported, a systematic search for possible causes began. This search led to a recent meeting sponsored by the American Institute of Steel Construction where a panel of design, welding, and research experts listened to findings presented by a number of engineers investigating connection failures in Southern California.An emerging consensus suggests that these highly stressed joints are extremely sensitive to even minor imperfections in the welds. At least at the bottom flange, the presence of the backing bar in the final connection appears to permit cracking to initiate at levels of strain that, taken for the section as a whole, are less than those required to develop significant plastic rotation in the section.The figure at right illustrates the areas of fused metal when the backing bar is left in place. In this figure, one can see the presence of a potential stress riser at the face of the backing bar and column flange below the area that is fused by the initial welding. Fracto-graphic examination of several failed welds suggests that cracking initiated at this point. The tendency of the stress riser to initiate cracking may be aggravated by theslag, or inclusions in the weld material.Additional research will be required to clearly describe the parameters that led to the different observed failures.Repair and New Design ConceptsBecause the causes of the connection failures are not precisely known, the most effective repair strategy also is uncertain. Nevertheless, structural engineers are being called upon to recommend both repair details for existing buildings and revisions to moment-frame connection details for new building construction. Suggestions include the following.1. Return to a fully welded connection (welded flange and welded web).2. Remove the backing bar (and run-off tab), gouge out the weld root, and reinforce the weld (figure below).3. Use weld metal with a specified toughness and alter weld-preparation procedures to include more thorough preheating and controlled cooling.4. Reduce the level of stress present in the weld itself with the intent to render less critical any imperfections in the weld. The figure following illustrates two approaches in using this technique: triangular ribs and cover plates.

A limited testing program is in progress at the University of Texas. This testing is partially funded by the owner of a damaged building that was under construction at the time of the Northridge Earthquake. Consequently, the program focuses on finding a solution for one particular project. Preliminary results from these tests indicate that suggestions 1 through 3 above are not sufficient to obtain the required ductility. In addition, tests using cover plates did not achieve satisfactory performance until column-flange stiffener (continuity) plates were included in the repair detail and a weld filler material with increased notch toughness was used.ConclusionsThe number of buildings discovered with weld connection failures continues to increase as more building owners and engineers become aware of this problem. Undoubtedly, there are other buildings that are yet to be discovered. In many structures, damage may not be obvious, so judgment will have to be used to develop the appropriate inspection and repair program for each building.It is clear that acomprehensivetest program of large-member, bolted-web, welded-flange frame joints is required. SEAOC, ATC, and CUREe are working together to organize, acquire funding, and direct a test program to determine the cause of observed failures. The goal is to develop professional practices and recommend standards for the repair, retrofit, and design of steel moment-frame buildings so that they provide reliable, cost-effective seismic performance in future earthquakes.The Northridge Earthquake has given the building industry the opportunity to thoughtfully assess the causes of unacceptable performance and suggest effective solutions to improve joint reliability. It is essential that the current concern of the engineering profession and the steel industries be maintained and translated into effective action until acceptable solutions are developed.A technical paper entitled "Northridge Earthquake, 17 January, 1994: Seismic Performance of Steel" was presented at theAmerican Iron and Steel Institute 1994 General Meeting.

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