Report No. SAC 95-07
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Technical Report: Case Studies of Steel Moment Frame Building Performance
in the Northridge Earthquake of January 17, 1994, by J.C. Anderson,
B.C. Gourley, M. Green, J.F. Hajjar, R. Johnston, R.T. Leon, D.P. OÕSullivan,
and J.E. Partridge, December, 1995.
Green investigated and repaired a three story building located in Santa Clarita. The building has a basically rectangular plan with dimensions of approximately 250 feet by 110 feet. Built in 1986, the steel framing is supported by conventional spread footing foundations. Member sizes of the steel moment frames range from W21 to W33 beams, generally of the lighter sections, and W14 columns ranging from 132 to 257 pounds per foot in weight. Floor construction consists of manufactured wood joists and a plywood diaphragm overlain by a layer of lightweight concrete. Initially, this building was thought to have only suffered nonstructural damage, until the removal of ceiling tiles uncovered sheared high strength bolts. This discovery initiated an in-depth structural inspection. Major damage to the moment connections was found to frames in the north-south direction. Other damage occurred at a number of simple shear connections that were not designed to resist lateral forces, and included both sheared web bolts and torn web connection plates. After determining the extent of the damage, the building was "red tagged", and the process of designing repairs was begun. The visual and ultrasonic inspection was completed and an initial repair plan was developed that called for the rewelding of the damaged connections and removal of the backing strips. Subsequent desires by the owner to increase the seismic capacity led to the development of a scheme to add diagonal braces at the moment connections between the girders and columns just above the ceiling level. All work was completed within the ninety day time limit needed to keep the tenant leases from being voided.
Hajjar, et. al., have provided a case study of a four story office structure located in Valencia that was only one year old at the time of the Northridge Earthquake. The office building portion of this complex is basically rectangular in plan with dimensions of approximately 187 feet by 110 feet, except for the second floor, which includes a large diaphragm and a curved projection along one elevation. Floor construction consists of metal decking and lightweight concrete fill supported by steel framing and a reinforced concrete spread footing and grade beam foundation. Three or four bay moment frames were located on the exterior frame lines. Moment frame member sizes ranged from light W24 sections at the roof to light W36 sections at the second floor, with W14 columns between 120 and 193 pounds per foot. Concentrated in the north-south frames, the damage suffered by this building was quite severe (approximately seventy-five per cent of the moment connections suffered brittle fractures), and required almost six months to repair at a cost exceeding one-third of the original construction cost. The damage included numerous column flange fractures and pull-out fractures of the column flange. This report presents detailed documentation of the inspection, evaluation, repair design, and repair process for this building. It also presents an analytical investigation that included both static and dynamic, and linear and nonlinear studies of the structure. Using ground motions postulated for this strongly shaken site, it was found that the results from a three-dimensional, second-order inelastic, dynamic analysis provided strong correlation with the observed damage patterns which resulted from the Northridge Earthquake. These dynamic analyses resulted in large ductility demands indicating that substantial redistribution of forces may have occurred and affected the final pattern of damage in the building. Significantly less correlation was found from elastic analyses, two-dimensional analyses and three-dimensional static analyses.
Anderson, et. al., performed an integrated experimental and analytical investigation on a two story building that was severely damaged in the Northridge Earthquake. This building, which was located in Santa Clarita and constructed in 1992, experienced a significant permanent lateral displacement in the first story as a result of the earthquake. The building is basically rectangular in plan with dimensions of approximately 100 feet on each side, and includes half bay setbacks on two elevations at the second story. Floor construction consists of metal decking and lightweight concrete fill supported by steel framing and a reinforced concrete spread footing and grade beam foundation. Moment frame member sizes ranged from light W24 sections at the roof to light W27 sections at the second floor, with light W14 column sections. Damage to the steel frame was concentrated in this story, with yielding noted at the base plate connections, and a number of through column fractures at the second floor moment connections. The extent of this damage was such that the building owner decided to demolish the structure above the foundation level. Four damaged connection specimens were obtained from the building during the demolition process. Cyclic tests were conducted on these specimens in both the damaged state and after being repaired. In addition, newly fabricated specimens of the same member sizes were tested for purpose of comparison with the damaged and repaired specimens. A series of thirteen tests were conducted on seven specimens. Tests of the damaged connections demonstrated residual strength and deformation capacity that could be useful in post-fracture response and in subsequent aftershocks. It should be noted that the amount of residual strength and deformation capacity is greatly dependent on the type of fracture that occurs. Repaired connection test specimens were able to restore the original strength and stiffness of the connections, and in a number of cases increase the rotation capacity. Some of the testing indicated that premature failure in the repaired connections could be precipitated by undetected existing cracks in the specimens. Ductility values for the repaired and newly fabricated specimens with the pre-Northridge "standard" moment connection detail were only in the range of two to four. The analytical investigation portion of this work consisted of linear and nonlinear, static and dynamic analysis of the building, in an effort to determine if the damage could have been predicted. Using representative Northridge ground motions for this site, both linear or nonlinear models predicted significant overloads in the first story of the structure. As an example, the Code base shear design value was on only the order of ten per cent of that obtained from the elastic dynamic analyses. In addition, individual connection demands at critical locations were overstressed by a factor of more than eight. Detailed nonlinear finite element analyses of the connections were also performed for proportional static loading. These analyses were performed to provide guidance to the test program and to indicate highly stressed areas in the various connection configurations considered in the study.
As a quality assurance measure, all SAC Steel Project Investigations are overseen by a Technical Advisory Panel (TAP). The panel for the Task 3 investigations included practicing engineers experienced in the inspection, evaluation and repair of damaged steel buildings and researchers with expertise in steel building performance in earthquakes and statistical data interpretation.
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