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SAC PublicationsReport No. SAC 95-07 |
Link Here for the Reference Library and Downloadable Documents |
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SAC PublicationsReport No. SAC 95-07 |
Link Here for the Reference Library and Downloadable Documents |
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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