SAC Phase 1 Analytical Studies of Building Performance
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Project Title:
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Dynamic Response Analyses of the 17-Story Canoga Building
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Sub-contractors:
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James C. Anderson, University of Southern California
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Filip C. Filippou, University of California at Berkeley
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Project Summary:
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This case study investigates the seismic behavior of a 17 story steel building
(18 stories with penthouse) which experienced cracking in the welded connections
of moment resistant frames. Both, response spectrum and time history elastic
dynamic analyses with a three dimensional model of the building were used
to calculate demand/capacity ratios for an equal hazard spectrum and four
recorded earthquake records. Nonlinear static (pushover) analyses were
conducted on two dimensional models using two different programs and three
different hysteretic models. These same programs were used for performing
nonlinear dynamic analyses for the recorded earthquake ground motions.
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Instruments recorded three components of motion at the roof level. Analyses
of these records indicated that the building period increased by 50% in
the north-south direction and by 25% in the east-west direction. The results
of elastic dynamic analyses indicated that damage generally occurred at
locations having high demand/capacity ratios. However, a high demand/capacity
ratio did not necessarily imply damage. While the predominate ground motion
was in the north-south direction, building eccentricity in the east-west
direction combined with a strong component of ground motion in that direction
made a significant contribution to the response in the critical north-south
frames. Inelastic dynamic analyses using ground motions recorded near the
building site show a much better correlation with the damage observed in
the building. The FEAP-STRUC program with its element library which can
consider the partial cracking through the section of a member shows considerable
potential for future two analyses of both two and three dimensional building
models.
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This study has investigated the dynamic behavior of an 18 story building
which has lateral resistance provided by two-bay moment frames on each
side. In the east-west direction these frames are offset from the center
of mass giving the building a significant eccentricity in this direction.
The building suffered 29 fractures to welded moment connections in the
frames on the east and west side of the building with 23 of these in the
frame on the east side. Following the earthquake a residual displacement
was measured to be six inches toward the north. The building was intrumented
with three accelerometers at the roof level which recorded two horizontal
and one vertical component of the roof motion. A moving window Fourier
analysis of this record indicates that in the north-south direction, the
period of vibration increased from 3.8 seconds initially to almost 5.8
seconds at the end of the record. The period in the east-west direction
also changed but not as much. Results from a two dimensional nonlinear
dynamic analysis of the building indicate that based on the recorded damage
pattern and the residual displacement at the roof, the Canoga ground motion
is representative of the actual base motion the building experienced which
was not recorded. Results also indicate that ground motions in other parts
of the San Fernando Valley (Sylmar) would have been considerably more damaging
to this structure.
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Based on the results obtained in the rather short period of this investigation,
the following conclusions are presented:
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The eccentricity of this building in the east-west direction makes it essential
to use a three dimensional model of the building when calculating demand/capacity
ratios.
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Elastic response analysis indicates that when the building is subjected
to two horizontal components of ground motion the effect of the east-west
eccentricity is to increase the demand in the east frame and to decrease
it in the west frame. This is consistennt with the damage pattern observed
in the building.
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Fourier analysis of the record on the roof of the building indicates a
substantial period shift (increase) during the earthquake. This may also
be consistent with the observed damage pattern but needs to be studied
further.
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Plastic rotation demands for the Canoga ground motion, which is thought
to be representative of that at the base of the building, were all less
than 0.87% and were contained primarily in the girders and not the more
vulnerable columns. Rotations of this magnitude should be sustainable with
proper welding and connection detail. However, under the Sylmar record,
obtained at the north end of the valley, the plastic rotation reached 1.6%
and the structure may have been close to forming a sway mechanism.
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A residual deformation of the building indicates that inelastic behavior
has occurred. However, the fact that there is no residual deformation does
not imply inelastic behavior did not occur. The number, sequence and magnitude
of yield excursions have a strong influence on residual displacement.
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Considering that this study was the first full scale use of the program,
FEAP-STRUC proved extremely useful in the nonlinear static and dynamic
analysis of the 18-story high-rise steel building. The structural element
library of the program includes modeling of partial section cracking and
shows considerable potential for future two analyses of both two and three
dimensional building models.
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Nonlinear Static Load-to-Collapse (Push-Over) Analyses are by themselves
of questionable value in the seismic evaluation of flexible, high-rise
frames which have a significant contribution from the higher modes in the
dynamic response unless the lateral load distribution is significantly
modified from the distribution that is stipulated in present codes of practice.
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Linear Elastic Dynamic Analyses also proved of questionable use in the
presence of localized damage. While weld fractures tend to occur at locations
of high Demand/Capacity ratios, a high demand capacity ratio does not necessarily
imply weld damage. The nonlinear dynamic analyses were much more accurate
in predicting a concentration of inelastic deformations in the upper stories
of the frames. The agreement with the observed weld fracture damage was
very good for the records obtained in the San Fernando Valley. The results
obtained using the Sylmar ground motion suggest that the building might
have experienced a stronger ground shaking than implied by the Canoga record
or the Oxnard record (closest to the site). The concentration of damage
in the upper stories of the frame was much more pronounced with the use
of a special girder/connection model with weld fracture that was specifically
developed for this project. The brevity of the study did not permit a thorough
evaluation of this fracture model on the dynamic response of the frame.
This should be an important future task of the SAC project, since the use
of such a realistic model will permit the assessment of the safety of many
high-rise buildings under future strong earthquakes.
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The viscous damping ratio does not play an important role in the first
ten seconds of the dynamic response where most damage seems to have taken
place, since the viscous energy dissipation, which depends on the inelastic
excursions of the frame members.
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The accurate modeling of gravity loads as distributed element loads and
the related effect of vertical ground accelerations might have a significant
effect on the response and should be an important future task of the SAC
project. The brevity of this study did not permit the assessment of this
effect, even though the distributed plasticity frame element in FEAP-STRUC
is ideally suited for the purpose. In any case, axial loads due to gravity
play an important role in the response of flexible, high-rise steel frames
in conjunction with second order effects of instability. In this respect
the leaning effect of the gravity load system on the lateral load resisting
system should be carefully assessed in future studies.
SAC Home
SAC Steel Project
c/o Earthquake Engineering Research Center
1301 South 46th Street
Richmond, CA 94804
(510) 231-9477
FAX: (510) 231-5664