SAC Phase 1 Analytical Studies of Building Performance

Project Title:
Dynamic Response Analyses of the 17-Story Canoga Building
James C. Anderson, University of Southern California
Filip C. Filippou, University of California at Berkeley
Project Summary:
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.


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.


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.


Based on the results obtained in the rather short period of this investigation, the following conclusions are presented:


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.


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.


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.


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.


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.


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.


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.


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.


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.


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 Logo 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