Technical Background

SAC Phase 1 Analytical Studies of Building Performance


Project Title:
Performance of a 13-Story Steel Moment-Resisting Frame Damaged in the 1994 Northridge Earthquake

Chia-Ming Uang, University of California, San Diego
Qi-Song Yu, University of California, San Diego
Ali Sadre, Esgil Corporation
David Bonowitz, Nabih Youssef and Associates
Nabih Youssef, Nabih Youssef and Associates

Project Summary:
This report summarizes the results from case study of a thirteen-story steel moment frame (SMF) building, which sustained extensive damage to its welded connections during the 1994 Northridge earthquake. The project is located approximately five kilometers from the epicenter, and the ground motion records in the basement, as well as the sixth floor and the twelfth floor were made available. The building had been surveyed and data were collected on the fractured joints. Original design drawings were also readily accessible. The structure has no pronouned irregularities or discontinuities in its load paths. The subject building is a prime candidate for this research for all of the above reasons

The primary objective was to verify the accuracy of present analytical tools in successfully predicting the pattern and severity of the documented connection failures. Absence of any notable torsional or stiffness irregularities in this building simplified the dynamic analyses, which along with access to recorded ground motions allowed the model to be calibrated with high precision. Statistical reports on the failed connections indicate that the building suffered more severe damage in the reference north-south (N-S) direction than the reference east-west (E-W) direction.

Results from elastic time-history analyses correlated well with the measured response in the E-W direction, but not so will in the N-S direction. Correlation began to deteriorate as the damage became more extensive and the elastic model was unable to simulate the fractures to the welded moment connections. The demand/capacity ratios (DCRs) of the beams computed from the response spectrum analyses were as high as 1.18 in the N-S direction, which was nearly double the amount in the E-W direction. Beams with higher DCRs were concentrated between the second and seventh floors, which coincided well with the observed damage on the west side of the building. In the E-W direction, the maximum story drift ratio was calculated at 1% with a roof drift ratio of 0.55%. These values suggested that the ground motion did not force the building into the post-yield state in the E-W direction and connections primarily fractured will before the plastic capacity of the beams were developed.

Modeling the panel zone was among the most significant adjustments for the inelastic analyses. The design base shear was calculated for the building per 1994 Uniform Building Code (UBC). Structural overstrength was then estimated to be 2.6 at ultimate level based on an inelastic static push-over analysis. Inelastic time-history analyses improved the correlation in the N-S direction. It also indicated that a significant number of panel zones had yielded. Thus, the authors speculated that panel zone must have been a major source of energy dissipation during the Northridge earthquake.

From this limited study, on a regular structure, it appears that current analytical and modeling tools for both elastic as well as inelastic analyses are quite reliable. They can be instrumental in predicting with a reasonable degree of accuracy the intensity and pattern of damage expected during severe seismic events.

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