SAC Phase 1 Analytical Studies of Building Performance


Project Title:
Analysis of a 17-Story Steel Moment Frame Building damaged by the Northridge Earthquake
Terrence F. Paret and Kent K. Sasaki; Wiss, Janney, Elstner Associates, Inc.
Project Summary:
The purpose of this analysis is to determine if commonly used structural analysis techniques can predict structural damage in steel moment resisting frame buildings, particularly fractures in beam flange-to-column welds. Computer programs were used to model the behavior of the building and then the results of these analyses were compared to the actual damage observed.

Some apparent but misleading correlation was found between the predicted and observed damage. The elastic and inelastic time history analyses were able to predict some of the observed damage in regions (stories) of the building, but not damage at a particular frame or at a specific beam-column location or in a particular flange. The lack of correlation may be attributed to the simplified model used and/or the actual field conditions of the pre-earthquake structure. Torsional effects of the building and composite action of the floors and beams may have significantly affected the building behavior, however, these effects were not modeled in this analysis. The actual strengths, specifically the strengths of welded beam-column connections, were probably significantly different than strengths assumed in this analysis.

The building studied is a modern 17-story office building located within five miles of the Northridge Earthquake epicenter. The building is approximately eight years old. The building plan is roughly rectangular with two steel moment resisting frames in each principal direction. The horizontal diaphragm at each floor consists of a composite concrete/metal deck floor system. Structural damage due to the Northridge Earthquake was limited to the two steel moment-resisting frames parallel to the north-south direction. The structural damage consisted of weld fractures at some of the beam bottom flange-to-column welds. No weld fractures were observed in any of the beam top flange-to-column welds. Surveys conducted after the Northridge Earthquake to check the plumbness of the building indicate that the building is currently leaning six inches to the north. Nonstructual damage was limited to gypsum board cracking in the stairwells and upper floors, cracking of some of the lobby wall tile, cracking of masonry walls at the mechanical rooms, fracturing of welds in the stair construction and minor breakage of some of the upper story windows. The strength and stiffness contributions of the masonry walls and stairwell construction were however neglected in the analysis.

The results of this study confirm our initial beliefs that analysis may have limited use in accurately predicting the locations of weld fractures in this building. The randomness of the observed fractures and the lack of correlation of the analysis results (such as D/C's, and drift ratios and plastic hinge rotations) to the obsrved weld fractures suggest that capacity side factors on the observed damage patterns in this building.

Based on the results of this study and on our knowledge of the damage in this building we believe that it would be inapropriate to rely on analysis for precluding the inspection of certain regions of the building. The analyses we performed were sufficiently inaccurate in so far as predicting damage that reliance exclusively on the analysis results for predicting damage would be unjustifiable.

Moreover, from these analyses, we believe that it is not possible to identify damge threshold values for D/Cs, interstory drifts and plastic hinge rotations which engineers could use in future evaluations of welded moment frame structures in order to predict the occurance of damage. Even if definite damage threshold values were able to be identified, we believe that those values would be only applicable to the building in this study. It is our belief that field conditions, welding processes and procedures have an overwhelming influence on damage threshold values and that if these construction conditions vary greatly as would be expected from building to building, the damage threshold values would likely vary greatly as well.

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