SAC Publications

Report No. SAC 95-05

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Parametric Analytical Investigations of Ground Motion and Structural Response, Northridge Earthquake of January 17, 1994, by S. Campbell, M. Englehardt, J. Hall, G. Hart, L. Ho, S. Huang, A. Husain, W. Iwan, H. Kim, K. Kim, R. Lobo, T. Sabol, M. Skokan, and J. Uzarski, December, 1995.

Executive Summary

This volume includes the technical reports documenting a number of parametric analytical investigations related to ground motions and the structural performance of welded steel moment frame construction in the January 17, 1994 Northridge Earthquake. These studies comprise Tasks 3.4 and 3.5 of Phase 1 of the SAC Steel Project. The goal of subtask 3.4 was to develop a joint fracture modeling element for incorporation in an available nonlinear analysis program, that could be used in individual building analyses performed in subtask 3.1 (See SAC 95-04 ). The contents of this volume and SAC 95-04 comprise the majority of the analytical investigations performed in Phase I of the SAC Steel Project.

A solicitation for proposals to perform parametric investigations of steel moment frame building performance (subtask 3.5) was circulated during December, 1994. Over 20 proposals were received for this task. A total of five Subcontractors were selected. These investigations included the following:

  1. A large number of inelastic building analyses on two model buildings based on extensive inelastic analytical models for both beam-column and column splice connections, varying assumptions for connection capacities, and a suite of ground motion time histories that included large, thrust fault motions.
  2. The development of drift demand spectra for important strong ground motion records of the Northridge earthquake and other ground motions, and the implications of these spectra on the design and evaluations of existing structures.
  3. The development of simplified nonlinear models for use in the development of nonlinear response spectra. This investigation was done in conjunction with one of the building analyses in task 3.I. (Note that the report for this study is included in SAC 95-04, in the paper by Krawinkler et. al.).
  4. Analyses to study the effects of vertical ground accelerations on the response of steel moment resisting frame buildings. This investigation was done in conjunction with one of the building analyses in task 3.1.
  5. Parametric studies on the sensitivity of expected building and connection performance due to variations in modeling assumptions and procedures.
The work done in subtask 3.4 by Campbell rose out of the need to analytically investigate the effect of damaged connections on the behavior of steel moment frame structures. None of the nonlinear analytical packages available for use by the investigators undertaking the detailed building analyses in subtask 3.1 included an element which simulated the fracture and post-fracture behavior of these connections. Modified beam-column elements were produced to model these effects within the DRAIN series of programs. Versions were produced for both two and three dimensional analyses. Example problems using the new elements were also developed and made available.

A parametric study by Hall focused on the effects of near fault motions from moderate size (Magnitude 7) earthquakes on moment resisting steel frame buildings. Analytical models of two buildings, one six stories and the other twenty stories, were subjected to a series of ground motions, including the Olive View Hospital free-field record in the Northridge earthquake, and three earthquake simulations that included blind thrust earthquakes in the Magnitude 7 range. The building models varied parameters such as material yielding, weld fracture, composite floor slabs, accumulation of damage from a second earthquake, and vertical ground motion. The basic results of the study were as follows:

  1. Large building deformations were predicted for many of the ground motions, especially for the six story building, even when all welds were assumed to be "perfect".
  2. The near fault ground motions appear to be well above the building code spectrum levels. Such effects should be considered in design.
  3. Including weld fracture into the model increased the building displacements, and the potential for excessive damage and collapse. In several of these analyses, the buildings collapsed.
  4. Including the effects of the floor slabs increased the amount of column yielding, but was not felt to improve the system behavior significantly.
  5. A second large earthquake can lead to increased levels of damage, especially if many fractured welds occurred during the first event.
  6. Vertical ground motions appeared to be of minor importance.
  7. Inelastic building response prediction is sensitive to the many modeling assumptions that are made. Careful analysis and interpretation of the results is therefore required.
lwan processed ground motion data for selected near field sites where high peak velocities were observed during the Northridge earthquake. Drift demand spectra were derived from this data, and also from synthetic Northridge ground motions. The results of these analyses are then compared with those on actual and prototype buildings to evaluate the efficacy of using the drift demand approach as a simple evaluation tool. The drift demand spectra developed indicate demands in the Northridge Earthquake in the range of two per cent, which appears to be consistent with more detailed analyses, and appears to be consistent with the level of damaged experienced by a number of these buildings.

Hart, et. al. performed a series of analyses to investigate the response of welded steel moment resisting frames subjected to both horizontal and vertical ground motion. A portion of a building that sustained damage during the Northridge Earthquake was modeled for use in these analyses. The sensitivity of this model to varying lateral and vertical mass was investigated using a suite of nine synthetic time histories in a response spectrum approach. This study indicated that the effects of vertical ground motion are likely more significant for interior columns than exterior columns. The effect on beam demands appeared to be insignificant for the models studied. The effects of varying horizontal ground motions indicated a very direct impact on element internal forces for all members, except perhaps interior columns.

The last report addresses a brief study examining the effects of various modeling assumptions on the predicted response of a steel moment resisting frame. A variety of modeling assumptions were used in a series of analyses of a six story single bay steel moment frame. A "baseline" model with bilinear representation of inelastic response and bare steel frame behavior and a "refined" model were developed. The "refined" model included hardening rules that better represent experimentally observed inelastic behavior, and included composite floor slab effects. These models were subjected to both a static pushover and a series of three strong ground motion records. The results indicated the following:

  1. The predicted response of these frames can be quite sensitive to modeling assumptions. Predicted levels of beam plastic rotations varied up to 100% between models in this study.
  2. The presence of a composite floor slab can significantly affect the predicted response of a steel moment frame, Neglecting these effects can substantially underestimate both the frame strength and stiffness, and can alter the expected distribution of plastic rotation demands.
  3. The assumed level of yield stress in steel members, and especially the relative value between the beams and columns can have a significant affect on the predicted response.
  4. The predicted location of yielding within a beam-column connection (joint panel zone, beam flanges, column flanges, etc.), can be shifted by varying the modeling assumptions.
  5. Local response prediction indices (e.g., beam plastic rotation) are more sensitive to variations in modeling assumptions than global indices such as interstory drift. This confirms that large uncertainties will exist in the prediction of plastic rotation demands, and the need to recognize these uncertainties in the course of steel moment frame building designs and evaluations.
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 were experienced in the fields of elastic and inelastic structural analysis, modeling techniques and structural steel system behavior.
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