Technical Background

SAC Phase 1 Analytical Studies of Building Performance


Project Title:
Analysis of a Damaged 4-Story Building and an Undamaged 2-Story Building

Helmut Krawinkler, Stanford University
Ali Alali, Stanford University
Charles C. Thiel, Jr., Telesis Consultants
John M. Dunlea, Lumar Engineering

Project Summary:
This case study focuses on predictions of seismic demands and correlation of these demands with fractures observed after the Northridge earthquake. Two adjacent buildings are used for this purpose. One of them, a 4-story building with perimeter frames, experienced many connection fractures in one of the NS perimeter frames during the Northridge earthquake. The other building, a 2-story building with perimeter frames in the NS direction, and perimeter frames plus one interior moment resisting frame in the EW direction, did not exhibit visible connection fractures.

Seven series of static (pushover) and dynamic (time history and spectral) analyses were performed, utilizing four recorded ground motions, and equal hazard spectrum, and nine simulated ground motions that were generated to represent the ground motions of the Northridge earthquake at the site of buildings. The analytical models were different in all seven analysis series, ranging from a simple elastic centerline analysis model to inelastic models that incorporate the contributions of the floor slab to the moment resisting and simple frames. Two of the models are preliminary attempts to model the post-fracture behavior of one of the frame structures.

This report summarizes damage observations in the case study buildings, presents analytical modeling issues, and then focuses on the analytical study, its interpretation, and the correlation of predicting demands with observed connection fractures. Many conclusions are listed at the end of the report; the most salient ones being the following:

All four "structures", i.e. the 2- and 4-story NS and EW steel frame structures, are much stronger than required by the 1988 UBC. The elastic base shear strengths are 3 to 4.5 times as large as the Code base shear requirements at the allowable stress level. The overstrength factor (ultimate strength over elastic strength) is in the order to two. Thus, the elastic D/C ratios and the plastic deformation demands are relatively small for the severe seismic inputs used in this case study.

A reasonable correlation was achieved between element plastic deformation demands and observed connection fractures. The low stress level predicted in the columns (by the inelastic analyses) justifies the absence of fractures across the column, which were observed in many other buildings. The observed fractures are at locations at which the predicted plastic deformation demands in either the beam or the joint panel zone are high. The difference in observed damage between the NS and EW frames of the 4-story building (many fractures in the NS frame and only one fracture in the EW frames) is in line with large differences predicted in deformation demands. However, significant plastic deformation demands are also predicted for the 2-story building in which no fractures were observed.

Element demand parameters that provide useful information on the likelihood of connection fractures include elastic demand/capacity ratios (with limitations), plastic rotation demands in beams and columns, and plastic shear distortions in joint panel zones.

Elastic demand/capacity ratios are meaningful indicators in some cases and misleading ones in others, particularly when joint panel zones are weak in shear. Their interpretation has to be done with great caution.

Modeling of the shear strength and stiffness properties of joint panel zones is critical for realistic demand predictions in steel moment resisting frames.

Excessive shear yielding of joint panel zones may be a source of connection fractures. It causes large curvatures in the column flanges at the corner of the joints, which in turn causes large strains around beam-to-column flange welds. In the 4-story case study building the observed fractures can only be explained by this phenomenon since the stress level in several of the beams with fractured connections is low.

The predictions of plastic rotation and distortion demands were not very sensitive to the refinements made in modeling the structure, provided that the strengths of beams, columns, and joint panel zones are represented realistically.

An accurate prediction of the occurrence and locations of fractured connections by means of a global elastic or inelastic analysis is an unrealistic expectation. The variations in the frequency content of input ground motions are large, the determination of the structural periods is far from perfect, and most important, the scatter in deformation capacities of welded connections is very large and is affected greatly by local detailing. Analytical predictions should serve as indicators that assist in identifying the existence of potential problems and in developing an inspection strategy once a potential problem has been identified.

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