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.
