Evaluation of the Vertical Distribution of Base Shear Force in Base-Isolated Structures


Department of Civil Engineering,Sharif University of Technology


Application of the base-isolation systems, as a means to limit the seismic-induced response
of structures, has attracted the attention of many engineers and researchers. Due to their
importance, the Uniform Building Code (UBC) has incorporated a special section for the
seismic analysis and design of base-isolated structures since its 1991 edition. The present
work investigates the vertical distribution of the lateral seismic force for base-isolated structures
provided by the 1997 edition of UBC (UBC97). Di erent 6 and 8-story, 3-D base-isolated
structural models with LRB isolators are considered, having a variety of e ective periods and
e ective damping ratios. The UBC97 analysis procedure for the base-isolated structures is used
to determine the minimum lateral seismic force and its vertical distribution for di erent
Since the number of stories above the isolation interface is more than four for the considered
isolated structural models, the response spectrum analysis is used, considering the equivalent
linear properties for isolation systems. Also, the UBC97 recommended that the 5%-damped
design spectra be properly modi ed to account for the actual modal damping ratios of an
isolated structure. Extensive nonlinear dynamic analyses were performed for 8 types of LRB
isolators, using appropriately normalized earthquake accelerograms recorded on SA and SB soil
pro les. Both the superstructure and the isolators are allowed to behave nonlinearly, in order to
evaluate the seismic induced demand shear force on di erent
oors. The peak base center and
corner displacements, maximum base and story shear forces and the maximum inter-story drifts
are determined for di erent base-isolation systems and earthquake records. The results, together
with their mean and mean plus one standard deviation values, are used for the evaluation of
UBC97 response spectrum analysis procedures for these buildings. The results indicate that
the UBC97 suitably predicts the seismic lateral forces for base-isolated buildings. However, it
does not provide a good estimate of the shear force distribution over the height, especially for
the highly damped base-isolation systems. Furthermore, the number of columns per story that
behaved nonlinearly during the time history analyses is included for comparison.