Higher-mode effect on the seismic responses of buildings with viscoelastic
dampers
Meng-Hao Tsai and Kuo-Chun Chang
Department of Civil Engineering, Taiwan University, Chinese Taipei
Abstract: In conventional modal analysis procedures, usually only a
few dominant modes are required to describe the dynamic behavior of
multi-degrees-of-freedom buildings. The number of modes needed in the dynamic
analysis depends on the higher-mode contribution to the structural response,
which is called the higher-mode effect. The modal analysis approach, however,
may not be directly applied to the dynamic analysis of viscoelastically damped
buildings. This is because the dynamic properties of the viscoelastic dampers
depend on their vibration frequency. Therefore, the structural stiffness and
damping contributed from those dampers would be different for each mode. In this
study, the higher-mode effect is referred to as the response difference induced
by the frequency-dependent property of viscoelastic dampers at higher modes.
Modal analysis procedures for buildings with viscoelastic dampers distributed
proportionally and non-proportionally to the stiffness of the buildings are
developed to consider the higher-mode effect. Numerical studies on shear-type
viscoelastically damped building models are conducted to examine the accuracy of
the proposed procedures and to investigate the significance of the higher-mode
effect on their seismic response. Two damper models are used to estimate the
peak damper forces in the proposed procedures. Study results reveal that the
higher-mode effect is significant for long-period viscoelastically damped
buildings. The higher-mode effect on base shear is less significant than on
story acceleration response. Maximum difference of the seismic response usually
occurs at the top story. Also, the higher-mode effect may not be reduced by
decreasing the damping ratio provided by the viscoelastic dampers. For practical
application, it is realized that the linear viscous damping model without
considering the higher-mode effect may predict larger damper forces and hence,
is on the conservative side.
Keywords: model analysis, viscoelastic dampers, higher-mode effect
