Application of mass/stiffness eccentricity to control response of structures subjected to earthquake ground motion
Department of Civil and Environmental Engineering
Doctor of Philosophy
Saadeghvaziri, M. Ala
Hsu, C.T. Thomas
Marhaba, Taha F.
This dissertation is driven by the concept that engaging more modes in the response of structures can be used to mitigate its translational dynamic response. One such an approach is to engage torsional modes through engineered eccentricity (mass/stiffness eccentricity), thus, introducing coupled translation-rotation response. This idea was first introduced in a paper published by MacBain and Spillers in 2004. As a follow up to the same idea this dissertation was an attempt to investigate and develop the theory concerning the application of mass/stiffness eccentricity to control the translational motion of structures subjected to earthquake ground motion.
Different discrete and continuous mathematical models of structures were used for this study. Discrete models are single story building and multi story building, and continuous models are shear beam and flexural beam.
Initially, the steady state behavior of eccentric structures was analyzed. This type of analysis proved to be revealing in terms of parameters that impact the response mitigation. A sufficient and necessary condition under which increasing eccentricity in a single story building always leads to mitigation of translational displacement was deducted. Moreover it was observed that in addition to the eccentricity the relationship between dominant translational frequency to dominant rotational frequency plays a significant role in the magnitude of reductions.
Furthermore through conducting a statistical analysis the seismic effectiveness of the proposed method was investigated. For this purpose the structural models were exposed to 16 real earthquake records. The records were selected in a way that a broad range of frequency content were covered. The records are applied to structures with different eccentricities and frequency ratios. Altogether 5632 analyses were performed. The results showed that eccentricity was indeed effective in reducing the average translational displacements up to 30%. Moreover, using the data obtained from time history analyses the variation of reductions with eccentricity and frequency ratio was studied.
The dissertation continued with proposing a systematic approach for finding the eccentricities and frequency ratio that lead to the maximum reduction in displacements. To address this issue an optimization problem in frequency domain was formulated. The mean square value of response was selected as the performance function. Two types of constraints including limitations on rotations and eccentricity were imposed. Kanai- Tajimi power spectral density function was used to model the ground motion. It was observed that this approach could be used to decrease the performance function up to 50%. Finally through a case study the performance of the proposed approach was compared with tuned mass dampers (TMD). The results showed that the proposed method could be as effective as TMDs. Even in some cases more reductions in displacements could be achieved.
njit-etd2011-005 (111 pages ~ 3,295 KB pdf)
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Created May 16, 2011