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UBC Theses and Dissertations

The dynamic behaviour of the centre of stiffness of R/C eccentric structures under seismic excitation Tong, Raymond K. W.


Under seismic excitation, the centre of stiffness of an eccentric structure is stationary as long as the structure remains elastic. Once yielding occurs, the centre of stiffness will begin to move away from its original position, as the torsional forces induced by the eccentricity cause uneven distribution of yielding among members. This movement of the centre of stiffness very often increases the eccentricity of the structure causing further damage. The purpose of this thesis is to determine the significance of this magnification of the initial eccentricity. A procedure for locating the centre of stiffness was developed and incorporated into a time-step dynamic analysis program code named DRAINTABS. Two models were chosen to describe the moment-rotation relationship of reinforced concrete members; the elasto-plastic model and the Takeda model. The former is a bilinear model whereas the latter takes into account the strength degradation of reinforced concrete members under cyclic loading. A number of reinforced concrete buildings were studied. It was found that the centre of stiffness did not always move towards the side of the structure which was more heavily loaded due to the torque induced by the eccenticity. Excursions in the other direction were possible when the translational motion was not in phase with the torsional motion at the instant of maximum excursion. Moreover, when the strength degrading characteristic of R/C members was modelled, the eccentricity of the structure increased gradually with increasing length of excitation. However, this increase was found to be small and therefore insignificant. The procedure for locating the centre of stiffness was also incorporated into another analysis program code named PITSA which utilizes the modified substitute structure method. The results obtained were compared to those obtained using DRAINTABS. Although PITSA has been proven to be a relatively inexpensive yet reliable alternative to a time step analysis, it failed to predict the maximum displacement of the centre of stiffness with any acceptable degree of accuracy.

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