既存中高層コンクリート系建物の上層減築に伴う下層の地震損傷集中化の可能性

抄録

 This paper discusses the seismic performance of concrete frame structures in which the upper stories are subtracted for seismic rehabilitation. Existing reinforced concrete buildings and steel reinforced concrete buildings are considered in this study. Most of structural engineers seemingly assume that the seismic performance of such a retrofitted building is improved due to decrement of the total inertial mass. Accordingly, this retrofitting technique has been implemented in old buildings which are insufficient to meet the current building standards law in Japan. This study clarifies how the seismic performance is changed by removing the upper stories through both theoretical approach and nonlinear time history analyses.
 Mid-to-high-rise buildings having seven-to-fourteen stories are employed. It is assumed that these buildings are designed in the current building standards law in Japan. A shear spring model with multi degree of freedom is employed in order to directly assess the distribution of the stiffness and strength of the stories along the height. Both a trilinear envelope curve and a degrading hysteresis rule are considered in each story. The variation of the seismic performance is evaluated in terms of maximum inter-story drift angles (IDA).
 Firstly, fundamental natural periods and corresponding vibration modes are discussed in elastic range. The distribution of the story stiffness is varied in accordance with the increasing of the number of the subtracted upper stories. As a result, eigenvalue analyses show that the approximately triangle shaped vibration mode is changed into a quarter sinusoidal shape after the several mass are removed from the top of the analysis model. This phenomenon is linked to an increase of the IDA in the lower stories in elastic range. Time history analyses also demonstrate that the IDA slightly increases at the bottom story if the natural period of the retrofitted building belongs to the range where spectral velocity is constant.
 Secondary, an evaluation method of IDA without time history analysis is proposed within the framework of the response spectrum method. This proposed method clarifies the relation between amplification factors of the bottom IDA and the number of the subtracted stories. Subsequently, the method is developed with the deviation coefficients of elastic story response and a yield drift angle for each story. Variation of ductility factors are summarized in terms of the number of the subtracted stories.
 Finally, both the proposed method and nonlinear time history analysis conclude that the bottom IDA increases by two times at most in comparison with the original building. The reason is that the ductility factor is spatially concentrated due to subtraction of the upper stories contrary to expectations. In other words, the subtractions of the upper stories possibly deteriorate the seismic performance.