既存壁式プレキャスト鉄筋コンクリート構造集合住宅建物の基礎と地盤を考慮した地震応答解析

Abstract

Existing wall-type precast reinforced concrete residential buildings maintain high structural quality, although they were constructed widely in Japan more than 40 years ago. A large number of the residential units in these existing buildings are standardized and typically small. Consequently, they do not suit for modern living uses. In order to utilize these buildings, seismic behavior was evaluated by static pushover analyses in past research. It was concluded that the failure mechanisms are rocking of the walls in the transverse direction and beam yielding in the longitudinal direction. In this research behavior of the foundations was not studied. Past large earthquakes such as Hyogoken-Nambu earthquake in 1995 showed that the WPC buildings maintained high structural seismic resisting performance and the foundations including piles were relatively weak. This fact implies need of analytical study of the seismic behavior of the buildings considering their foundation and soil. It is more true on the evaluation of their performance in the transverse direction, where the behavior of the foundation is more influential to the overall structural behavior than the longitudinal direction. In this research, time-history seismic response analyses of the WPC buildings under large earthquakes with 500 year of the recurrence period are conducted. Two cases of combinations of the soil condition and foundation system are studied. One is continuous footing on hard soil and the other is pile in soft soil. 2-D sway-rocking models were created for the two cases with corresponding sway and rocking soil springs. The superstructure is the model which is identical in the cases and composed of inelastic joint springs and elastic precast concrete panels. Pile stresses are evaluated by applying the seismic deformation method. Findings in this research are shown below: (1) In the study case for the building with the continuous foundation, uplift of the building by the overturning moment (OTM) is observed. It is referred that the failure mechanism is rocking of shear walls associated with failure of the vertical joints in past study of pushover analyses with the fixed-base model. In the time-history analyses, the maximum base shear coefficient (CQ1MAX), which is defined as the ratio of lateral force to the building weight, is 0.73. (2) In the study case with the pile foundation, the piles can fail in shear before the collapse of the superstructure. While CQ1MAX reaches 0.81 with the elastic pile model, it would be lower in studies considering inelastic shear failure of piles. (3) In the study case for the building with the continuous foundation, shear stress of shear walls is no more than 1.8N/mm2, which is lower than probable ultimate shear stress (2.0-2.7N/mm2). Although, in the study case with pile foundation, the shear stress is at most 2.1N/mm2, this is the lower limit of ultimate shear stress and would decrease considering shear failure of piles. Therefore, shear walls would not fail under the large earthquakes, while foundation may be damaged. This analytical finding agrees with the past seismic damage of this type of buildings.