Preliminary Evaluation of Seismic Capacity and Torsional Irregularity of Uto City Hall Damaged in the 2016 Kumamoto Earthquake

Abstract

In this paper, the seismic capacity and torsional irregularity of the main building of Uto City Hall are evaluated by using a simple method based on the building’s structural drawing. The simplified evaluation method of seismic capacity, which was proposed by Shiga in the 1970s, is based on the wall-area index and the average shear stress in walls and columns. For evaluation of its seismic capacity, the following two cases are considered: the building is assumed to behave as a unit building, and each of the structural blocks responding independently. The evaluation of the torsional parameters, stiffness eccentricity and radius of torsional stiffness with respect to the center of stiffness are based on the sectional area of the columns and walls, which is presented in the Japanese Standard for the seismic evaluation of existing reinforced concrete (RC) buildings. The main findings of this paper are as follows. (a) The seismic capacity of the main building of Uto City Hall is insufficient to survive severe earthquakes. However, the evaluated results of both cases cannot explain the damage observed in upper stories. (b) The ratio of the stiffness eccentricity to radius of torsional stiffness evaluated in each story exceeds 0.15, while the radius ratio of the torsional stiffness with respect to center of stiffness to the gyration of the whole mass above the considered story is smaller than 1. Therefore, the main building of Uto City Hall is sensitive to torsional response: it may be classified as a “torsionally flexible building”.