Structural Engineering Award of Excellence: Sumitomo Fudosan Roppongi Grand Tower, Tokyo

Yuichi Koitabashi
Engineering Senior Director, Structural Design Division, Engineering Department
NIKKEN SEKKEI LTD, Tokyo

A Case Study of Mid-Story Isolation for Seismic Resistance

Roppongi Grand Tower achieves an engineering breakthrough with the integration of a hybrid mid-story isolation system, despite being over 200 meters tall. The system used here consists of a mid-story isolation layer installed at 130 meters above ground, and viscous wall dampers installed at each floor below the isolation level. A complex arrangement of seismic elements at the isolation layer includes soil dampers, installed so that they connect the upper-level suspended columns to the lower-level short columns. Steel dampers are located above or below some short column locations. The intricate arrangement causes the columns to be shared by steel and oil dampers, and the cost of steel used is reduced. Simultaneously, decreasing the number of columns improved maintainability in the isolation level. Below the isolation level, the lower structure is designed such that the lateral stiffness is relatively low compared to the structure above it. The input seismic energy is therefore reduced, and seismic isolation effects are further enhanced. This scheme also improves the effect of seismic isolation, because the larger movements and viscous damping in the lower structure improve seismic energy absorption. The effectiveness of the overall system was verified by undertaking time-hysteresis analyses and studies of steady vibration using a transfer function method.

In studying the seismic performance against large earthquakes, the response velocities and story shear forces due to the employed system are 30 percent lower than comparable vibration-control alternatives. Additionally, since almost all the seismic energy is absorbed by the isolation layer, installing dampers along all stories is unnecessary. Therefore, the mid-story isolation system offers effective cost savings in terms of framing and quantity of seismic devices used.

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