Koji Okada
Design Director, Architectural Design Department
NIKKEN SEKKEI LTD, Tokyo

Daiki Nakamizo
Associate, Structural Design Division, Engineering Department
NIKKEN SEKKEI LTD, Tokyo

Mid-Story Isolation Success

Addressing one of Tokyo’s biggest urban problems—a significant lack of buildable area—the DaiyaGate Ikebukuro straddles a set of active railroad tracks. The building is 20 stories tall, with two basement levels and rail lines passing through it at ground level. The structure rests over the lines with a deck at the second story, with a mid-story seismic isolation system integrated into the fourth floor. The upper and lower structures of this isolation layer have high horizontal stiffness and lateral capacity, employing outer bracing for seismic resistance.

For this project, site constraints presented significant challenges. The development is the largest over-track building in Japan, with few precedents, even internationally. Because train operation could not be interrupted, the construction of the basement levels or other substructures beneath the railroad lines was not allowed. As such, the ground substructures had to be separated, with one substructure system along the western side of the railroad tracks, and another on the eastern side. The decision to adopt this mid-story isolation system was taken in order to reduce vibration effects due to train operation, and also to decouple the relative displacement between the lower stories and upper office floors.

The isolation system grants significant response reduction and absorbs almost all of the seismic energy, since the drift of the second layer is small. The building has sufficient toughness to mitigate train vibration effects, and sufficient redundancy in case of trains derailing and impacting the structure. Additionally, the outer bracing (installed above the isolation layer) and V-shaped columns (below the isolation layer) are engaged during large earthquakes to reduce story drift.

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