This study elucidates the three-dimensional seismic responses of the steel frame structures having single-layer latticed cylindrical roof. A finite element modal analysis using acceleration motion recorded at the Kobe Maritime Meteorological Observatory is performed. It is shown that the dominant components in the seismic response are the free vibration modes having relatively large participation factor. Large vertical acceleration on the roof occurs due to horizontal seismic motion. Large

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... forces of the braces in the sub-structures are also shown to result from the vertical vibration of the roof.