The 2003 Tokachi-oki earthquake caused the severe damage to oil storage tanks by liquid sloshing. Especially at Tomakomai in Hokkaido, the ground motions at the periods of 3 to 8 sec predominated, which were harmonized with the natural period of liquid sloshing of oil storage tanks, then seven single-deck-type floating roofs were damaged and sank. For the 30,000kl FRT(φ 42.7m), one of those tanks, with about 7 see of fundamental sloshing period, maximum sloshing wave height was estimated 3m and over. On the other hand, for the 100,000kl FRT(φ 78.2m) with about 12 sec of fundamental sloshing period, maximum sloshing wave height was estimated about 1.5m and the excitation of 2nd sloshing mode was considered to be strongly excited. Considering both of nonlinear behavior of a large amplitude wave of 1st sloshing mode and nonlinear effects of large deflection of a deck plate at 2nd sloshing mode, we established the simplified method of seismic design of single-deck-type floating roofs using modified velocity response spectrum. This spectrum was based on many studies, investigated by Zama [1] and others, of the prediction of long-period strong ground motion and of liquid sloshing of oil tanks in Japan.

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