A major earthquake that occurred in Japan was used as an experiment to verify the adequacy of seismic design. We studied our current seismic design margins based on the elastic theory and using data observed in Hyogo-ken Nanbu Earthquake. While a study of this sort should be treated statistically, the fact that there was only one set of data led us to employ quantitative analysis. In some cases, we found a margin of 1.4 to 4.8 times between the actual seismic behavior and the design basis (e. g., carbon steel pipe STR400). The reduction effect of the input ratio of waves depends largely on the foundation structures. In the case of liquid storage tanks, the liquid level during an earthquake is an important margin factor. The strength margin depends largely on the margins of stress evaluation and material strength. The margin should not be used to cover the uncertainty of the designer or operator. The margin should be used to cope with the uncertainty of earthquake as a natural phenomenon. Anti-fracture design should be taken to the important structures like tanks.
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