During intense seismic activity, strong horizontal excitation forces acting at the bottom of the unanchored oil tank shell can create an overturning moment and uplift displacement. To explore this, we carried out a nonlinear FEM static analysis to quantify the uplift displacement of an 110, 000m3 oil storage tank when subjected to the maximum response pressure load of the “Far-Off-Sanriku Earthquake” (1994) . A 3-dimensional shell model and maximum ground acceleration value of 293gal were used for the analysis.
The maximum uplift displacement of the tank shell was calculated to be 188mm. For the circumferential distribution of this displacement, the first Fourier coefficient of cosine mode, a1, was about 40%; the second, a2, about 25%; a0 about 18% and the third, a3, about 14%. The Fourier coefficients for circumferential distribution of shell radial displacement were 30% respectively for cosine modes a1, a2 and a3. The analysis also showed that inward deformation of the shell occurred in the upper section of the tank at the uplift maximum part.
A simple static model analysis showed that the maximum uplift displacement of the tank shell was about 100mm. However, no allowance was made for tank shell deformation in this analysis.
A comparison of the uplift displacement of the 3D model with that of the simple model shows that the displacement simulated by the simple model corresponds to the summation of cosine modes a0 and a1 from the 3D shell model analysis. The a0 and a1 cosine modes are also regarded as rigid modes of shell plate.
The ratio of total mode uplift displacement to (a0+a1) varies from 1. 2 to 1. 8 as the seismic load accelerates to full load (293gal) . This ratio is defined as the “mode correction factor of shell uplift”, and the real uplift displacement can be calculated by multiplying the value for uplift displacement obtained from the simple model by the “mode correction factor”.
