Application of nonlinear fluid-structure interaction methods to seismic analysis of anchored and unanchored tanks

Seismic response of liquid storage tanks is quite different from conventional structures not only due to hydrodynamic effects acting on the tank shell but also owing to many sources of nonlinear behavior mechanisms of tanks such as buckling of the tank shell, large amplitude nonlinear sloshing, nonlinear tank-soil interaction, material yielding, plastic rotation of base plate and successive contact and separation between tank base and soil. For the assessment of earthquake response of such structures, numerical methods are indispensable tools since they offer a concise way of accurately considering all these nonlinearities in the same model. Yet, before using numerical techniques for design purposes for the evaluation of different configurations of the fluid-tank systems when subjected to several earthquake ground motions, they should be validated by experimental results. For this purpose, in this study, a fully nonlinear fluid-structure interaction algorithm of finite element method is employed for the seismic analysis of anchored and unanchored steel liquid storage tanks. An existing experimental study carried out on anchored and unanchored tanks is utilized for the verification of the numerical procedure. In the numerical models, Arbitrary Lagrangian Eulerian (ALE) description of the liquid-structure interface which is described by the mesh nodes at the boundary is employed. Fluid motion is governed by compressible Navier-Stokes equations. Both material and geometric nonlinearities are considered in order to accurately determine stress, strain and strain rate distributions throughout the tank. Successive contact and separation between base plate and foundation along with friction effects are taken into account by contact modeling algorithm. Response parameters of tanks, such as free surface sloshing wave height, pressure time histories, base plate uplifting and shell stresses obtained by numerical simulations are correlated with those of experimentally observed. By means of experimental and numerical results, the principal effects of base uplifting on the seismic response of unanchored cylindrical steel tank are highlighted with comparing that of rigidly restrained tank. Moreover, the provisions given in the current tank seismic design codes for metal cylindrical tanks are discussed and the response parameters of tanks are computed per minimum requirements of these codes. The results are compared with findings of numerical and experimental studies and the comments on the consistency of these results are explained in detail. Strong correlation between the experimental and numerical results is obtained for investigated response parameters although code requirements present deviations from the reference experimental study especially for uplift displacement and free surface wave height.