Soil–structure interaction in dynamic behaviour of elevated tanks with alternate frame staging configurations

The frame stagings with a single row of columns placed along the periphery of a circle, are generally adopted for elevated water tanks to support the tank container. Apart from this usual staging configuration, some alternate configurations are also used in practice. These alternate configurations are made by adding few structural members to the usual configuration. These staging configurations are advantageous for adoption from a few different viewpoints. The present paper aims to observe the effect of soil–structure interaction on two dynamic characteristics namely, the impulsive lateral period which regulates lateral seismic behaviour and the impulsive torsional-to-lateral period ratio which regulates torsional vulnerability of the structure. The analytical expressions for these two dynamic characteristics have been derived considering the effect of soil-flexibility for elevated water tanks with these alternate configurations. These formulations have been validated against the results of finite element analysis for a few example tanks. A parametric study with limited example tanks based on these formulations shows that the frame staging with all kinds of alternate configurations having less panel heights, more number of columns, larger column diameter and stiffer circumferential beams compared to columns encounters the strongest influence of soil–structure interaction effect. The study on the example tanks with different alternate configurations shows that the design of elevated tanks based on a fixed base assumption may lead to wrong assessment of seismic base shear. The underestimation of base shear may lead to unsafe design whereas overestimation may cause uneconomic design. Neglecting soil-flexibility may also cause overlooking the possibility of occurring axial tension in columns and wrong assessment of torsional vulnerability of the staging structures.