Base uplifting analysis of flexibly supported liquid-storage tanks

An approximate method is presented for the analysis of earthquake induced uplifting in base plates of unanchored liquid-storage tanks that are supported directly on flexible soil foundations. The method takes due account of the non-linearities arising from the continuous variation of base contact area, membrane action and plastic yielding in the base plate. First, an ‘exact’ solution is presented for a uniformly loaded flexibly supported prismatic beam, uplifted by a concentrated force applied at one end. A series of such beams are later used in an approximate model of the base plate. The method is highly efficient and believed to be accurate for the level of uplifts encountered in practice. The results are presented for a range of foundation stiffness, representing loose sandy soils to rigid concrete mat. It is shown that, for the same overturning base moment, the flexibly supported tanks, in comparison with the rigidly supported tanks, experience significantly smaller axial compressive stress, but larger base uplift, foundation penetration, plastic rotation at plate boundary and hoop compressive stress in the tank wall.