Stiffness requirements for wind girders in open-top cylindrical steel tanks

Abstract

Aboveground cylindrical storage tanks are very thin shell structures which are prone to buckling when subjected to wind pressure. Both external and internal pressures are developed due to wind action, and the magnitudes of the pressures generally vary around the circumference. A wind girder is generally used in open top cylindrical tanks to increase their resistance against buckling. The wind girder must have adequate strength and stiffness in order to fulfill its function properly. Widely used design specifications provide recommendations for the design of the wind girder. Strength requirements are given in terms of stress resultants, while a minimum second moment of area is provided as a stiffness requirement. The recommendations for the stiffness requirement were developed by conducting Linear Elastic Bifurcation Analysis (LBA) of cylindrical shells subjected to uniform external pressure. This paper explores stiffness requirements for the wind girder in open top cylindrical steel tanks. The relationship between the buckling strength and the second moment of area of the wind girder was explored through a finite element parametric study. Tank shells with different radius-to thickness and height-to-diameter ratios and having different wind girder sizes were investigated by means of LBA. A buckling strength curve was developed based on the LBA results where the strengths can be predicted with 90% accuracy. In addition, a new second moment of area requirement was proposed. The parametric study was extended to investigate the behavior using Geometrically Nonlinear Analysis including Imperfections (GNIA). The same tank geometries were studied using GNIA and considering different imperfection amplitudes. A buckling strength curve, which takes into account imperfection amplitudes, was developed to be used in design.