Unanchored steel storage tanks, commonly used in industrial facilities, can suffer damage during major earthquakes due to various failures. To better understand the seismic behaviour of such ...structures, a pushover-based seismic performance assessment of four tanks with varying slenderness ratios was performed. The emphasis was placed on understanding the relationship between engineering demand parameters, tank slenderness ratio, and wall geometrical imperfections, which were, however, imposed only to the lower course around the tank circumference to assess the upper limit of the effect of geometrical imperfections on the elephant-foot buckling (EFB). The findings reveal that axial compressive stress in the tank wall correlates with increased slenderness and geometrical imperfections. This implies that the axial compressive stress in the wall of broader tanks is relatively low, and the bulging at the bottom of the wall is mainly due to high hydrodynamic pressure and the resulting hoop stress. In contrast, the wall of slender tanks buckles primarily due to high axial stresses, leading to EFB. Through dynamic analysis, the study showed that the pushover analysis can underestimate the axial stress if the tank’s base plate is uplifted significantly before EFB occurs. The effect of the impact should thus be considered, especially in the case of slender tanks, because the base plate uplift mechanism is more pronounced than in broader tanks. Further research is needed for a more accurate prediction of the axial compressive stress in slender tanks. However, the safety margin in the post-yielding range is low because the yielding area of the tank wall rapidly increases after the occurrence of steel yielding. In the absence of a detailed 3D model of tanks, simplified formulas for estimating stresses in the tank wall may be used for the broader tanks but not for more slender tanks because of their inability to simulate the highly non-linear relationship between the ground motion intensity and the stresses observed in the plastic region of the tank wall.
•Slenderness and geometrical imperfection correlated to axial stress in tank walls..•Slender tanks buckle from axial stress, broader ones also due to high hoop stress.•Dynamic analysis revealed base plate impact triggers EFB, mainly in slender tanks..•Dynamic analysis including base plate-foundation impact necessitates further research..•Simplified formulas to evaluate slender tank stresses overestimate seismic capacity.
Abstract
Unanchored steel storage tanks are used in industrial plants to store oil and other petrochemical liquids. In a major earthquake, such tanks may sustain different types of damage, including ...elasto‐plastic buckling in the shape of an elephant's foot, which is the object of the research. The methodology introduced in this paper consists of cloud‐based 1D site response analysis (SRA) and pushover‐based seismic performance assessment of the tank wall using a 3D non‐linear model of the tank. The SRA, which considers the recorded ground motions on the rock outcrop and the sample of interval shear‐wave velocity profiles, is carried out to estimate spectral accelerations at the tank impulsive period. By adopting the code‐based model of hydrodynamic pressures, the seismic demand on the tank wall is then calculated by pushover analysis. The risk‐targeted decision model is used for safety verification. The proposed methodology is demonstrated through an example of a large liquid storage tank for which it is shown that the fragility function for peak ground acceleration at the rock outcrop is on the left side of the target fragility function, indicating that the tank cannot be considered safe from elephant‐foot buckling. The introduced assessment process is relatively simple if the cloud‐based SRA is automated. However, further experimental and numerical investigations are required to confirm the validity range of the pushover analysis for the seismic performance assessment of tanks.
