The current seismic designs of aboveground storage tanks (ASTs) are conducted based on Fire Service Act of Japan, API Standard 650 Annex E, or Eurocode 8 etc. All of these methods evaluate the beam-type vibration, and the deformation of shell plates has a circumferential wave number m = 1. In ASTs, non-axisymmetric local mass / rigidity such as nozzles, stairs and so on, initial imperfections, or nonlinear strong vibrations also cause the oval-type vibrations of m ≥ 2, and the shell plates vibrate in a petal shape.&nbsp;&nbsp;In the oval-type vibration of ASTs, several studies reported that the initial stress due to hydrostatic pressure increased the natural frequency in high m range. The main stress of shell plates is the circumferential tensile stress occurred by the hydrostatic pressure. Tensile stress increases the rigidity of shell plates, and as a result, the natural frequency also increases.&nbsp; Several reinforcing rings in the circumferential direction called "wind girder" are installed on the upper part of shell plates of ASTs to prevent buckling due to wind pressure. There are two types of wind girders : top wind girder and intermediate wind girder. The intermediate wind girder is smaller than the top wind girder. The floating roof tank, which is open top tank, is installed both the top wind girders and the intermediate wind girders, and the fixed roof tank is installed the intermediate wind girders. It is considered that the wind girder also increases circumferential rigidity of the shell plates and has some influence on the natural frequency of oval-type vibration.&nbsp; In this paper, considering the initial stress of the shell plates due to hydrostatic pressure, the effect of wind girder on the natural frequency of oval-type vibration is studied for a large floating roof tank with a capacity of 90,000 cubic m. The method is a free vibration analysis by the axisymmetric finite element method considering the coupled vibration of the fluid and the shell plate. The fluid motion is formulated by the Eulerian approach.&nbsp; As a result, the following conclusions are obtained.1) The wind girder significantly increases the natural frequency of oval-type vibration in the high circumferential wave number region.2) Along with the top wind girder, the intermediate wind girder also has a significant influence on the natural frequency in the high circumferential wave number region.

Natural Frequency of Oval-Type Vibration of an Aboveground Storage Tank Installed With Wind Girders

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

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