NATURAL VIBRATIONS OF A STEEL-CONCRETE CYLINDRICAL SHELL IN A SOIL MEDIUM

Abstract

Introduction. Cylindrical shells embedded in the soil medium are generally used in pipeline transportation. To prevent damage to pipelines by concrete weights when the structure surfaces in a waterlogged environment, it is proposed to use concrete pipe products, with the inner part made of steel and the outer part formed by a concrete layer 30–50 mm thick. In this case, the designer faces the question of which calculation method to use for determining the natural vibration frequencies. Purpose of the study: To compare the values of natural vibration frequencies of a large-diameter steel-concrete gas pipeline in the ground, obtained using an analytical dependency, with the values determined in the Lira software package. Methods: The first method of determining frequency is based on an analytical expression obtained using the semi-momentless theory of cylindrical shells. The second method is based on the finite element method with the construction of a computational model in the Lira-SAPR software. Modeling of steel and concrete layers of the composite shell in the software package was carried out using 4-node plates, which are combined into a common structure with the help of perfectly rigid bodies (PRB). In the first case, the calculation for the soil medium surrounding the shell was carried out by creating a mass (measuring 5.3×5.3 meters) using volumetric bodies, while in the second case, it was done by setting a coefficient of subgrade reaction for the concrete layer. Results: We established that the second method of setting soil conditions allows a 5–6 times reduction in data entry time while achieving the same results. The discrepancy in the natural vibration frequencies for the research object, determined by the analytical method and the finite element method (FEM), does not exceed 10%, and for the first three frequencies of the spectrum, it is no more than 6%. Therefore, all methods are applicable. However, the use of an analytical expression allows calculations to be performed 10 times faster and does not require specialized software, making it more advantageous in the design based on frequency characteristics.