مطالعه ي آزمايشگاهي و تئوري رفتار كمانشي پوسته‌هاي استوانه‌يي فولادي با ضخامت متغير تحت بارگذاري مركب فشار محوري و خارجي

Experimental and theoretical study of buckling behavior of steel cylindrical shells with variable wall thicknesses under combined loading of axial compression and external pressure

به دلايل اقتصادي، پوسته‌هاي استوانه‌يي فولادي بزرگ ذخيره‌ي مايع با ضخامت متغير در ارتفاع ساخته مي‌شوند. پوسته‌هاي مذكور در طول عمر خود تحت انواع بارگذاري‌ها قرار مي‌گيرند. در نوشتار حاضر، دو مطالعه‌ي آزمايشگاهي براي بررسي آثار شروع كمانش در مرحله‌ي اعمال بار فشاري محوري و همچنين در مرحله‌ي اعمال فشار خارجي در رفتار كمانشي پوسته‌هاي استوانه‌يي كه تحت بارگذاري تركيبي (بار فشارهاي محوري و خارجي) قرار گرفته‌اند، انجام شده است. بر اساس نتايج حاصله زماني كه شروع كمانش در مرحله‌ي اعمال بار فشاري محوري رخ مي‌دهد، ظرفيت كمانشي پوسته‌هاي استوانه‌يي كاهش مي‌يابد. روابط تئوري براي پيش‌بيني بار كمانشي پوسته‌ي استوانه‌يي با ضخامت متغير گام به گام تحت بارگذاري مركب فشارهاي محوري و خارجي و با درنظر گرفتن ناكاملي هندسي بررسي شده‌اند كه يكي از روابط تئوري به كار رفته، همبستگي نزديكي با نتايج آزمايشگاهي داشت.

Thin-walled steel cylindrical shells are industrial structures that play an important role in the storage of petroleum and refineries, potable water supply, and fire extinguishing systems. Steel storage cylindrical shells are manufactured with variable thicknesses in height for economic reasons. Each steel cylindrical shell structure is made of several individual cylindrical parts of constant thickness. These shells with fixed roofs are subjected to axial pressure due to wind effect and snow accumulating on the roof. The cylindrical shells are also subjected to external pressure due to wind load and/or vacuum load when the containing liquid is discharged. The combination of axial compression and external pressure may lead to the failure of the shell structures. In this paper, two experimental studies were performed to investigate the effect of initial buckling during the axial compressive preloading or at the external pressure phase on the buckling behavior of cylindrical shells with stepwise wall thickness under combined loading of axial compressive preloading and external pressure. The results showed that the buckling capacity decreased under axial preloading and external pressure when the initial buckling occurred during the application of axial compressive preloading. Also, more deformations and buckling waves formed in the thinner individual section of cylindrical shell and also failure occurred faster and the structure became unstable faster, when initial buckling occurred during the axial compressive preloading. Theoretical relationships, in which geometric imperfection is considered, were employed to predict the buckling load of cylindrical shells with variable thicknesses subjected to combined loading of axial compression and external pressure. One of these two relationships shows a closer correlation with experimental results. Also, the results showed that the buckling behavior of cylindrical shells was very sensitive to the applied axial compressive preloading; therefore, as the axial compressive preloading increases, the quasi-empirical theoretical relationships become more conservative.