كمانش پوستههاي استوانهاي كوتاه قامت فولادي تحت بارگذاري عرضي

Buckling of short cylindrical steel shells under transverse loading

مخازن استوانه اي جداره نازك فولادي بدليل ضخامت كم ديواره، مستعد كمانش تحت بارهاي وارده مي باشند. بار باد و زلزله از جمله بارهايي هستند كه باعث بارگذاري عرضي بر سازه مي گردند. در اين مطالعه رفتار مخازن دانه اي كوتاه قامت تحت سه توزيع فشار عرضي بررسي شده است. اين سه توزيع فشار درنظر دارد، فشار لرزه اي ناشي از محتويات دانه اي بر ديواره سازه را نمايندگي كند. آيين نامه اروپا در زمينه مقاومت و پايداري پوسته ها، از پيشروترين آيين نامه هاي بين المللي در اين حيطه مي باشد. روش تحليل كه برگرفته از اين آيين نامه است، پايداري استاتيكي اين نوع مخازن را با انجام انواع تحليل هاي خطي و غيرخطي كمانشي ارزيابي مي نمايد. تحت بارگذاري انجام شده در اين مطالعه، دو مُد كمانشي پاي فيل و چين خوردگي مورب برشي در پاي سازه، در مخازن كوتاه قامت مشاهده مي گردد. پژوهش حاضر ضمن معرفي روش تحليل آيين نامه اروپا، اطلاعات ارزشمند ديگري نيز در تأثير وقوع پلاستيسته و وجود نقص يا اعوجاج سازه از هندسه استوانه اي، در مقاومت كمانشي ارائه مي نمايد.

Thin-walled cylindrical shells are extensively used in civil engineering. Due to thin wall thickness, they are vulnerable to stability failure in the form of shell buckling. The European Standard in design of steel structures is considered to be a pioneer in strength and stability assessment of shell structures. Wind pressure and seismic action lead to non-uniform distributed transverse loading on cylindrical shells. It has been shown that non-uniform loading may have a significant and deleterious effect on the structural stability of these structures. The present study deals with buckling behavior of short cylindrical shells under three non-uniform distributed transverse pressures. The loading patterns were adopted in a way to simulate the normal pressures due to ensiled materials in excited situation. It was done with respect to Eurocode design provisions for earthquake resistance of circular silos. Its aim is to produce useful information for the design of cylindrical shells against buckling under general transverse loading. An overview of Eurocode treatments of shell stability using finite element analysis is presented. In addition, the paper explores the effects of different forms of transverse loading on stability response of the structure. The numerical approach was selected to fulfill the stability evaluations. Eurocode has many provisions for the global analysis of shell structures using finite element analysis. Hence, a full suite of computational shell buckling calculations was performed according to this standard. Linear bifurcation analysis was undertaken, firstly. It served as a benchmark for further evaluations. Two different linear bifurcation eigenmodes were observed. The main mode of buckling was diagonal shear wrinkles near the base of silo with partial extension in circumferential direction. The other mode was local axial compression buckle at the foot of the shell. A wide range of imperfection sensitivity studies using these eigenmodes were conducted. The imperfections can take many forms and can have different amplitudes. Some imperfection forms may result in higher strength of the shell. This makes identifying the worst condition very challenging. A sample parametric study on imperfection amplitude in forms of eigenmodes, illustrated this kind of analysis. Additionally, the effect of plasticity was explored through the ideal elastic-plastic model for steel. It was shown that due to loading pattern, the plasticity may cause different amount of reduction in elastic load factor. To establish the actual plastic collapse load, the modified Southwell plots were used. To achieve more realistic evaluation of buckling and post-buckling behavior of thin-walled cylinders, the finite element analyses should include all possible source of strength reduction in stability of the shell structures. To this end the geometrically and materially non-linear analysis with explicit inclusion of imperfections (GMNIA) is considered to be the most advanced form of numerical analysis. The load factors derived from GMNIA analyses showed a stability reduction more than half as compared to linear bifurcation analyses in two load cases. The non-linear incremental buckling modes were also explored. Finally, the general shape of buckled short cylinders under transverse loading was characterized by combination of diagonal shear wrinkles and elephant’s foot buckling mode.