کمانش پوسته های استوانه ای کوتاه قامت فولادی تحت بارگذاری عرضی

مخازن استوانه ای جداره نازک فولادی بدلیل ضخامت کم دیواره، مستعد کمانش تحت بارهای وارده می باشند. بار باد و زلزله از جمله بارهایی هستند که باعث بارگذاری عرضی بر سازه می گردند. در این مطالعه رفتار مخازن دانه ای کوتاه قامت تحت سه توزیع فشار عرضی بررسی شده است. این سه توزیع فشار درنظر دارد، فشار لرزه ای ناشی از محتویات دانه ای بر دیواره سازه را نمایندگی کند. آیین نامه اروپا در زمینه مقاومت و پایداری پوسته ها، از پیشروترین آیین نامه های بین المللی در این حیطه می باشد. روش تحلیل که برگرفته از این آیین نامه است، پایداری استاتیکی این نوع مخازن را با انجام انواع تحلیل های خطی و غیرخطی کمانشی ارزیابی می نماید. تحت بارگذاری انجام شده در این مطالعه، دو مُد کمانشی پای فیل و چین خوردگی مورب برشی در پای سازه، در مخازن کوتاه قامت مشاهده می گردد. پژوهش حاضر ضمن معرفی روش تحلیل آیین نامه اروپا، اطلاعات ارزشمند دیگری نیز در تاثیر وقوع پلاستیسته و وجود نقص یا اعوجاج سازه از هندسه استوانه ای، در مقاومت کمانشی ارائه می نماید.

Buckling of short cylindrical steel shells under transverse loading

Thinwalled 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 nonuniform distributed transverse loading on cylindrical shells. It has been shown that nonuniform 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 nonuniform 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 elasticplastic 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 postbuckling behavior of thinwalled 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 nonlinear 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 nonlinear 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 rsquo;s foot buckling mode.