كليدواژه :
سيلوهاي استوانه اي فولادي , كمانش , آيين نامه اروپا , فشار تخليه , نرم افزار آباكوس
چكيده فارسي :
ﺳﯿﻠﻮﻫﺎي اﺳﺘﻮاﻧﻪاي ﻓﻮﻻدي از ﺟﻤﻠﻪ ﺳﺎزهﻫﺎي ﮐﻠﯿﺪي در ﺑﯿﺸﺘﺮ ﺑﺨﺶﻫﺎي ﺻﻨﻌﺘﯽ و ﮐﺸﺎورزي ﺑﻪﺷﻤﺎر ﻣﯽروﻧﺪ ﮐﻪ وﻇﯿﻔﻪ ﻧﮕﻬﺪاري ﻣﻮاد داﻧﻪاي را ﺑﺮﻋﻬﺪه دارﻧﺪ. ﺑﺎرﻫﺎي ﻣﺨﺘﻠﻔﯽ ﻫﻤﭽﻮن ﺑﺎرﻫﺎي ﻧﺎﺷﯽ از ﻓﺮآﯾﻨﺪ ﺑﺎرﮔﯿﺮي و ﺗﺨﻠﯿﻪ ﺳﯿﻠﻮﻫﺎ، ﺑﺎر ﺑﺎد، ﺑﺎر زﻟﺰﻟﻪ و ﺑﺎرﻫﺎي ﺣﺮارﺗﯽ، ﻻزم اﺳﺖ در ﻃﺮح ﺳﯿﻠﻮﻫﺎ درﻧﻈﺮ ﮔﺮﻓﺘﻪ ﺷﻮد. ﺑﺎ اﯾﻦ وﺟﻮد، ﻧﻈﺮ ﺑﻪ ﮐﺎرﮐﺮد اﺻﻠﯽ ﺳﯿﻠﻮﻫﺎ ﮐﻪ ذﺧﯿﺮهﺳﺎزي ﻣﺤﺘﻮﯾﺎت داﻧﻪاي اﺳﺖ، ﭘﺮ ﺗﮑﺮارﺗﺮﯾﻦ ﺑﺎري ﮐﻪ در ﭼﺮﺧﻪ ﻋﻤﺮ اﯾﻦ ﺳﺎزهﻫﺎ ﺗﺠﺮﺑﻪ ﻣﯽﺷﻮد، ﺑﺎرﻫﺎي ﻧﺎﺷﯽ از ﻓﺮآﯾﻨﺪ ﺑﺎرﮔﯿﺮي و ﺗﺨﻠﯿﻪ اﺳﺖ. ازﻃﺮﻓﯽ، ﻣﻌﻤﻮﻻً ﻓﺸﺎر ﺑﺰرگﺗﺮي ﻃﯽ ﻣﺮﺣﻠﻪ ﺗﺨﻠﯿﻪ در ﻗﯿﺎس ﺑﺎ ﻣﺮﺣﻠﻪ ﺑﺎرﮔﯿﺮي ﺑﺮ ﺟﺪاره ﺳﯿﻠﻮﻫﺎ وارد ﻣﯽﺷﻮد؛ از اﯾﻦ رو، اﺳﺎس ﻃﺮح اوﻟﯿﻪ ﺳﯿﻠﻮﻫﺎي ﻓﻮﻻدي ﺑﺮ ﭘﺎﯾﻪ ﻓﺸﺎرﻫﺎي ﺗﺨﻠﯿﻪ اﺳﺖ. اﯾﻦ درﺣﺎﻟﯽ اﺳﺖ ﮐﻪ ﺑﻨﺎﺑﺮ ﺿﺨﺎﻣﺖ اﻧﺪك ﺟﺪاره، ﺑﺤﺚ ﺧﺮاﺑﯽ ﮐﻤﺎﻧﺸﯽ در اﯾﻦ ﺳﺎزهﻫﺎ ﻣﻄﺮح اﺳﺖ. ﺗﺤﺖ ﻓﺸﺎر ﺗﺨﻠﯿﻪ ﻧﯿﺰ ﮐﻪ ﺑﺎ اﯾﺠﺎد ﻓﺸﺎر ﻣﺤﻮري و ﮐﺸﺶ ﭘﯿﺮاﻣﻮﻧﯽ ﻗﺎﺑﻞ ﺗﻮﺟﻪ در ﭘﺎي ﺳﯿﻠﻮﻫﺎ ﻫﻤﺮاه اﺳﺖ، ﻣُﺪ ﮐﻤﺎﻧﺶ اﻻﺳﺘﯿﮏ-ﭘﻼﺳﺘﯿﮏ ﭘﺎﻓﯿﻠﯽ ﻣﺘﺼﻮر اﺳﺖ. ﺿﺮﯾﺐ اﺻﻄﮑﺎك ﺟﺪاري ﺑﯿﻦ ﺳﯿﻠﻮ و ﻣﺤﺘﻮﯾﺎت داﻧﻪاي، ﭘﺎراﻣﺘﺮي ﺗﺄﺛﯿﺮﮔﺬار در ﺗﻌﯿﯿﻦ اﻧﺪازه و ﺗﻮزﯾﻊ ﻓﺸﺎر ﺗﺨﻠﯿﻪ در ﺳﯿﻠﻮﻫﺎ ﺑﻪ ﺣﺴﺎب ﻣﯽآﯾﺪ. ﺑﺮاي ﺗﻌﯿﯿﻦ ﭼﮕﻮﻧﮕﯽ اﺛﺮﮔﺬاري اﯾﻦ ﭘﺎراﻣﺘﺮ ﺑﺮ ﻧﺘﺎﯾﺞ ﻣﻘﺎوﻣﺖ ﮐﻤﺎﻧﺸﯽ ﺳﯿﻠﻮﻫﺎي ورق ﺻﺎف ﻓﻮﻻدي، ﺳﻪ ﻧﻤﻮﻧﻪ ﺳﯿﻠﻮي ﺟﺪاره ﻣﺘﻐﯿﺮ در اﯾﻦ ﭘﮋوﻫﺶ درﻧﻈﺮ ﮔﺮﻓﺘﻪ ﺷﺪ. ﻫﺮ ﯾﮏ از ﺳﯿﻠﻮﻫﺎ ﺗﺤﺖ ﺑﺎرﮔﺬاري ﺗﺨﻠﯿﻪ ﭘﯿﺸﻨﻬﺎدي آﯾﯿﻦﻧﺎﻣﻪ اروﭘﺎ ﻗﺮار ﮔﺮﻓﺘﻨﺪ. ﺑﺎ اﻧﺠﺎم ﺗﺤﻠﯿﻞﻫﺎي ﺳﻪﺑُﻌﺪي ﺧﻄﯽ و ﻏﯿﺮﺧﻄﯽ ﮐﻤﺎﻧﺸﯽ در ﻧﺮماﻓﺰار آﺑﺎﮐﻮس، ﻣﻘﺎوﻣﺖ و رﻓﺘﺎر ﮐﻤﺎﻧﺸﯽ اﯾﻦ ﺳﻪ ﻧﻤﻮﻧﻪ ﺳﯿﻠﻮ ﺑﻪ ازاي ﻣﻘﺎدﯾﺮ ﻣﺨﺘﻠﻒ ﺿﺮﯾﺐ اﺻﻄﮑﺎك ﺟﺪاري ﮐﻪ ﺑﯿﻦ 0/2 ﺗﺎ 0/6 ﺗﻐﯿﯿﺮ ﻣﯽﮐﻨﺪ، ارزﯾﺎﺑﯽ ﺷﺪ. ﻫﻤﭽﻨﯿﻦ ﮐﻔﺎﯾﺖ رواﺑﻂ آﯾﯿﻦﻧﺎﻣﻪ اروﭘﺎ در ﻃﺮح ﺗﻨﺸﯽ ﭘﻮﺳﺘﻪﻫﺎي اﺳﺘﻮاﻧﻪاي ﺗﺤﺖ ﻓﺸﺎر ﻣﺤﻮري ﺗﻮأم ﺑﺎ ﻓﺸﺎر داﺧﻠﯽ، ﻣﻮرد ﺳﻨﺠﺶ ﻗﺮار ﮔﺮﻓﺖ. ﺑﺎ ﺗﻮﺟﻪ ﺑﻪ ﻧﺘﺎﯾﺞ ﺣﺎﺻﻞ، ﺑﺎ وﺟﻮد ﺗﻮزﯾﻊ ﻓﺸﺎرﻫﺎي ﻣﺘﻔﺎوت ﻧﺎﺷﯽ از ﺗﻐﯿﯿﺮ در ﺿﺮﯾﺐ اﺻﻄﮑﺎك ﺟﺪاري، اﺛﺮ اﯾﻦ ﺿﺮﯾﺐ در ﻣﻘﺎوﻣﺖ ﮐﻤﺎﻧﺸﯽ ﺗﺤﺖ ﺑﺎر ﺗﺨﻠﯿﻪ ﻧﻤﻮﻧﻪ ﺳﯿﻠﻮﻫﺎي ﻣﻄﺎﻟﻌﻪ ﺷﺪه، ﮐﺎﻣﻼً اﻧﺪك ﺑﺎ ﺑﯿﺸﯿﻨﻪ ﺗﻔﺎوت ﺗﺎ 8% ارزﯾﺎﺑﯽ ﺷﺪ. ﺑﻪ ﻋﻼوه، ﺣﺪاﮐﺜﺮ ﺗﻔﺎوت در ﺗﺨﻤﯿﻦ ﻣﻘﺎوﻣﺖ ﭘﺎﻓﯿﻠﯽ ﻧﻤﻮﻧﻪ ﺳﯿﻠﻮﻫﺎ ﺑﺎ روش آﯾﯿﻦﻧﺎﻣﻪ ﻧﺴﺒﺖ ﺑﻪ روش تحليل كامپيوتري، محدود به 25 %و همواره در جهت ايمني محاسبه شد كه نشانگر عملكرد مناسب روابط آييننامه است
چكيده لاتين :
Thin-walled cylindrical steel silos are one of the major storage structures in most of industrial and agricultural sectors. There are different load cases that should be considered in design of silos, such as, filling and discharge loads, wind load, seismic load and thermal loads. Nevertheless, during the life cycle of a silo, filling and discharge of particulate solids exert the most frequent loads on the silo walls. Due to larger values of discharge pressures as compared with those of filling pressures, discharge loads are considered for structural design of silos. Considering small wall thickness of steel silos, they are susceptible to buckling failure. Under discharge pressures, high meridional (axial) compression and internal pressure form at the base of silos that can lead to elastic-plastic elephant’s foot buckling mode. Therefore, it is deemed as the main buckling failure mode under discharge loads of silos. The wall friction coefficient of silos mainly depends on the wall surface characteristic and type of the ensiled material. This coefficient is a key variable in determination of magnitude and distribution of discharge pressures. To assess the effect of this variable on buckling capacity of steel silos, three example silos with different aspect ratios were considered. Each silo was loaded by the concentric discharge pressures in accordance to Eurocode. Subsequently, 3D linear and non-linear buckling analyses (i.e., LBA and GMNA analyses, respectively) were performed for different amounts of wall friction coefficient that varied between 0.2 and 0.6. Considering the results obtained, LBA analyses predicted an elastic axial compression buckling mode in the upper edge of base strake, where there is a change in shell wall thickness. Also, an elastic-plastic elephant’s foot buckling mode at the base strake of each silo was predicted by the GMNA analyses. Moreover, the load-path curves of example silos extracted from the GMNA analyses showed a bifurcation buckling that was followed by a dramatic reduction in post-buckling resistance. This held true for all three silos and all different values of wall friction coefficient considered in this study. The discharge buckling resistances estimated by the LBA were up to three times larger than those predicted by the GMNA. Therefore, including non-linearity in discharge buckling assessment of silos is urgently required. The effect of wall friction coefficient on buckling capacities of steel silos was significant for the LBA analyses that governed by axial compression. However, the elephant’s foot buckling mode observed under discharge load is affected by the both axial and internal pressures. As a result, adopting more sophisticated analyzing procedure that includes geometrically and materially non-linearity in the calculations (i.e., GMNA analyses) showed quite marginal effects for this coefficient (with the maximum difference of 8% in buckling capacity). As an extra investigation, the Eurocode provisions on stress design of steel silos under meridional compression with coexistent of internal pressure have also been examined. Eurocode recommends a reduction in critical axial buckling stress due to accompanying internal pressure, in terms of the plastic pressurised imperfection reduction factor αpp. As compared with the finite element results, for all the cases considered in this paper, the critical axial membrane stress calculated with respect to the Eurocode provisions yielded satisfactory predictions.
