ارزيابي پارامترهاي مؤثر بر ظرفيت باربري ستون هاي فولادي جعبه اي مقاوم سازي شده تحت بار

Evaluation of Effective Parameters on Axial Load Bearing Capacity of Preloaded Strengthened Steel Box Columns

مقاوم سازي سازه هاي موجود يكي از مسائل مهم در حوزه مهندسي عمران به شمار مي آيد. در بيشتر موارد مقاوم سازي زماني صورت مي گيرد كه سازه در حال بهره برداري بوده و عضو تحت بارهاي سرويس قرار دارد؛ بنابراين يكي از نكات مهم در بحث مقاوم سازي كه غالباً از آن چشم پوشي مي شود، نيروي محوري قابل توجه موجود در ستون، قبل از مقاوم سازي آن مي باشد. در اين مقاله با در نظر گرفتن نيروي محوري در ستون با مقطع مربعي و مقاوم سازي آن با ورق هاي تقويتي فولادي، ميزان تاثير پارامترهايي نظير انحناي ستون در زمان مقاوم سازي، مقدار پيش بار، نسبت لاغري و نسبت سطح مقطع ورق هاي تقويتي به ستون پايه بر حداكثر ظرفيت باربري اين نوع ستون ها مورد بررسي قرارگرفته است. از آنچا كه هدف اصلي در اين پژوهش تعيين ميزان تغييرات ظرفيت باربري نسبت به مقادير آيين نامه مي باشد، حداكثر ظرفيت باربري مدل هاي بدون پيش بار منطبق بر نتايج حاصل از آيين نامه در نظر گرفته شده و انحناي اوليه مناسب براي هر نمونه محاسبه شده است. بر اساس نتايج بدست آمده اعمال پيش بار به ستون تقويت نشده سبب افزايش انحناي موجود و به تبع آن كاهش حداكثر ظرفيت ستون تقويت شده مي شود. همچنين در يك پيش بار ثابت حداكثر ميزان كاهش در ظرفيت باربري نمونه ها در لاغري هاي ميانه اتفاق افتاده و با افزايش مقدار پيش بار، تاثير ميزان تغييرات در نسبت لاغري بر كاهش ظرفيت باربري نمونه ها افزايش مي يابد. در انتها نيز يك رابطه كاربردي جهت محاسبه ميزان كاهش در ظرفيت باربري اين نوع ستون ها بر اساس نسبت لاغري و ميزان پيش بار، پيشنهاد شده است.

Strengthening existing structures is one of the most important issues in the field of structural engineering. For the purpose of avoiding any interruption in serviceability of a structure and economic issues, strengthening process is usually performed when a member is under service loads. On the other hand, it is very difficult to weld plates after unloading a loaded steel column. Therefore one of the important issues in strengthening steel column is the presence of significant axial load caused by service loads. This is usually neglected in the redesign process of retrofitted columns. This paper aims at numerical investigation of the behavior and ultimate load bearing capacity of in-service strengthened steel box columns with continuous welded plates. Effects of different parameters on the capacity of preloaded strengthened columns are presented and discussed. Results are discussed by the effects of initial imperfection; magnitude of preload before strengthening; slenderness ratio of the strengthened column and ratio between cross sectional area of reinforcing plate and un-strengthened column. To investigate the effect of these parameters, each un-retrofitted specimen is exposed to the preloading levels of 0.0, 0.2, 0.4 and 0.6 of the load carrying capacity of un-strengthened column. Then results of this preliminary analysis are defined as a predefined field for the column of the same retrofitted model. Ultimate bearing capacity of the strengthened model is calculated using a modified Risk analysis method. The critical load carrying capacity of models without pre-existing axial load was set to the theoretical value presented in ANSI/AISC 360-10 and suitable imperfection for each model was calculated. This is because the main objective of this study is the variation of results with respect to the existing design curves. Based on the results of numerical analysis, application of preload to unstrengthened column magnifies the initial geometric imperfection of the column and consequently decreases the ultimate bearing capacity of strengthened column. Also as the magnitude of axial load existing prior to addition of reinforcing plates increases, the ultimate bearing capacity of the strengthened column decreases with respect to the calculated theoretical value. The maximum amount of this reduction for the preload ratios of 0.2, 0.4 and 0.6, is respectively up to 2%, 5% and 9.5% of the load-bearing capacity of strengthened column. As another result, slenderness ratio is one of the main parameters that affect the bearing capacity of specimens with a specified preload level. This means that at a constant preload level the maximum reduction in bearing capacity occurs for models with median slenderness ratio. Also models with cross sectional ratio of reinforcing plates ranging from 0.4 to 1.0 were studied and it was shown that inside this range, the cross sectional ratio of reinforcing plates parameter does not have remarkable effect on the ultimate bearing capacity of column. At the end, an empirical relation is proposed to calculate the reduction of ultimate bearing capacity for columns with different slenderness ratios and preload levels. Results of this study may be utilized to increase the accuracy of redesigning process during in-service strengthening of steel box columns.