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Strengthening of existing structures is one of the most important issues in the field of structural engineering. Due to avoiding any interruption of service on a structure and economic issues, strengthening process usually occurs when a member is under service loads. On the other hand in the loaded steel columns, it is really difficult to weld plates after unloading the column from existing loads, Therefore one of the important issues being neglected in the redesign process of strengthened columns is the significant axial load existing in the column, caused by service loads before strengthening them. This paper aims at numerically investigating 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. Included in the result are 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 unstrengthened 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 unstrengthened column. Then results of this preliminary analysis are defined as a predefined field for the column of same retrofitted model and ultimate bearing capacity of the strengthened model is calculated using a modified Riks 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 reduction of ultimate bearing capacity for columns with different slenderness ratios and preload level. Results of this study may be utilized to increase the accuracy of redesigning process during in-service strengthening of steel box columns.

Keywords: Strengthening, Load Carrying Capacity, preload, Steel box column

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