Strengthening is a type of retrofit works, in which a material is added to a base cross-section in order to transfer additional load. The use of welded plates to strengthen steel members is a common practice because it is fast and cheap. Unloading the structure before strengthening may often be almost unfeasible or economically inconvenient, in which case strengthening under load is carried out (for case study examples see [1–3]). Although there are several rules of thumb, current normative documents do not refer to this problem and no reliable analytical solution has been developed to date . In most of the experiments detailed in literature (e.g., [5–7]), the load resistance of columns strengthened under load was very similar to that of reference columns strengthened without preloading. However, there is no consensus among researchers. Some (Brown , Ricker , Spal , and Unterweger ) believe that load resistance is decreased by preloading, others (Tall , O’Sullivan , Tide , Wu and Grondin , and Bhowmick and Grondin ) claim that well designed and conducted column strengthening under load does not decrease its load resistance.
The paper presents experimental and numerical research into the strengthening of columns under load using welded plates. Experimental research of other investigators (e.g., [6, 7, 16]) was focused on flexural buckling of columns. Also other works of authors investigated flexural buckling of wide flange columns strengthened under load with intermittent welds . All the experimental research into flexural buckling showed that the preload had low effect on the load resistance of the column strengthened under load. Therefore, columns susceptible to local and torsional-flexural buckling were selected. Additionally, the theoretical axis of loading did not pass the column centre of gravity which caused a bending moment. The experiments were conducted at the laboratory of the Institute of Metal and Timber Structures at Brno University of Technology in April 2015. The purpose of this paper and the presented experiments is to contribute to the existing body of knowledge concerning strengthening under load that aims to make retrofit design both safe and economical.
Three sets of columns were selected for the research. The experimental sets of columns comprise three columns each. Set (D) includes columns labelled D1, D2 and D3, set (E) columns E1, E2 and E3, and set (F) columns F1, F2 and F3. Numerical models of columns are labelled D, E and F. The columns were subjected to a compressive force (see Figure 1). The axis of loading passing through axis
did not pass the centre of gravity (intersection of axes and ) which caused additional bending moment. The cross-sections are presented in Figure 2. All columns were welded with continuous welds; the throat thickness of which was 4 mm. Set (D) included columns with a T-shaped cross-section welded from the flange with the dimensions 140 × 8.1 mm and from the web with the dimensions 200 × 5.4 mm. The columns from set (D) were class 4 according to the classification of cross-sections in EN 1993-1-1 . Set (E) comprised columns with a monosymmetric I-shaped cross-section. The flanges had the dimensions 140 × 8.1 mm and 80 × 7.9 mm, and the web was 200 × 5.4 mm in size. Sets (D) and (E) were necessary for the comparison of behaviour and resistance. Set (F) contained columns with a T-shaped cross-section with the same dimensions as the columns in set (D). The columns from set (F) were first loaded to 70 kN and then strengthened under this load with a second flange. The preload magnitude was chosen as the half of the average experimental load resistance of set (D). The preload ratio (ratio of the preload magnitude and the base section load resistance) 0.5 is considered in Czech technical recommendations  as the maximum limit value for strengthening under load. The resulting cross-section was the same as that of the columns from set (E). The columns from sets (E) and (F) were class 3.