The paper presents experimental and numerical research into the strengthening of steel columns under load using welded plates. So far, the experimental research in this field has been limited mostly to flexural buckling of columns and the preload had low effect on the column load resistance. This paper focuses on the local buckling and torsional-flexural buckling of columns. Three sets of three columns each were tested. Two sets corresponding to the base section (D) and strengthened section (E) were tested without preloading and were used for comparison. Columns from set (F) were first preloaded to the load corresponding to the half of the load resistance of the base section (D). Then the columns were strengthened and after they cooled, they were loaded to failure. The columns strengthened under load (F) had similar average resistance as the columns welded without preloading (E), meaning the preload affects even members susceptible to local buckling and torsional-flexural buckling only slightly. This is the same behaviour as of the tested columns from previous research into flexural buckling. The study includes results gained from finite element models of the problem created in ANSYS software. The results obtained from the experiments and numerical simulations were compared.

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 [

The paper presents experimental and numerical research into the strengthening of columns under load using welded plates. Experimental research of other investigators (e.g., [

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

Column specimen strengthened under load in the test set-up.

Column sets with dimensions, axes, strain gauge (SG) positions, and draw wire sensors (DWS) positions.

Buckling resistance of investigated beam-columns was determined using Cl. 6.3.4 general method for lateral and lateral torsional buckling of structural components in Eurocode 1993-1-1 [

Each set comprised three columns (labelled 1, 2, and 3) selected for use in experimental research. All columns were 3 m long and loaded by loading cylinder. The load was transferred through knife-edge bearings (see Figure

Knife-edge boundary conditions.

Axial force and column length changes during the spot welding (orange background colour), welding process (red colour), and cooling (blue colour).

Tension coupon tests were conducted to determine steel mechanical properties. Three coupons were machined from webs, three from longer flanges, and three from shorter flanges. The precise thickness of the plates was measured with callipers at several spots and the average value was used for numerical analysis. 1-LY11/6/350 strain gauges (SG) were used (see Figure

Numerical simulations were performed to complement the values which had not been measured in the experiment. ANSYS software [

The column strengthened under load F was modelled using the following procedure. First, the ideal geometry was created. Pinned boundary conditions around the axis

For the purpose of comparison, the models of columns D and E were also created using a similar procedure. The geometry of columns D and E updated the same imperfections as column F. The same program and similar procedure were used by Liu and Gannon [

The effect of residual stress was investigated in greater detail because it is the source of unknown variables. The temperature load that was determined to correspond to the measured deformations of specimens (throat thickness 4 mm, area of the weld ^{2}; see Figures ^{2}) and 4.6 mm (area of the weld ^{2}) for temperature load 33% lower and 33% higher, respectively. A little difference in throat thickness leads to a relatively big difference in the area of the weld. Other variables are the travel speed of the welding arc, number and length of pauses, and so forth. These variables are impossible to account for in field welding. The results of this parametric study are described further.

Stress distribution in longitudinal direction

The results of mechanical properties were averaged. Young’s modulus of elasticity, yield strength, and ultimate strength were 210 GPa,

Stresses recalculated from strain gauges and deflections obtained from draw wire sensors (DWS) at the moment of collapse are plotted in Figure

Experimental results at the moment of collapse: load resistance, stresses recalculated from strain gauges [MPa], and deflections.

Local buckling of column D2 (before collapse occurred) and torsional-flexural buckling of columns E3 and F3 (after collapse).

Graphs of deflections in the middle of the column in the direction of principal axes

Residual stresses in the longitudinal direction

The welding process caused deformation of the flanges. The weld caused shrinkage and the flanges inclined by approximately 3° in the direction towards the web. The average stress on the flange of the base T-shaped cross-section (the average of SG1 and SG2) at 70 kN of load was only −13.4 MPa (compression). The stress after the welding process even switched into tension, to an average value of 13.9 MPa (tension). The stress on the web of the base T-shaped cross-section (SG3) was −85.9 MPa, which is still a low value compared to the yield stress. Unfortunately, the welding process near SG3_T destroyed the strain gauge but the weld clearly caused shrinkage. The specimens from set (F) were deflected at midheight by welding by an average value of −8.63 mm in the direction of axis

The average load resistance of set (D) was 157 kN, which is much larger value than the one calculated from EC 3 [

The unloading and loading load-displacement curves of columns F1 and F2 followed the same path and the load resistances of both specimens were the same as of column F3. Therefore, it is presumed the unloading to 10 kN did not affect the column behaviour. From the graphs in Figure

The goal was to achieve similar distortion and stress distribution as described in Section

Residual stresses caused by welding are shown in Figure

Longitudinal stresses on strain gauges during loading, comparison of numerical model and experimental results.

Stresses in the longitudinal direction

The results of the parametric study concerning the temperature load causing residual stress distribution are summarised in Table

Results of parametric study: load resistance

Column | Temperature load | | | |
---|---|---|---|---|

D | 0.67 × exp | 134 | 6.3 | |

exp | 138 | 8.5 | ||

1.33 × exp | 140 | 10.3 | ||

| ||||

E | 0.67 × exp | 420 | 6.3 | −7.2 |

exp | 399 | 8.5 | −9.9 | |

1.33 × exp | 387 | 10.3 | −12.4 | |

| ||||

F | 0.67 × exp | 405 | 6.3 | −7.4 |

exp | 383 | 8.5 | −10.5 | |

1.33 × exp | 374 | 10.3 | −13.4 |

The experiments on slender columns susceptible to local and global buckling proved that the method of strengthening T-shaped columns with a second flange under preload magnitude up to 50% of the base section’s buckling resistance is safe and feasible. These six experiments support the claim that the load resistance of a column strengthened under load is nearly the same as that of a column welded without preloading. The numerical simulation was managed to determine values that had not been measured in the experiment. The following conclusions can be drawn from the experiments and research involving numerical models:

The force changes rapidly during the shielded metal arc welding process with thick electrode core wire. Temperature changes must be carefully monitored and stress changes considered especially if the column is less stiff than surrounding structural components. Use of thin electrode core wire or intermittent welds could reduce these changes.

The residual stresses and deformations caused by welding are often much higher than the stresses caused by preloading. Especially in cases with slender base sections, the stresses during axial loading rise slowly with increasing load but plummet at the most stressed fibres in the cross-section when the load on the column is nearing its critical load. Therefore, with preload magnitudes up to 50% of the base section’s load resistance, the buckling of the base section (both local and global) hardly affects the load resistance of the strengthened section and strengthening plates can prevent further local buckling.

Welding causes shrinkage and tensional residual stresses near the weld. If the strengthening method is designed so that the position of the welds is near the most compressed fibres or causes deformations which have a positive influence, the welds can increase buckling resistance.

Similarly, if possible, strengthening plates should be positioned outside the base cross-section in order to increase the moment of inertia. In the case of columns strengthened under load, these plates reach the yield point later, thus increasing buckling resistance.

The validated numerical model will find application in the behavioural analysis of members strengthened under load in future research in this area. Such research is currently in progress, and other experiments have already been planned to verify the above-mentioned conclusions.

The authors declare that there is no conflict of interests regarding the publication of this paper.

The financial support of Project no. LO1408 “AdMaS UP-Advanced Materials, Structures and Technologies,” supported by the Ministry of Education, Youth and Sports under “National Sustainability Programme I” is gratefully acknowledged.