Keywords: truss structure, optimum design, finite element analysis.

This paper concerns the renewal of trussed columns for overhead lines for power transmission. The collapse of middle voltage electric columns is due to the reduction of the cross-section area near the foundation. The reason for the cross sectional reduction is the corrosion. In this study, the load carrying capacity of columns is assessed and studied. Two models were used for the columns: truss structures and solid beams. By the use of calculations we observed that the columns had a reserve load carrying capacity of approximately 20%.

If the reduction of the cross sectional area is smaller than 20%, it is not necessary to fasten the legs of the column. It is only necessary to ensure the prevention of corrosion. If the reduction of the cross section is greater, it is necessary to fasten the foot. While the trussed columns are being renewed one foot of the four loses the load carrying capacity. In this case a supporting system has to be built. At the same time several columns will be renewed this is why several supporting systems are necessary. The main target is the minimum weight (volume) of the supporting system.

To calculate the internal bar forces a finite element analysis has to be carried out. First a truss and beam element is selected to determine the load carrying capacity. If we use a supporting system we connect the bars of the supporting systems to the columns nodes. The cross section of the bars are of square hollow section (SHS). The dimension of the SHS is determined by optimal design. The objective function is the cross sectional area of the bars and the restrictions are the flexural buckling and the local buckling. After the minimum cross section areas of the bars have been obtained, we make a new optimal design. In this case, the vertical displacement of the supporting system is limited. The limitation of the vertical displacement is important during the reconstruction. We made a new calculation using the optimal values.

Finally there are two solutions for the renewal of column tags. One of them is applied when the reduction of the cross section is smaller. The second one is applied when using a supporting system. This research provides new information concerning the determination of the load carrying capacity of trussed columns. When a supporting structure is necessary, it is important to calculate the cross sections which fulfil the constraints and provide for minimum weight of the supporting system.

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