Keywords: reinforced concrete columns, strengthening, steel caging, finite element modelling, bending moment, axial force.

Steel caging is at the present time one of the most commonly used methods of strengthening reinforced columns (RC) columns. It consists of placing steel angle pieces at each corner of the column and joining them by a series of strips. Steel capitals are often welded to the ends of the columns in contact with the beam-column joint to improve the transmission of loads to the strengthening.

This paper describes a series of finite element (FE) models, carried out after a series of experimental tests, of an RC column strengthened using steel caging subjected to combined bending and axial loads.

The FE model was constructed using the ABAQUS software, taking into account: second-order geometric effects, the non-linear behaviour of concrete, steel reinforcement and steel caging, also the existence of an interface between the steel caging and concrete or mortar. The model was validated by comparing the results of the experimental tests with the numerical results. Both sets showed a good degree of fit.

The FE model was used to obtain the N-M diagram of two types of RC column strengthened with steel caging: one had capitals fixed to the ends (as in the case of the experimental tests) and the other with no connection between the strengthening and the beam-column joint. The analysis of these diagrams showed the capacity of the strengthening to improve column resistance. The steel caging was seen to increase both maximum axial load and maximum bending moment, especially when capitals are fitted to the ends of the column.

The FE model was also used for a parametric study with over 160 simulations of the influence of different parameters on the behaviour of the strengthened column, including: angle dimensions, yield stress of the steel used in the strengthening, number of strips, dimensions of the capitals, compressive strength of the concrete in the columns and the mechanical quantity of the column reinforcement. It was observed that in the specimens without capitals, variations in both the angles and yield stress of the steel strengthening had little effect on the resistance of the strengthened column. However, when capitals are attached, this resistance increases with the angle dimensions and with the yield stress of the steel strengthening. With regard to concrete compressive strength or the quantity of reinforcement used in the column, it was seen that the higher the value of these parameters, the higher the resistance of the strengthened column.

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