Add to ORKGA model for analysing the nonlinear behaviour of steel concrete composite beams prestressed by slipping cables, taking into account the deformability of the interface shear connection, is proposed. By assuming an admissible displacement field for the composite beam, the balance condition is obtained by the virtual work principle. The solution is numerically achieved by approximating the unknown displacement functions as series of known shape functions according to the Ritz method. The non-linear problem is then solved by the Newton-Raphson method. The model is applied to real cases by showing the consequences of a weak connection in the global behaviour (ultimate carrying load capacity) and in the local behaviour near the anchorages