In this study, theoretical and experimental results are presented which were obtained during an investigation of the influence of the $P-{Delta}$ effect that was caused by the simultaneous changing of the axial load P of the column and the lateral displacement ${Delta}$ in the external beam-column joints. The increase or decrease of ${Delta}$ was simultaneous with the increase or decrease of the axial compression load P and caused an additional influence on the aseismic mechanical properties of the joint. A total of 12 reinforced concrete exterior beam-column subassemblies were examined. A new model, which predicts the beam-column joint ultimate shear strength, was used in order to predict the seismic behaviour of beam-column joints subjected to earthquake-type loading plus variable axial load and $P-{Delta}$ effect. Test data and analytical research demonstrated that axial load changes and $P-{Delta}$ effect during an earthquake cause significant deterioration in the earthquake-resistance of these structural elements. It was demonstrated that inclined bars in the joint region were effective for reducing the unfavourable impact of the $P-{Delta}$ effect and axial load changes in these structural elements.