The research objective is focused in developing a strength analytical method of a thinwalled steel square pipe (Square Hollow Section/SHS) affected by the interaction of concentratedcompressive load and bending moment. This strength analytical method is based on two different approaches, namely plastic mechanisms and elastic theories. This is called the method of "cut-off strength". In this research, it has also been carried out to test the strength of the investigated beam under the interaction of concentrated-compressive load and bending moment. In order to essess the accuracy of the analytical method developed, estimate data of this method is also verified by comparing it to the actual one measured from experiments. The verification indicated that the estimated data, on average, deviates from the experimental one by 5%.