**6. Conclusions**

A new technique to determine the strength of compressed CFST was proposed. Based on the known principles of deformation calculation, it takes into account the specific features of CFST adequately. The methodology uses new dependencies to determine the strength and the ultimate deformation of a concrete core, as well as the way of concrete deformation diagram development. It allows to perform the combined calculation of CFST strength, taking into account their flexibility and the calculation of possible stability loss. There is no need for an empirical formula to determine the critical force proposed by modern design standards for composite structural steel structures in the practical application of the method.

The versatility of this method should be emphasized separately. The method is acceptable for CFST columns made of various types of concrete using various technologies.

The practical use of the proposed method gives a reliable estimate of the stress–strain state and the strength of concrete filled steel tube columns.
