**1. Introduction**

Concrete filled steel tubes columns (CFST) are composite structures. They feature a variety of advantages. CFST have significant constructive, technological, economic advantages and at the same time an architecturally expressive appearance [1–5]. Such obvious CFST advantages as decreased labor consumption of their production due to lack of forms and reinforcement cages and high speed of building erection are quite attractive for construction specialists. Besides, mechanical features of a steel shell and a concrete core combine quite rationally in these columns. The strong steel shell serves as a reliable frame for the concrete core ensuring good volumetric load conditions for it. Due to this, concrete strength of columns with circular cross-section increases 1.8÷2.5 times in average. Concrete, in its turn, protects the walls of the steel shell from loss of stability and corrosion from inside. As a result, concrete and steel mutually increase load-carrying ability of each other and that of the whole element.

In case of emergency (explosions, earthquakes, etc.), another important feature of such columns, high survivability, comes to the fore. It is ensured by high deformability of the concrete core, which, together with its high strength, ensures absorption of
