*Bearing Capacity of Concrete Filled Steel Tube Columns DOI: http://dx.doi.org/10.5772/intechopen.99650*

the first case the value of the bearing capacity turned out to be underestimated, and in the second case - overestimated.

The proposed criterion for achieving the bearing capacity of CFST columns is important for practical calculations. The use of this criterion makes it possible to identify the cases when the strength properties of a concrete core cannot be fully used. Calculation by the method of limiting efforts does not always reflect the physical essence of the process and can lead to significant errors.

From the point of view of modern concepts of solid mechanics, steel-reinforced concrete structures refer to nonlinear and non-equilibrium deformable systems. The feature of such system calculation is the need to refine the values of the existing forces and displacements consistently, since the internal forces and the rigidity of the structures are interdependent.

The proposed method of CFST load capacity calculation allows to take into account these features. Considering flexibility the higher stiffness of the compressed rod is taken into account at the sites located closer to its supports. In this regard, it is obvious that the correct implementation of this method in practice will allow to obtain more reliable calculation results in comparison with the currently used semi-empirical approach.

Besides, this method makes it possible to perform the calculations of normal cross section and stability strength from a unified point of view. During the calculation, it is possible to track (in terms of longitudinal deformation value) the completeness of concrete and steel strength property use. If the material deformations reach the maximum permissible values, it can be concluded that the strength of the structures is lost. If this is not observed in the loss of the load-bearing capacity of the structure, a conclusion can be made about the loss of stability of the second kind.

It is especially important, that the proposed method with an appropriate refinement can be used for calculating the compressed structures made of various constructional materials.

One more important circumstance should be noted. It is known that in CFST columns, even before the onset of complete loss of bearing capacity, axial deformations can reach excessively large values at which the operation of real structures becomes impossible. In these cases, the limiting deformation can become dominant, determining ULS. In this regard, during the calculation of bearing capacity the axial deformations of the compressed CFST elements should be limited. This approach can be implemented only when calculating with the use of a nonlinear deformation model of reinforced concrete.

The proposed method can be effectively used to calculate long-term load columns [25].
