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The aim of this work is to propose a technique to calculate the strength of short concrete-filled steel tube columns under the short-term action of a compressive load, based on the phenomenological approach and the theoretical positions of reinforced concrete mechanics. The main dependencies that allow the realization of the deformation calculation model in practice are considered. A distinctive feature of the proposed approach is the method of the multipoint construction of deformation diagrams for a concrete core and steel shell. In this case, two main factors are taken into account. First, the steel shell and the concrete core work under conditions of a complex stress state. Since the proposed dependencies to determine the strength and the ultimate relative strain of volumetrically compressed concrete are obtained phenomenologically, they are more versatile than the commonly used empirical formulas. In particular, they can be used for self-stressing, fine-grained and other types of concrete. Second, with a step-by-step increase in the relative deformation, the lateral pressure on a concrete core and a steel shell constantly change. Thus, the parametric points of the concrete and steel deformation diagrams also change at each step. This circumstance was not taken into account in earlier calculations. A comparison of the theoretical and experimental results indicates that the practical application of the developed calculation procedure gives a reliable and fairly stable estimate of the stress-strain state and the strength of concrete-filled steel tube columns.

This paper presents a new approach to recognize a long-term strength of concrete. We introduce into the calculation a new concept of energy integrity of the specific energy of W-maximum resistance to the destruction. In the formation and aging process the concrete is considered to be a non-conservative system. Strength development implies the increasing of the demanded specific work for fracture. The results of the calculations show that for critical reinforced concrete constructions operate at a significant level of long-term loading. The coefficient of the working conditions should be given as differentiating, taking into account the predicted concrete creep.

This work is dedicated to an experimental research of strength and resistance characteristics of axially loaded concrete filled steel tube elements of self-stressing concrete, including high-performance concrete. Labcrete specimens of circular section 112 mm in diameter and 1000 mm in length were used in experiments. The research shows that the use of self-stressing concrete increased the strength of the specimens by approximately 10%, and enhanced elastic behavior limit by 20-33%. In the course of testing self-stressing concrete specimens, registered longitudinal deformations were within 0.52-0.75%. Analysis of the obtained results indicates a significantly greater case effect in the pre-stressed specimens. This effect is somewhat less pronounced in high-performance concrete specimens, yet still prominent.