IMPROVING THE CALCULATION OF DEFLECTIONS OF FLEXIBLE REINFORCED CONCRETE ELEMENTS TAKING INTO ACCOUNT THE LONG-TERM EFFECT OF EXTERNAL LOAD

Abstract

The article analyzes the existing methods of determining deflections of bending elements taking into account long-term processes. It is noted that the main factor affecting the deflections of the elements over time is the creep of concrete. With prolonged action of loads due to concrete, creep of the compressed zone of concrete, the neutral line moves towards the stretched section zone, the stresses in the compressed concrete decrease, and the stresses in the reinforcement increase, while the deflections of the elements increase.

Based on the analysis of the results of calculation of deflections of flexural elements taking into account the duration of the load, it was established that the method of determining deflections according to Eurocode 2 significantly underestimates the influence of concrete creep, and the DSTU method overestimates it. This is explained by the fact that the creep characteristics of concrete are not presented in the Eurocode 2 formulas, and the duration of the load is taken into account by one constant coefficient, the value of which is not justified. In the DSTU method, the increase in the deflection of the elements is taken into account by multiplying the deflection from the short-term load by the creep coefficient of concrete, which does not correspond to the real state of change in the stress-strain state of the section over time.

The article proposes to improve the method of calculating deflections by introducing elements of creep deformations into the stressed-strained state of the cross-section, taking into account the completeness of the stress plot in the concrete of the compressed zone. This is due to the fact that the creep coefficient of concrete is normalized based on the results of long-term tests of concrete prisms under uniform compression, and during bending there is a case when the stresses are not distributed evenly along the cross-section height. The formula for determining the coefficient of completeness of the stress plot in concrete is proposed.

The method of determining the deflections of flexural reinforced concrete elements based on the deformation technique is given, which consists in building a matrix of data on the stress-strain state of the elements depending on the gradual increase in the deformations of the extreme concrete fiber of the compressed cross-sectional area over the entire possible range of external load.

Attention is drawn to the need to continue research on the effect of concrete creep on deflections of flexural reinforced concrete elements with long-term experimental tests of prototypes made of modern materials.