In design of seismic resistant reinforced concrete constructions it is necessary to take into account the influence of plastic strains on stress-strain state and on possibility of appearance of open cracks in compressed area of concrete while the sign change of the load. In order to evaluate the influence of this factor the design of reinforced concrete beams models with regard to elastoplastic diagrams of materials has been carried out. The dependences of residual cracks height, of bearing capacity and of limit value of coefficient of plasticity in the second half-cycle on the maximum coefficient of plasticity in the first half-cycle have been built. The conclusion has been made about the decrease of bearing capacity of bending elements, caused by the development of plastic strains and due to them the presence of open cracks in compressed area of concrete. The limit state criterion has been offered, preventing brittle fracture on following cycles of loading.
One of the most important issues when calculating buildings and structures for seismic effects is taking into account the presence of damage in the compressed zone of concrete. It is known that the current norms of the Russian Federation [4] on earthquake-resistant construction assume the development of plastic deformations in structural elements. When determining the loads, the possibility of developing plastic deformations in structural elements is taken into account by introducing a decreasing coefficient K1, while neglecting the effect of plastic deformations on the strength of bearing reinforced concrete elements. The presence of plastic deformations in the reinforcement leads to the appearance of residual cracks in the compressed zone of concrete and, as a consequence, to a decrease in the bearing capacity of bending elements in subsequent loading cycles. The influence of unclosed cracks in compressed zone of concrete of bending elements on their bearing capacity for symmetrical and non-symmetrical reinforcement, for different values of reinforcement coefficient and for different values of coefficient of plasticity is considered. Based on the results of calculations the bearing capacity data for bending reinforced concrete elements with residual cracks in compressed zone of concrete have been obtained. The approximate method of determination of residual cracks depth in compressed zone of concrete in terms of coefficient of plasticity in the first semi cycle of loading is proposed.
A significant part of the territory of Russian Federation refers to seismically dangerous areas. In the current code in design of reinforced concrete buildings for seismic loads the development of plastic strains is supposed. They are taken into account while determining loads by introduction of reducing factor K1, but their influence on the strength of elements is neglected. Plastic strains of reinforcement lead to appearance of residual cracks in compressed zone of concrete and due to this the reduction of bearing capacity of bending elements takes place on following loading cycles. The approximate method of determination of the depth of open cracks and of the residual height of sections when changing the sign of internal forces after reaching the maximum deflection has been proposed. The depth of residual crack is determined from the condition of equilibrium of longitudinal forces with regard to stress-strain state of a section at three stages of loading: at a moment of achievement of maximum plastic strains, at the unloading stage and at a moment of the beginning of crack formation after changing the sign of internal forces. The comparison of results obtained by the approximate method and results of calculation of finite elements model of a beam has been carried out.
The Russian standards for earthquake resistant constructions suppose the development of plastic strains in reinforced concrete constructions under seismic effects. Their presence influences significantly on the stress-strain state, bearing capacity and mechanism of destruction of reinforced concrete constructions under alternation loads of high intensity. The methodology and main results of experimental tests connected with the evaluation of plastic deformations influence on the behavior of reinforced concrete bending elements when changing the force sign is represented. The information about the bearing capacity reduction when changing the force sign with the increase of plastic strains in the first semi cycle of loading is given. The coefficients of plasticity for reinforcement deformations limit values are obtained, corresponding to the beginning of compressed zone concrete destruction when changing the force sign and the plots of normal sections deformations distribution of experimental specimens under direct and reverse loading have been built.
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