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It is well known, that buildings and their bearing structures are subject to ageing, including corrosion, deterioration, etc. When faults in bearing structure are detected, disposal-at-failure maintenance should be made. But before that, it is necessary to assess the rate of deterioration.The author suggests to use finite element method for calculation of the safety margin of reinforced concrete bearing structures, because the finite element method is widely used in engineering practice of structural design. In the process of engineering inspection of reinforced concrete structures all defects of the inspected structure should be clearly specified. The article suggests to create the FEM-Model of the inspected structure in view of the fact that this structure is defected. In order to achieve this effect, the stiffness matrix of some finite elements should be changed and the FEM-Model must be created of volumetric finite elements (the article speaks about eight-node parallelepiped elements).At first the FEM-Model will be created of eight-node parallelepiped elements with standard descriptions for the reinforced concrete; then finite elements in damage area must be changed. On the basis of integral estimation of the mode of deformation, deformation ratio will be calculated, which is essential for the description assignment of the changes in stiffness matrix. The formulation of the deformation ratio includes all the possible defects of structure through indexes, which must be analytically calculated depending on the concrete defect.The method described in the article is useful in the process of engineering inspection of the reinforced concrete structures. Using this method can sufficiently specify the safety margin of a defected structure and forecast the future operational integrity of this structure under the acting load.

In the article the finite elements were developed that allow to take into consideration the design features of structures and the specific deformation of reinforced concrete without complicating the design scheme. The results of flat structures calculation under different types of loading are presented.The flat finite elements, which are used today in most widespread software systems for the calculation of the majority of buildings and structures, have significant drawbacks due to the peculiarities of the finite element method computational model. The two major drawbacks are: first, stiffness characteristics are specified as for rectangular cross-section and, second, constant stiffness characteristics over the entire area of finite elements is presupposed.These drawbacks are particularly evident in the process of calculating reinforced concrete structures and they significantly complicate the support systems design of multi-storey buildings. The simplifications used by the calculators are dangerous, as it is practically impossible to evaluate the resulting inaccuracies.The calculation model of the finite element method can be represented as a collection of nodes connected in one system with the help of finite elements, which conditionally replace the corresponding parts of a structure.Elasticity theory problems are solved with the help of finite elements of the shells and spatial finite elements. Here the accuracy of the results increases with the increase in breakdown frequency, and one of the main criteria for evaluating the effectiveness of discrete models is their convergence: the worse is the convergence — the higher breakdown frequency is needed to achieve the required accuracy of homogeneous structures calculations.In order to consider the factors affecting the calculations accuracy without increasing the complexity of making the design scheme, it is advisable to arrange a more detailed structure discretization on the stage of developing computational model. This concept is implemented in the discrete-braced computational model, which supposes replacement of the structure sections by the discreet braces combined in nodal points. The main advantages of discrete-braced model are determined by the possibility of multilevel discretization of a structure, achieved in terms of geometrical dimensions and in terms of the direction of digital communication, components of the stressstrain state, stiffness characteristics of digital communications, variable along the longitudinal axis and changing layer by layer in cross section.The basic diagram of the discrete-braced model is: the calculated structure is conditionally replaced by a set of nodes located at the layout grid lines crossing and linked in pairs by the discrete braces, which limit the mutual displacement of the nodal points for all the considered degrees of freedom.The stiffness characteristics of braces are set independently for each brace and each type of deformation on the basis of geometrical and deformational characteristics of the construction sections replaced by braces.In order to determine these sections, conventional boundary lines are traced on the structure, that are located between the grid lines. It is believed that these lines demarcate the structure sections that influence the stiffness parameters of the neighboring connections of one direction. Thus each out-of-node structure point belongs simultaneously to two sections. Stress-strain state of the structure, stiffness characteristics of the braces along the X and Y axes are defined independently of one another. The distributed internal forces arising in front sections of the braces are brought to concentrated generalized forces transmitted through the nodes between the braces in both directions.In the general case, each node of the obtained flat system has six degrees of freedom — three linear and three angular. Generalized displacements inside connections are described by linear functions. Each connection resists six types of deformations — tension and compression, shear in plane of the structure, shear out of the plane, torsion, rotation (bending in plane) and bending out of the plane. In the process of braces deformation, the efforts relevant to deformations appear in them: axial force , two shear forces, torque and two bending moments, and the stress-strain states during deformation of braces in plane and out of plane of the structure are independent from one another.It is offered to determine stress-strain state of the obtained discrete braced-noded system using the method of shifts by means of composing and solving the system of 6n linear algebraic equations (n — the number of nodes ).The accuracy and convergence of the calculation results for discrete-braced model of structural homogeneous isotropic elements is not inferior, and in some cases exceeds the accuracy and convergence of the finite element method results. The use of discretebraced model provides additional opportunities, in particular for non-linear calculations of reinforced concrete structures, which can significantly simplify the numerical schemes used, and thus significantly reduce the calculation complexity.

Reinforced concrete is uninterruptedly developing progressive type of building materials. One of the most important advantages of reinforced concrete is the possibility of using it with reinforcing steel or composite materials of increased and high strength.As a result occurs substantial permanent growth in production, increase in strength and other service characteristics of steel rolling used for reinforcing concrete.Production and application of the modern types of reinforcement in our country started not long ago, much later, than in the USA and European countries. Until 1950 deformed reinforcement was not produced and used in our country; the production of hot-rolling reinforcement of the A400 (A-III) class started only in 1956.But already in 1960 the application of this reinforcement was 1.0 million tons a year, and in 1970 — 3.4 million tons a year.Up to the year 2012, the production and application of the deformed reinforcement of the classes A-400, A-500C and A-600C of all kinds exceeded 8.0 million tons. In order to ensure economic efficiency and competitive ability of national construction, the process of increasing the strength and workability of domestic reinforcing bar is continuously taking place. The results of this process in respect of the common untensioned reinforcement of reinforced concrete structures are discussed in the present article.We suggest to consider the mechanical and service characteristics of deformed reinforcement, which is manufactured according to the standards of our country GOST P 52544, classes A500C and B500C, GOST 5781, class A400, and Technical specifications 14-1-5596—2010, class Ан600С, grade 20Г2СФБA.For the comparative analysis we use the standard data for similar reinforcement established by EN 10080-2005 and Eurocode 2, as well as by standards ÖNORM B-420 of Austria, BC 4449/2005 of Great Britain, DIN 488 of Germany, A706M of the USA and G3142 of Japan.The standards of the above-mentioned countries slightly differ from the standardsEN 10080 and Eurocode 2, and from the Russian standards.We consider the statistical data of the real properties of hot-rolling, coldolling and thermo mechanically strengthened deformed reinforcement manufactured and certified according to GOST R 52544 and GOST 5781, produced in Russia, Byelorussia, Moldavia, Latvia, Poland, Turkey and Egypt.The fundamental difference of modern European standards from Russian standards and the standards of other countries considered in this article is that the requirements of EN 10080 and Eurocode 2 are unified for all reinforcement with the yield point of 400 to600 H/mm2 regardless of its production method.At the same time it is stated, that the actual properties of reinforcement of all groups according to EN 10080, differ essentially from those specified by this Standard and they better correspond to the Russian, Austrian and German standards.The Standard EN 10080 in the version of the year 2005 is inconvenient, because it does not determine technical classes. As a result, many European countries use their own, but not the European standards.Conclusion.The comparative analysis of our national and foreign standards of deformed rolled steel used for reinforcing concrete demonstrates that the physical and mechanical properties of the Russian and European reinforcing steel are almost the same, but for the following facts:Standard requirements established according to GOST 5781, GOST 10884 andGOST R 52544 are a bit higher than the standards of EN 10080;Reinforcement of the classes A400, A500 B500C and A600C, manufactured according to the Russian standards, can be used without recounting instead of reinforcement of the same strength classes according to EN 10080 and to the standards of other countries all over the world.