## Vol 15, No 1 (2019)

**Year:**2019**Articles:**8**URL:**http://journals.rudn.ru/structural-mechanics/issue/view/1167**DOI:**https://doi.org/10.22363/1815-5235-2019-15-1

###### Abstract

Aim of the research. The essence of the failure of the globally widespread theory of long-term resistance of reinforced concrete is defined and analyzed. Methods. This failure includes the following interconnected parts: 1) the set of ten basic fundamental properties of structural concrete is completely distorted (for example, instantaneous linear properties are Maxwell scheme); 2) mathematical rules are violated when recording the rates of elastic deformation and creep deformation, due to a misunderstanding of the Boltzmann principle (these violations distort the whole structure of the theory); 3) the rules of classical mechanics are violated, what is caused by substitution of fundamental properties of concrete with various “chain models” (for example, the principle of independence of action of forces, which is the fourth fundamental law of Galileo - Newton, is violated); 4) sections of the general “world theory of creep of reinforced concrete”, based on its algebraization, in their essence reject the fundamental law of natural science - Newton's second law: not only the inertial component is rejected, but also forces depending on speed (in this way the “world theory of creep of reinforced concrete” is degraded to the level of Aristotle’s mechanics); 5) unacceptably idealized creep theories and structural models that endow concrete with unrealizable properties, especially flagrant in zones of cracks, are incorporated in the normative calculations of structures; 6) solid design companies of the world show that concrete creep is not a scientific theory: this is a warning to designers. Results. The performed analysis is accompanied by necessary mathematical calculations and experimental estimates.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):3-24

###### Abstract

Introduction. Structural stability is an essential part of design process for steel structures and checking the overall stability is very important for the determination of the optimum steel beams section. Lateral torsional buckling (LTB) normally associated with beams subject to vertical loading, buckling out of the plane of the applied loads and it is a primary consideration in the design of steel structures, consequently it may reduce the load currying capacity. Methods. There are several national codes to verify the steel beam against LTB. All specifications have different approach for the treatment of LTB and this paper is concentrated on three different methods: America Institute of Steel Construction (AISC), Eurocode (EC) and Russian Code (SP). The attention is focused to the methods of developing LTB curves and their characteristics. Results. AISC specification identifies three regimes of buckling depending on the unbraced length of the member ( Lb ). However, EC and SP utilize a reduction factor (χ LT ) to treat lateral torsional buckling problem. In general, flexural capacities according to AISC are higher than those of EC and SP for non-compact sections.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):25-32

###### Abstract

Relevance. A new technical solution of bent-closed profiles relating to light steel thin-walled structures (LSTS), which have high technical and economic indicators and are widely used in industrial and civil construction, is presented. Aim of the research. The characteristics of thin-walled structures can be further enhanced by shaping the modification of profiles, combining in its composition the outlines of closed and open outlines. Methods. By means of developmental studies and optimization and design calculations for bent-closed profiles, their new technical solution was developed, the originality of which was confirmed by patent examination. Results. New curved closed profiles (CCP) consist of a tubular part of a single thickness and a double-thickness rib. For their manufacture without welded, bolted or riveted joints, the sheet blank is made along the entire length with serrated longitudinal edges, the teeth of which are arranged relative to each other in a staggered manner and mutually bent in grooves between themselves after the curved profile is closed along its edge. The bends of the gear fasteners increase the thickness of the collapse and provide an increase in the shear strength of the joints of thin-walled elements. With equal dimensions in height and width of the CCP, they are optimized by the criterion of uniform stability, which is the same from the plane and in the plane of the supporting structure. Additionally, a series of pentagonal, triangular and trapezoidal profiles are presen- ted, in which the lateral faces are inclined relative to the vertical at angles of 45 and 60 degrees.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):33-43

###### Abstract

Introduction. The behavior in the course of a time of geogrid material with random nonlinear viscoelastic characteristics under tension is analysed. Parameters of viscoelasticity are represented in form of Gaussian random vector. The components of this vector are taken from experimental data. Aim of the research. The objective of this research is the analysis of influence of different factors (value of applied load and the application of load in the form of random value instead of dead one, number of realizations, change of given level of strain) on providing of needed service life of geogrid material with given reliability level. Here reliability is interpreted as function of probability of non-failure. The first crossing of some given level by random strain is considered as a failure. The strain value corresponding to yield limit of geogrid material is accepted as the given level of longitudinal strain. Methods. The realizations of Gaussian random vector of viscoelastic parameters of material with given correlation matrix were imitated by means of linear transformation method. Results. It is demonstrated that longitudinal strain is Gaussian nonstationary random process which stochastic analysis can be made on base of 10 000 realizations. The dependencies on time of mathematical expectation and standard deviation of random longitudinal strain as well as function of probability of non-failure are found. Conclusion. It is shown that durability estimation found on base of the deterministic problem solution is overestimated in comparison with stochastic problem solution if the condition of given service life providing with some reliability level is set up.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):44-50

###### Abstract

Aim of the research to verify the proposed calculating apparatus and accumulate new experimental data on the complex resistance of reinforced concrete structures, experimental studies of such structures made of high-strength concrete of circular and circular cross-section were conducted at the testing base of the South-West State University. Method is experimental-theoretical. Results of experimental research the plots of the deflections and rotation angles, the dependency of deformations of concrete according to the testimony of the outlets of electrodesorption with respect to the calculated cross section 1-1. The main deformations of elongation and shortening of concrete were determined; the reinforcement was selected in such a way that in the stage preceding the destruction, it reached fluidity, so the stresses in the reinforcement are known. It is established that for reinforced concrete structures made of high-strength concrete of circular cross-section, as a rule, there is the development of two cracks, i.e. the round shape of the cross-section slightly reduces the concentration due to the structure of high-strength concrete. For the annular section there were several cracks, of which stands out the one on which the destruction occurs. On the steps preceding the destruction, this crack begins to prevail over the rest and has a maximum opening width. On the basis of experimental studies of reinforced concrete structures made of high-strength concrete of square and box sections, reliable data on the complex stress-strain state in the studied areas of resistance, such as: the values of the generalized load of cracking , and destruction ,, its level relative to the limit load; the distance between the cracks at different levels of cracking (up to the moment of destruction, as a rule, two or three levels are formed); crack widths at the level of the axis of the working armature, at a distance of two diameters from the axes of the armature and along the entire crack profile at various stages of loading, from which it follows that the crack opening at the level of the axis of reinforcement in 2-3 times less compared with the crack opening on the removal of 1.5-2 diameters of the working axis (longitudinal and transverse) reinforcement; the coordinates of the spatial formation of cracks; schematic drawings on tablets of education, development and opening of cracks of reinforced concrete constructions in torsion with bending. Thus, the experimental studies and the result provide an opportunity to test the developed computational model and its working hypotheses for assessing the resistance of reinforced concrete structures made of high-strength concrete in torsion with bending.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):51-61

###### Abstract

Relevance . Dynamic analysis of complex structures using numerical methods leads to the solution of the algebraic problem of eigenvalues and the corresponding eigenvectors of high orders. The solution of this problem for high order matrices is performed using reduction methods. One of the most effective methods is the method of sequential frequency-dynamic condensation, which allows partial consideration of the dynamic properties of the structure in the minor degrees of freedom. This allows for more accurate results compared to static condensation. Frequency-dynamic condensation is traditionally used to reduce frequency equations derived from the finite element method in the form of the displacement method or the force method. Methods. The authors have developed an algorithm for the frequency-dynamic condensation method for the frequency equation obtained on the basis of the FEM in the form of the classical mixed method. That allows to obtain not only the spectrum of the lower vibration frequencies, but also the corresponding vibration modes and the stress-strain state of the structure. Results . This article describes the algorithm and its practical implementation in the problem of dynamic analysis of a rectangular plate. The results of the numerical analysis of the problem are presented. An assessment of the accuracy of the method and recommendations for its use are given.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):62-68

###### Abstract

Aim of the research. Free axisymmetric fluctuation of a cylindrical orthotropic cover, the infinite length contacting to the infinite elastic medium and filled with liquid is investigated. Methods. At design of the thin-walled shell designs which are widely applied in aviation, the missile and space equipment and various fields of the industry, an important task is dynamic calculation of the intense deformed condition of these designs. At a research of dynamics of covers it is necessary to determine own frequencies and forms of small fluctuations, and frequencies from the lower part of a range are of the greatest interest. It is supposed that the rigidity of material of a cover is a little more than rigidity of material of the environment. The solution of the equations of movements of the environment is considered in two options. Results. The frequency equation is received. The analysis of frequency and a form of fluctuations of a cover is carried out. The schedule of dependence of frequency of own axisymmetric fluctuations of a system on wave formation in the longitudinal direction is constructed. By means of an asymptotic method the frequency equations of the ridge cylindrical covers filled with liquid are constructed, the approximate frequencies of the equation and simple settlement formulas allowing to find values of the minimum own frequencies of fluctuations of the considered system are received, the forced fluctuations of the supported cover filled with liquid are investigated and defined is amplitude frequency characteristics of the considered oscillatory processes.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):69-74

###### Abstract

Aim of the research. Investigation of fire danger of seismic expansion joints, which have recently become almost the main component of seismic protection of buildings and structures. Identify the degree of fire danger of the building structure. Develop preventive measures to ensure fire safety of buildings and structures where seismic expansion joints are used. The relevance of this problem lies in the fact that the destruction of deformation seismic joints in seismic conditions for the spread of combustion (usually secondary occur in earthquakes) from one compartment of the building to another (protected by partitions, fire compartments, locks). That is, combustion opens the possibility to bypass special protections put in its way, in order to avoid its spread. Methods. Antiseismic deformation joints have been studied from the point of view of fire safety. The work mainly deals with the expansion joints used in the installation of aluminum panels for the design of buildings and structures. The connection of fire danger of antiseismic joints combined with technological and operational errors of performers is shown. Tests of various seismic protection joints for fire danger have been carried out. Results. It is concluded that it is necessary to develop a special regulatory documentation that introduces specific rules for the execution of expansion joints, especially designed to improve the seismic safety of buildings and structures.

**Structural Mechanics of Engineering Constructions and Buildings**. 2019;15(1):75-80