This paper presents the results of a study of pliable flange connections in steel frame. Calculation of the load carrying capacity and ductility of flange connections is an important design stage and requires consideration of various factors such as analysis of mechanical properties of materials, geometry, operating conditions and type of loads. The main objective of structural design is to ensure structural safety while minimizing cost. For this purpose, it is necessary to use calculation schemes that correspond to the real physical operation of structures. In solving this problem, the following were used: full-scale experiments, finite element method and analytical method of limit equilibrium (LEM), which is traditionally used in structural fracture mechanics. The influence of bending stiffness of the flange on the process of joint failure is investigated. A new design solution of a pliable beam-column assembly is proposed, which is characterized by simplicity of calculation and assembly.

The actual problem of resistance of near-reinforcement zone of concrete is solved as a problem of volumetric stress-strain state with "closure" of output integral parameters of this zone on the rod scheme of the whole reinforced concrete element synthesizing hypotheses and dependencies of various disciplines of mechanics of solid deformation body, including fracture mechanics. The calculation model of the reinforced concrete element takes into account the effect of reinforced concrete of prof. Vl.I. Kolchunov describing the mechanism of formation and development of transverse and longitudinal cracks. In this case, generalized hypotheses of linear and angular deformations for warping and gradients of jumps of relative mutual displacements of reinforcement and concrete are adopted. New functionals of reinforced concrete are constructed, which are consistent with the ideas about resistance of crosssections of rod elements in near-reinforcement zones. Physical equations for a concrete matrix modeled between transverse cracks are written.

The displacement components for the near-reinforcement area are found in relation to the crack opening width at the boundary of the "concrete-reinforcement" contact in transverse, longitudinal and radial cracks, respectively. The use of the accepted assumptions and multi-level calculation scheme for the near-reinforcement region significantly brings the calculation model closer to a real assessment of physical phenomena.

This article is a generalization of known solutions to the problem of determining the reduced geometric stiffness of sections of elastic prismatic beams using geometric arguments: the shape coefficient and the ratio of conformal radii (internal to external). Analytical dependences are constructed for all considered sections (regular polygons, ellipses, rectangles, isosceles and right triangles): reduced geometric stiffness is the coefficient of shape and reduced geometric stiffness is the ratio of conformal radii. Formulas in the form of polynomials for determining the ratio of conformal radii are also constructed for these sections. An analysis of the changes in both arguments under various geometric transformations showed that the ratio of conformal radii has similar isoperimetric properties as the shape coefficient. This allows to determine the geometric torsional stiffness of the sections by interpolation.

An improved design of concrete filled steel tube element of rectangular cross-section subjected to bending has been proposed, which has greater strength and requires significantly greater energy consumption for destruction compared to known analogues. To test the effectiveness of the proposed design, experimental studies were carried out on the strength of normal sections and the beam rigidity of concrete filled steel tube beams under four-point bending. Research has shown that by simultaneously strengthening the compressed and tensile zones, it was possible to increase the strength of normal sections of beams. The increase in strength of beams of the improved design averaged 42%. The rigidity of non-reinforced concrete filled steel tube beams turned out to be significantly higher compared to beams without filling the steel pipe with concrete. In the improved beams, the stiffness was still approximately 20% higher. The results of comparing the calculated strength of concrete filled steel tube beams using the limit force method with experimental data indicate their satisfactory agreement.

This paper discusses the applicability of a nonlinear deformation model for determining the parameters of the stress-strain state of eccentrically compressed concrete-filled steel tube elements. Adjustments to the deformation diagrams of the concrete core and the steel tube were introduced to account for the complex stress state of the materials. The cross-section of the concretefilled steel tube element is considered as a collection of elementary areas. As the criterion for calculating compressive resistance at the ultimate strength stage, it is proposed to use the maximum force calculated based on the compatibility condition of steel and concrete deformations. The advantage of this criterion is that it eliminates the need to normalize the ultimate compressibility of concrete and accounts for the high degree of force redistribution in cross-sections. The method was verified by comparing the calculated and experimentally obtained ultimate forces on a sample of experimental data. The applicability of the nonlinear deformation model based on deformation diagrams, considering the multiaxial stress state of concrete and the steel tube, was confirmed for the analysis of eccentrically compressed concrete-filled steel tube elements.

This study examines the relationship between the maximum deflection and natural frequency of vibration in a three-layer cross-laminated timber (CLT) board at variable thicknesses of the inner and transverse plank layers under different boundary conditions. The finite element method (FEM), implemented in the SCAD++ computer complex by creating a finite element computational model in the form of a composite plate with anisotropic properties of the layers and rigid bonds between the plates, was used for the investigations. As a result, the values of maximum deflections and natural vibration frequencies for plates with different cross-sectional parameters were obtained. The obtained results are compared with the analytical values of the fundamental dependence obtained by Professor V.I. Korobko describing the relationship between the maximum deflection of a composite plate and the frequency of natural vibrations. It was found that the deviation of the numerical values of the K coefficient does not exceed 3% under different boundary conditions of structures and parameters of their cross-section.

A mathematical model of the dynamic process excited in a statically loaded beam-base system by a sudden change in the beam's bending rigidity is constructed. It is assumed that either the beam material's elastic modulus or the beam's cross-sectional axial moment of inertia changes when it is rotated by 90 degrees relative to the beam's longitudinal axis while maintaining the load direction. Forced vibrations are investigated by decomposing the load and static deflection of the initial beam into rows according to the beam's natural vibration modes with changed parameters. Natural frequencies and corresponding displacement and bending moment modes are determined by the initial parameter method using a vector-matrix representation of the states of arbitrary beam sections. Numerical results are provided to demonstrate the capabilities of the approach.

Each building and structure are a complex technical system with predetermined technical parameters that must be monitored throughout the entire life cycle. The purpose of the study is to develop a methodology for quantitatively assessing the technical condition of load-bearing systems of buildings to improve their structural safety and operational suitability. The dependence of the actual wear of a building on the period of natural oscillations is determined based on the theory of predicting the risk of an accident, which allows determining an increase in the period of natural oscillations of load-bearing systems of buildings to assess the category of their technical condition. The limits of increase of the period of natural vibrations of bearing systems of buildings are defined. They allow quantitatively estimate the category of the technical condition (0-4% - normative technical condition, from 5-10% - serviceable, from 11-49% - limited serviceable, 50-95% and higher - emergency).

The speed of global changes in the field of economics and politics, in the social and spiritual spheres is the main factor that requires consideration and a special approach not only in judging and stating, but also for anticipating and predicting events. Globalization and possible risks of environmental and economic disasters require prompt solutions.  The dynamics of interaction between the developer, the owner and the designer is increasing. The sustainability of an object's development is determined not by its static nature, but by its ability to change parameters, dimensions, functions, etc. In conditions of change, the construction site must also change. Strategic economic development programs aimed at the transition to a closed-loop economy indicate that only 8.6% of the produced product is returned to reuse. This leads to an increase in the workload not only in the field of extraction and processing of resources, but also to an increase in labor costs. The globality of the environmental and economic catastrophe formed in the future requires prompt solutions. Thus, further transformation and adaptation of buildings becomes a mandatory requirement during their design and operation. The lack of the possibility of rapid changes in the life cycle of an object leads to the appearance of abandoned buildings and territories and to an increase in the proportion of unfinished construction. The growing number of such facilities dictates the need, firstly, to create conditions for an operational change of functional purpose, and secondly, to eliminate the imbalance between the growing demand for new areas and the availability of unused ones. One of the ways to solve this problem is the temporary return of unfinished spaces to operation, before the implementation of the main measures for the development of abandoned territories. As a result of the conducted research, three main problems of the appearance of unfinished construction have been identified, firstly, a significant difference between the life cycles of the functions of objects and the needs for these functions, secondly, the mutual influence of the environment on objects and objects on the environment, thirdly, the lack of long-term forecasting. Solutions for each of the identified problems are proposed.

Taking into account the constantly growing requirements for the quality, reliability and durability of concrete structures for hydraulic structures, there is a need to develop hydraulic concrete compositions with improved performance properties. The aim of the study is to improve the physico-mechanical and deformative properties of hydraulic concrete using finely dispersed aluminosilicate rocks – perlites and colloidal additives in the form of silicic acid sol. The object of the study is modified hydraulic engineering concrete based on a composite binder using finely dispersed vitreous and crystallized perlite, silicic acid sol and a polycarboxylate-based superplasticizer «Polyplast». Results of the study: The choice of silica-containing additives is substantiated and it is shown that their use has a positive effect on the properties of hydraulic concrete. The effect of complex modification on the properties of hydraulic concrete was established by reducing the content of Portland cement and replacing it with finely dispersed glassy perlite, pre-crushed to a specific surface of 300-600 m2/kg, introducing silicic acid sol and the superplasticizer "Polyplast", which make it possible to improve the physical, mechanical, deformation and hydrophysical properties: compressive strength - 53.4 MPa; ultimate tensile strength in bending - 10.9 MPa; crack resistance coefficient - 0.20; prismatic strength - 46.3 MPa; modulus of elasticity 37.345 MPa × 103; Poisson's ratio - 0.199, water absorption - by weight - 2.43%; water resistance grade - W16. It has been proven that by adding finely dispersed perlite, silica sol and superplasticizer Polyplast to Portland cement, it is possible to obtain hydraulic concrete characterized by strength indicators not inferior in strength to the control composition, and with increased indicators of water resistance and crack resistance compared to traditional compositions.