Analysis of the functional state of flanged joints

. This article discusses related aspects of the formation and provision of the functional quality of the load-bearing elements of frame systems. The subject of the study is the analysis of the state of steel structures with the possible manifestation of violations of the shape of structural elements in flanged joints. The formation and provision of the functional qualities of metal structures and their connections is carried out at all main stages of the life cycle. The scientific hypothesis consists in the assumption that the analysis of the parameters of the stress-strain state in metal structures, characterized by the presence of deviations from the established indicators of functional quality, contributes to an accurate assessment of the parameters of the state and service life. The conditions and circumstances characterizing the possibility of loss of functional qualities of flanged joints of steel structures are determined. A block diagram of the analysis of risks associated with manifestations of the shrinkage distortion of flanges, which reduce the operational reliability of steel structures, has been developed. It is advisable to supplement the methodological basis for the development of design solutions for steel structure units with a forecast (modeling) of the state and operational suitability of a flange connection in the presence of shrinkage distortion of flanges. The results of the analysis of possible states of loss of bearing capacity using a predictive model will contribute to the development of rational parameters of labor intensity and material consumption of manufacturing flanges of steel structures.


Introduction
Characterization of possible deviations from the functional quality indicators established for flanges of steel bearing elements.
The arrangement of building structures from structural elements (shipment-sized set of details) is one of the most defining technical and technological features of steel structures. Architectural (construction) systems made of steel structural elements make it possible to develop and implement rational design solutions for standard and unique construction objects of various functional and technological purposes [1][2][3].
Rational organization of the interaction of individual structural elements (shipmentsized set of details) of different functional purposes as part of an architectural system is carried out by means of butt joints or nodes of steel structures. Nowadays, welded and bolted (using flanges and high-strength bolts) joints of steel structure assemblies have become widespread (Figure 1). a) bolted (using flanges) b) welded Flanged joints are characterized by a relatively high manufacturability and efficiency of the organization of processes for the construction of steel structural elements with their use, simplicity and accessibility of methods for controlling the final joint [4][5][6].
A flanged joint can be considered as a systemic formation, including a number of structural elements (flanges, connected steel elements adjacent to the flanges, bolts, welds), the joint work of which ensures the functional quality of the joint.
The functional efficiency of flanged joints of steel structures (individual structural elements) is determined by the quality of design and construction solutions, technical and technological excellence in manufacturing, transportation, installation, reliability and serviceability indicators. Accordingly, partial or complete loss of functional quality (as a result of the manifestation of deviations in state parameters) of flanged joints is a consequence of the effects of certain types of unfavorable factors [6][7][8]. Figure 2 shows the main types of factors that determine the functional quality of flanged joints in steel structures. The main forms of manifestation of the factors that determine the functional effectiveness of the structural elements of flanged joints (see Figure 2) can be identified through the following main categories of deviations from the originally established design solution and/or operational state [6,9,10]: defect -deviation of the functional quality indicators of the steel structure (including the structural elements of the flanged joint) from the design values that occurred during the design, manufacture, transportation and erection (installation); damage -deviation of the functional quality indicators of the steel structure (including the structural elements of the flanged joint assembly) from the design values that occurred during operation.
The presence of defects characterizes the initial (unsafe for subsequent operation) state of the structural element of the flanged joint, which is the source of the formation and/or development of damage processes and material fatigue phenomena and can lead to accidental consequences. The formation and development of damage depends on the design of the flanged joint, the duration and operating conditions, as well as the intensity and magnitude of industrial impacts.
The formation and subsequent development of defects and damage in combination with natural physical aging (fatigue) of the material (steel) are signs of (destructive failure) degradation processes and the main factors that lead to a decrease in functional efficiency and serviceability of both structural elements of a flanged joint and constructive system as a whole [10,11].
Welded seams constitute the main technological feature of the interaction of structural elements as part of the nodal connection. The arrangement of welded joints in the vast majority of cases leads to the formation of violations (residual deformations) of the geometric shapes of structural elements, which are called the shrinkage distortion of a flange ( Figure 3) [12]. Accordingly, the presence of the flange shrinkage distortion is considered as a significant deviation (in the form of a defect) from the established functional quality of the flanged joint, which can lead to significant difficulties during the subsequent installation and operation of the structural system [6,10,13].
At the moment, a fairly large number of scientists have studied the behavior of flanged joints. Among them are the works of V.V. Kalenov, [13] V.M. Gorpinchenko, A.G. Soskin, O.I. Ganiz, V.B Glauberman and others.
Based on the research of the above scientists, recommendations were developed for the calculation, design, manufacture and installation of flanged joints of steel building structures [6] and chapter 27 of the manual for the design of steel structures (to SNiP II-23-81*) [14]. The manual and recommendations do not apply to the following flanged joints: -perceiving alternating loads, as well as repeatedly acting movable, vibrational or other types of loads with a number of cycles over 105 with a stress asymmetry coefficient in the connected elements r = (smin/smax)³ * 0.8; -operated in a highly aggressive environment. These joints include flanged joints of crane beams. Crane beams can be attributed to the elements of an open profile.
-operated taking into account defects and damage to structures.

Materials and methods
Risk assessment of shrinkage distortion manifestations at various stages of the formation of the functional quality of flanged joints of steel structures. The risks associated with the manifestations of defects, including shrinkage distortion of flanges, are capable of displaying the allowed deviations of the established values (indicators) of the functional quality of structural elements and are capable of forming the prerequisites for: • increase in the cost of the building structure "in action"; • partial or complete non-compliance of the parameters with the initially established (design) functions and parameters; • increased costs to maintain the required level of serviceability. Figure 4 shows a generalized block diagram of a system engineering analysis of hazards and negative consequences of manifestations of defects in structural elements of flanged joints, including the risks of shrinkage distortion of flanges [7,14,15]. A systematic approach to assessing the risks of hazards and the consequences of manifestations of negative deviations of the parameters of steel structures from the initial values of the functional quality seems to be a rational methodological basis for developing measures aimed at improving the reliability and safety of using flanged joints [16,17].
The most obvious and effective design and technological solutions to ensure the functional quality of flanged joints (and, accordingly, minimize the risk of quality loss) include: • increase in thickness of flanges; • use of material (steel) with guaranteed strength in relation to the specified rolled thickness; • performance of special technological operations to eliminate the appearance of flange shrinkage distortion based on the results of welding seams.
Defects and damage to steel structural elements (including shrinkage distortion of flanges) made during their manufacture at construction industry enterprises are not always possible to detect and eliminate in a timely manner under the conditions of a construction site [7,18,19].
The presence of a defect or damage to the butt joint may require a labor-intensive and costly set of measures aimed at eliminating the identified deviation.
Shrinkage distortion of flanges are usually referred to such categories of defects and damage, which constitute a significant danger to the functioning of load-bearing steel structures [20][21][22].
In the total number of causes of manifestations of flange shrinkage distortion during the operation of steel structures, the following are most often distinguished: • violations of the established rules and regulations for the technical operation of building structures and production (technological) processes; • application of power (mechanical), temperature loads, impacts not provided for by the design conditions for the operation of load-bearing steel structures; • carrying out activities (during repairs and reconstruction) associated with a change in the original (after manufacture and erection) structural and spatial integrity; • changes in the original design scheme for the operation of load-bearing structural elements of steel structures; • violations of design and/or installation parameters, deterioration of the quality of welded joints, presence of cracks and damage to bolts and places where flanges adjoin bolts; • violations of design parameters, quality deterioration, presence of cracks, absence of bolts.
During operation, periodic analysis of the compliance with the actual indicators of functional quality and serviceability of load-bearing steel structural elements (including flanged joints of assemblies) is provided. It is generally accepted that the amount of material resources allocated to parry defects and damage that occurred at this and previous stages of the life cycle (including errors and inaccuracies made during the design and construction) determines the conditions for ensuring the indicators of the functional efficiency of load-bearing steel structures.
Along with this, it can be noted that a satisfactory technical condition is a positive characteristic of the functional quality of construction products, but not a sign of unambiguous rationality and adequacy of design solutions for flanged joints.
When developing an engineering calculation method, the approach proposed by Vladimir Viktorovich Kalenov was used, that any connection of open profile elements in a constructive form can be divided into an order of elementary T-shaped flanged joints. Numerical analysis carried out by Kalenov V.V. showed that taking into account the compliance of the base in the calculation has little effect on the stress-strain state of the flanged joint.
But when modeling the features of the operation of a flanged joint, taking into account shrinkage distortion, it is impossible not to take into account the compliance of the base.
Therefore, it is proposed to consider the T-shaped flanged joint as a beam on an elastic foundation.
Due to the fact that the flange thickness tp is always significantly greater than the beam web thickness ts, in the first approximation, we can assume that the beam web, when   Below, we will assume that the flange wall rotates around the point C, as if it rested on an elastic foundation (see Fig. 6

) with conditional stiffness λ(x):
Where p is the pressure created by the elastic base; ay = φxvertical displacement; In the general case, all quantities included in relation (2) can be variables in the x coordinate. For definiteness, we can require that the differential moment from the elementary area bzdx does not depend on the x-coordinate: (4) Which gives Then Moment М0 along the length wz at point С on the forces of elasticity of the base will be equal to: (5) We equate Мс=М0 and get the following expression (6) where coefficient of elastic pinching, which characterizes the decrease in the bearing capacity of the flanged joint, taking into account its shrinkage distortion, depending on the angle, forming the deflection of the structure tensile forces, .bolt tensile strength. Thus, the stress-strain state of an open-section flange joint that perceives tensile forces, taking into account the shrinkage distortion of the flanges, depends on the elastic pinching coefficient, the thickness of the flange and the thickness of the beam wall, and the angle of deflection of the structure.
The results of numerical calculations for varying these values are presented in Table 1. During the numerical experiment, the Ansys 2020 R2 computer system was used. The main hypotheses adopted by the formation of the calculation scheme: 1) All bolts have the same preload 2) The bolts of the outer zone are divided into elementary T-shaped jonts.
3) The calculation was performed in a non-linear formulation. 4) The contact plane of symmetry is set using an elastic foundation. 5) Geometric characteristics of the first section: flange thickness 20mm, wall thickness 7mm, bolt diameter 24mm (Bo = 250kN) 6) The applied force T is 100kN.

Research results
Methods for determining the parameters of the functional quality of steel structures in the presence or forecast of manifestations of the shrinkage distortion of flanged joints.
The term "forecast" implies a probabilistic assumption about the state and indicators of the functional quality of the research object (for example, a flanged joint of load-bearing steel structures, with identified or expected deviations from the initial values) as of a certain perspective point in time [23,24].
The result of the application of the scientific method of forecasting is the quantitative indicators of the hazards of the manifestations of possible deviations from the design characteristics of the functional quality of structural elements and systems, obtained in the conditions of partial or complete absence of information about the object of research, features of manufacture, installation and operating conditions.
Depending on the qualitative and quantitative composition of the information support of the research object, forecasting methods are divided into types: • extrapolation of results obtained for similar facilities or operating conditions; • expert assessments from experts in this field; • prediction of the service life using a wide variety of input data under conditions of partial or complete uncertainty.
The analysis of the parameters of the technical condition and serviceability, carried out within the framework of predicting the effect of flange shrinkage distortion on the parameters of the stress-strain state of the structural elements of the butt joint, can be carried out using physical (experimental) and non-physical (mathematical) models [25][26][27][28][29].
The result of the application of experimental models is the indicators of the stress-strain state of the structural elements of the flanged joints, obtained in the presence of a sufficient amount of information about the object of study and/or conditions of its operational state. It is the adequacy of the experimental model-analogue of the expected operating conditions on the test bench that determines the reliability and limits of the application of the promising method of experimental research. A feature of the experimental models is the limitation of the possibility of direct use of research results for the design of new types of structural elements of flanged joints, materials and technologies for the manufacture of steel structures.
The result of the application of mathematical models is the indicators of the stress-strain state of the structural elements of the flanged joint, obtained in the conditions of partial or complete absence of information about the object of study and/or the expected conditions of its operational state. A feature of mathematical (predictive) models is the possibility of preventive analysis of the deterioration of operating conditions using a wide range of data and/or hypotheses for the formation of parameters of the stress-strain state of structural elements of a flanged joint under conditions of partial or complete uncertainty (Figure 7).
The reverse statement of the problem is also valid: determination of the required parameters of the object of study (geometrical and physical-mechanical characteristics of flanges) for known or established by the design assignment operating conditions, as well as the presence of defects or damage to the structural elements of the flanged joint (including flange shrinkage distortion) [5, 12.21].
The scope of the method of modeling the parameters of the stress-strain state is a complete list of tasks, the solution of which is necessary when analyzing the issues of design, manufacture and operation of flanged joints of load-bearing steel structures. The use of predictive (computational) and experimental methods for studying random processes of shrinkage distortion seems to be a rational approach for optimizing the practical methods of formation and ensuring the functional quality, reliability conditions and operational efficiency of steel building structures. dependent events; independent events.

Conclusion and discussion
Solving the issues of designing, manufacturing and evaluating the operational efficiency of structural elements of flanged joints remains an urgent multifactorial task of ensuring the functional quality of steel structures.
The current practice of solving the problems of ensuring the functional quality of flange joints, focused on a preventive increase in the thickness of the flanges and the production of technological operations to eliminate residual welded deformations (shrinkage distortion) of the flanges, obviously leads to an increase in the material consumption and labor intensity of the manufacture and installation of construction products.
The "technological" solution to the problem of ensuring functional quality in the manufacture of flanges is not a guarantee of the complete elimination of the dangers associated with the manifestation of shrinkage distortion during the construction and subsequent operation of steel structures.
Within the framework of this study, the parameters of the operational suitability of a flange were determined from the condition of the presence of the main forms of flange shrinkage distortion, and the dependence of the shape of the initial deflection of the structure on the criterion for reducing the bearing capacity of the flanged joint was found.
According to the analysis of formulas (4), (5), the destruction of the flange with shrinkage distortion has an avalanche-like character.