Group Of Limit States Research Articles

MODELING OF A LIGHTWEIGHT FLAT FLOOR WITH VOIDS MADE OF PLASTIC BALLS

Formulation of the problem. Prefabricated monolithic construction in domestic and global practice in recent years has occupied a small share in the field of construction. A fairly large number of designs of prefabricated monolithic lightweight floors, as well as its individual structural elements, have been proposed. The use of lightweight floors in the construction of residential and public buildings can significantly reduce the consumption of materials and the dead weight of structures and increase the size of live loads and overlapping spans. Despite this, it cannot be said that effective design solutions have been found that can maximally satisfy the requirements of consumers, architects and builders. Based on this, a constructive solution for a lightweight prefabricated monolithic floor with voids made of plastic balls, quite effective from the point of view of construction and subsequent operation, was proposed and investigated. The proposed design solution for a lightweight prefabricated monolithic flat floor with voids made of plastic balls has less mass than solid floors and optimal use of material resources, namely the consumption of concrete and reinforcement. Therefore, research to determine the optimal design parameters of a flat lightweight floor with void formers made of plastic balls is relevant. The purpose of the article is to conduct numerous studies of the stress-strain state of the proposed design of a prefabricated monolithic floor with voids made of plastic balls. Conclusion. As a result of the research, it was established that it is rational to use hollow balls made from recycled materials as a material for making voids from balls in flat floors. Moreover, all studied floor options with spans of 6 m, 7 m, 8 m and void sizes made of balls with diameters of 180 mm, 315 mm and 500 mm meet the requirements of the first and second groups of limit states.

SELECTION OF OPTIMAL PARAMETERS OF A LIGHTWEIGHT FLAT FLOOR WITH VOIDS MADE OF PLASTIC BALLS

Formulation of the problem. Architectural and construction design of buildings using reinforced concrete has led to an increase in the number of objects with an individual space-planning structure, a rich variety of facade and volumetric solutions. Today the question of the effectiveness of using flat prefabricated monolithic lightweight floors in the construction of multi-storey buildings with individual space-planning and architectural solutions is relevant. In a market economy interest in the issue of rational use of resources, namely the cost of concrete and reinforcement in construction, has sharply increased, which is directly related to attracting investment. As it is known, when designing buildings from prefabricated monolithic reinforced concrete, in contrast to prefabricated concrete, there is greater freedom in adopting the space-planning parameters of the building, and there may also be simpler design solutions, in particular for floors. The constructive solution of floors depends on numerous requirements for the building as a whole and directly for the floor. One of the requirements that influences the attraction of investments is a small material-intensive floor covering with significant spans of buildings of various structural systems. The effective use of lightweight precast monolithic reinforced concrete floors with voids made of plastic balls in construction requires a comprehensive technical and economic analysis of this innovative solution for the precast monolithic reinforced concrete floor structure. The purpose of the article is to perform a technical and economic comparison of various design options for a flat lightweight prefabricated monolithic floor. The influence of floor parameters on the consumption of concrete and reinforcement is considered. Conclusion. As a result of the research, it was established that it is rational to use hollow balls made from recycled materials as a material for making voids from balls in flat floors. Moreover, all studied floor options with spans of 6 m, 7 m, 8 m and void sizes made of balls with diameters of 180 mm, 315 mm and 50 0mm meet the requirements of the first and second groups of limit states.

Обеспечение сейсмической безопасности строительных объектов при массовых промышленных взрывах с учетом предельных состояний первой группы

In certain situations, seismic safety during open-pit mine blasts can be ensured only on the condition of coordination of the explosion technology regulatory documents with the provisions of the construction field regulatory documents. The distinctive feature of the construction documents is the application of probabilistic approaches to load specification. The limit state method is used to calculate the structures. The probability of non-exceeding the design values of the main loads is adopted as not less than 0.98 for the calculations of the first group of limit states. The explosion technology regulatory documents take into account the second group limit states at which the operation of protected objects is possible but hampered, for example, as a result of the increased frequency of objects’ repairs. At the same time, the probability of minor damages is 0.9–0.95. The methodology to ensure the seismic safety of protected objects during open-pit mine blasts, based on regression analysis of the experimental data and considering the probabilistic nature of such effects, is presented. The methodology considers all features of explosion technology applied at a given enterprise, as well as the structure of the geological environment along the way of seismic wave propagation, and other factors. Despite many advantages, however, the probabilistic approaches limit the minimum number of experimental points required to build a regression line and the limits of confidence intervals. Therefore, the increase in the confidence level of peak particle velocity from 0.95 to 0.98 must be accompanied by the increase in the minimum number of experimental points used to build the regression from 80 to 200, which significantly increases the scope and cost of the required experimental studies. The article considers the possibility of an increase in the confidence level of peak particle velocity caused by open-pit mine blasts from 0.95 to 0.98 without increasing the volume of the experimental sample; the conditions under which it becomes possible are analyzed as well.

Bearing capacity of a steel frame of a multi-storey modular building with consideration of the rigidity of quick-assembled connections

The article is devoted to the analysis of calculation and design problems and features that arise when designing multi-storey modular buildings with a steel frame. The article describes the main features of the classification and trends in the construction of volumetric block buildings in Russia and the world.
 The object of the study is quick-assembly connections of compressed-bent load-bearing elements of frame modular systems.
 The main purpose of the study is to obtain reliable analytical and theoretical data to determine the characteristics of the rotational stiffness of module connection nodes and assess its influence on the stress-strain state of the frame and its elements.
 Within the work, the concept of an experimental modular building was developed using I-beam columns as part of the frame, as well as inter-module joints for their connection. Flange connections with high-strength bolts without controlled tightening are considered as the main design solution for the inter-module connections. The designed connections include inspection windows to access the hardware.
 A numerical study was performed to evaluate the bearing capacity and stiffness of the connections. The influence of the stiffness on the behavior of the frame in relation to two groups of limit states was assessed using the finite element method.
 The paper substantiates the relevance of using I-beam profiles as columns of the frame of a modular building, which is not a common solution in modern practice.
 The developed concept of quick-assembly inter-module flange connections makes it possible to ensure the safety of the building while reducing the time and labor costs for its installation.

Nonlinear deformations of the compacted soil base under the broadening of piles in punched holes

The peculiarity of piles in punched holes with broadening (PPH), performed according to the technology of foundations in rammed pits, is considered to be a high load-bearing capacity compared to traditional driven piles. At the same time, from the long-term practice of using the PPH, the actual excess of the calculated settlement is noted. This is due to a significant excess of pressure of the order of 1,000-3,000 kPa under the widening of the pile over the calculated resistance of the compacted soil layer. This makes it necessary to calculate such a foundation for the second group of limit states, taking into account the nonlinearity within the limits of the calculated allowable pressure. The article compares three methods of calculating settlement taking into account non-linearity: according to the tables of the standard, the methods of N.V. Ornatsky and M.V. Malyshev. It is noted that the methods do not always correspond to the actual work of the foundation in the ground, which can reduce the reliability of calculations. The authors proposed to modernize the existing methods of calculating settlement due to the peculiarities of the work of the PPH. In the calculation scheme of the PPH, to determine the settlement, it is necessary to take into account the soil layer with an increased modulus of deformation created during the punching of the well and the formation of broadening. Zones of marginal equilibrium develop with a margin within the boundaries of the compacted layer under broadening. It is proposed to take into account the nonlinear dependence of soil deformations on stresses within the specified boundaries, and to take into account the remaining deformations of the compressible thickness in a linear formulation. Taking into account the non-linearity in the determination of settlement leads to an increase in the reliability of the application of the piles in question, especially in weak clay water-saturated soils, and in relatively strong soils allows us to confirm the reliable use of PPH.

Improved precast reinforced concrete crossbar of reduced height. Results of full-scale loading tests

The object of the study is an improved design of a reinforced concrete floor crossbar as part of a prefabricated frame of a residential building. The crossbar has a reduced height with undercuts in the support zones, which in combination with modified prefabricated multi-hollow plates allows you to perform the lower surface of the overlap without protruding ribs. This solution allows you to use precast reinforced concrete with all its advantages and compete with monolithic reinforced concrete girderless frames. To ensure the strength of inclined sections to the action of transverse force and bending moment, cage using rigid inserts made of sheet steel are used as transverse reinforcement. The results of the tests showed that the developed crossbar meets the stated requirements for both the first and the second group of limit states (strength, stiffness, crack resistance). Transverse reinforcement in the form of cages with rigid inserts made of sheet steel provides the required bearing capacity of inclined sections. The test confirmed the reliable joint work of concrete and rigid inserts from the beginning of loading to the ultimate state of strength.

Метод продольной надвижки с плавучей опорой металлических комбинированных пролетных строений мостов

Purpose: To consider the necessity of applying the erection technology of metal combined superstructure in the form of a lattice truss with a triangular lattice and a rigid bottom chord with driving underneath single span bridges by the incremental launching method with the use of floating support. To show the significance and efficiency of this method. To give the sequence of the main installation operations. To draw a general scheme of launching. To outline the procedure for checking the superstructure at all stages of erection. To show the method of checking the floating support according to two groups of limit states. Methods: Comparison of incremental launching using a floating support with incremental launching using auxiliary supports, as well as construction of the span on falsework. Analysis of the stress values obtained during the inspection of the superstructure at all stages of erection, as well as of the analytical expressions of the floating support inspection methodology. Results: The advantages of the incremental launching with the use of floating support in comparison with other types of installation of metal combined superstructure are specified. The main operations and features of metal combined superstructure erection are outlined. The general scheme of installation of facilities is given on the example of construction of the highway bridge over the river Psezuapse, as well as comments on the verification of the metal combined superstructure are given. The basic formulas for checking the floating support are expressed. Practical significance: Features and advantages of this method of installation of metal combined superstructure will simplify the construction of single-span bridges with the superstructure of this kind, as well as increase the economic efficiency of such projects. The method of floating support testing can be recommended for practical use.

Influence analysis of the connective elements failures on the road bridges steel reinforced superstructures service reliability

The paper considers the influence issues of connective elements failures, in particular flexible pin stops, on the road bridges steel reinforced superstructures service reliability. The authors have formulated possible criteria for the superstructure failure both for the first and the second groups of limit states, and also developed scenarios of connective elements exclusion from the structures operation, that are modeling the failure of individual connective elements in the operation process. For five finite element models of road bridges steel reinforced superstructures a numerical experiment on modeling of the connective elements exclusion from the operation of the structure was carried out. The calculation results for each scenario and for the partial scenario realization levels were analyzed in terms of possible superstructures failures as a whole. Numerical experiment results have shown that at the exclusion of a separate small group of connective elements increases the maximum stresses in the over-supported beams sections by up to 3 %, and further development of the connective elements disconnection scenario can lead to an increase in stresses by 20 %. It is concluded that for existing steel reinforced superstructures the risk of superstructures failure under normal operating conditions due to disconnection of connective elements is low, even in the case of almost unlikely complete disconnection of all connective elements. Nevertheless, the exclusion from operation of connective elements may be more critical in cases of abnormal bridge structure operation or in cases of significantly lower engineering reserves of the structure in terms of strength and stiffness.

THE EFFECT OF HOLLOW STRUCTURE PARAMETER ON THE 3D-PRINTED WALL BEARING CAPACITY. EXPERIMENTAL MODEL

We present the results of experimental studies and modelling of the evaluation of the bearing capacity of hollow 3D-printed walls with the printed shell performing bearing functions. The bearing capacity of hollow 3D-printed walls was experimentally assessed depending on the ratio of the void areas and casting layers in the wall structure. It was established that in case of central loading, a 3D-printed wall with bearing casting layers can serve as a bearing wall similar to traditional types of masonry construction without filling voids with structural concrete and reinforcement. We established the value of strength reduction of hollow 3D-printed walls, which amounted to ~0.1 – 0.25 MPa per 1 % of the increased area of voids. The limit value of the hollow structure parameter was determined, which must not exceed K = 0.75 in order to ensure the bearing capacity of self-bearing and non-bearing 3D-printed walls. We obtained an experimental model of the relationship between the hollow structure parameter and the bearing capacity, which allowed predicting the bearing capacity of a 3D-printed wall under central loading. It was suggested to take into account the hollow structure parameter K when calculating the elements of unreinforced 3D-printed walls under central compression according to the first group of limit states.

To the Definition the Deflections of Flexible Non-centrally Compressed Reinforced Concrete Struts Strengthened in the Transverse Direction with Composite Materials

An analysis of the calculation methods found in the normative literature on strengthening reinforced concrete structures with composite materials arranged in the transverse direction by the method of wrapping structures showed that there is no calculation method for the second group of limit states.The Code of Rules 164.1325800.2014 does not consider options for strengthening flexible compressed reinforced concrete structures, with the arrangement of composite materials in the transverse direction, introducing restrictions to the dimensions of structures. However, our research suggests the opposite. In this paper, experimental values of deflections of reinforced concrete struts in the transverse direction by composite materials, characteristics and testing methods of prototypes are given. The results of the theoretical calculation of deflections are presented and, based on the comparison of experimental and theoretical values of deflections, an algorithm for calculating the second group of limit states was developed.

Calculation of Pre-Stressed Structures using High-strength reinforcement without concrete-to-steel bond

The article deals with the method of calculation of a pre-stressed beam for the first group of limit states without adhesion of wire-rope reinforcement with concrete (monostrand) in the curved position. To verify the plausibility of hypothesis, a numerical simulation was done in the software complex ABAQUS for stress-strain state with such beam until the destruction. The analysis of the received results is executed and the conclusion of the performance a structures of their kind.

Forecasting the Durability of Composite Materials Products under the Combined Action of Mechanical Loads and Chemically Aggressive Media

Structures made of composite materials with a complex structure during operation are subjected to the combined effects of mechanical loads and aggressive media, which initiate the accumulation of damage in the material. As a result, premature destruction of structures, buildings, structures are possible. To prevent emergencies, it is necessary to be able to assess the resource of structures at any time. The article analyzes the previously proposed methodology of mathematical modeling of degradation processes, calculation and prediction of durability, resource structures, based on the application of the fundamental laws of mechanics of deformable solids and physico-chemical kinetics. The mechanisms of degradation of composites, methods of experimental determination of the main degradation parameters, conditions for calculating the durability of two groups of limit states are considered.

Compilation of calculated combinations in according with EN 1990

The article addresses one of the main issues that arise in the planned development of a new version of domestic standards, which is aimed at closer convergence with the European normative document EN 1990 in terms of calculation methods. If these methods are not fully accepted, then at least the principled positions are agreed, if possible preserving the domestic tradition
 An overview of the approaches of European standards EN 1990 to the compilation of design combinations of effects on building structures is given. The information on the method of choosing the calculated combination of influences of different groups of limit states taking into account different types of representative values of variable influences is generalized. Discrepancies with the methods of normative documents in force on the territory of Ukraine (DBN) on the calculation of building structures are indicated. Attention is drawn to the variability of Eurocode rules for the construction of settlement connections in relation to the situations under consideration, such variability is absent in the DBN.
 A compromise approach is proposed, which combines the principles of EN 1990 with the accumulated domestic experience and allows to significantly reduce the growth of design loads. The examples show that the increase in design effort causes significantly less increase in the cost of building materials.

Parametric optimization of frequency oscillation minimum surface shell on a rectangular contour under thermal load

Providing all parameters according to two groups of limit states at the design stage is a multi–criteria task. Optimal design helps to perform such tasks. When the phenomenon of resonance occurs, it is necessary to change the frequency of forced oscillations of the load–bearing structures of various tanks and coatings in order to ensure sufficient strength and stability of the structure and the building in general. For this purpose, there is a separate type of problem in optimal design that allows us to change the forced frequencies of oscillations using the example of the shell of the minimum surface on a rectangular contour.
 In mathematical point of view, optimal design tasks are optimization tasks, finding the extremum of the target function (maximum or minimum). Methods of solving optimal design problems can be conditionally divided into two groups. One of them will include methods that are based on using necessary conditions for the extremum of the objective function. The second group includes methods: linear, convex and dynamic programming, random search methods. The application of modern methods of mathematical optimization requires powerful PCs with a large RAM, therefore, when choosing optimization, it is necessary to take into account the capabilities of computer equipment.
 The optimization algorithm for single–criteria parametric optimization is performed as follows: the objective function is the weight of the shell of the minimum surface on a rectangular contour, the design variables are the shell thickness from 1 to 50 mm, the constraints are presented with first forced oscillation frequency is 0,15 Hz.
 Using software complex Femap with Nastran and our own software, a single–criterion parametric optimization was made. The results of changing the objective function are reducing the weight of the shell by 2300 kg of C240 steel, which is equivalent to 10,3% without losing the strength and stability of the minimum surface shell on a rectangular contour. Using author's methodology and own software, it is possible to perform an effective optimization calculation for the minimum surface shell on a rectangular contour.

Limit state according to the condition of loss of stability of equilibrium forms

Objective. The purpose of the study is to determine the group of the limiting state according to the condition of loss of stability of the equilibrium form of structures. Method. The study is based on the provisions of the theory of stability of equilibrium states of building structures; branching theory of solutions of nonlinear equations; perturbation method; methods of catastrophe theory.Result. The results of the analysis of the post-critical behavior of structures based on the solution of the problem in higher approximations and from the fundamental provisions of the theory of catastrophes are generalized. It is proved that the study of the stability of equilibrium forms of structures using algebraic means and geometric images of the theory of catastrophes makes it possible to unambiguously determine the type of critical bifurcation points, predict the nature of the behavior of the structure, and determine the limit state group to which the state reached by the structure should be attributed. Conclusion. It seems necessary to rename the ordinal numbers of the types of critical points of bifurcations so that they coincide with the numbers of the groups of limit states corresponding to them.

Modeling of the deformation impact of the main structure framework on the stress and strain state of its individual parts

The paper presents the calculation of the metal frame of the stairwell under the action of static loads, which was performed using the Dlubal RFEM application software package. The computer modeling technique for the study of metal structures in this software environment was adapted in accordance with building regulations. The general stress calculation in software for the finite element analysis was performed and the selected sections were checked in the RF-STEEL EC3 calculation module for two groups of limit states. Various options for calculating a stairwell made of a metal frame are analyzed. The influence of deformations of the main framework of the structure on the stress-strain state of its individual part was evaluated using software.

DETAILS OF NORMAL SECTIONS STRENGTH CALCULATION OF FLEXIBLE REINFORCED CONCRETE STRUCTURES BY THE WOOD’S METHOD IN PC "LIRA SAPR"

Problem statement. The problem of realization of the calculation method of normal cross-sections strength of reinforced concrete constructions under flat bending, which is established in the current building codes of Ukraine, is considered. The main attention is paid to atypical and practically not considered calculation cases, typical for automated algorithms in the environment of SP "LIRA SAPR". The purpose of the article. Analysis of the feasibility of using the calculation method of current building codes with further development of recommendations, based on the specifics of computerized calculations. Methodology. Within the framework of the performed research, rectangular cross-sections of reinforced concrete structures with single and double reinforcement (provided a significant increase in the area of reinforcement of the compressed cross-sectional area) with variation of concrete classes, reinforcement coefficient and ratio of reinforcement areas were considered. The stress-strain diagrams of concrete and reinforcement are bilinear with characteristic values set for the first group of limit states. The character of change of cross-sections’ status diagrams "M - εc(1) " is investigated. Research results. It is found that for single-reinforced sections with decreasing reinforcement area there is a decrease of the value of deformation of the compressed fiber of concrete, which is used to find solutions for systems of nonlinear equilibrium equations of the deformation method. This leads to an increase of the execution time of calculations of the flat elements’ reinforcement by the Wood method. It is established that for sections with double reinforcement at relatively large values of the ratios of the reinforcement areas, the equilibrium of the section is at the maximum deformations of the compressed concrete fiber. Conclusions. An approach aimed at accelerating the calculation of sections with single reinforcement, which is based on the use of the relationship between the percentage (area) of reinforcement and the deformation of the most compressed fiber of the reinforced concrete element. Features of analytical algorithms for calculating the selected sections are taken into account by implementing this technique in the PC "LIRA SAPR", optimization and acceleration of automated algorithms for calculating reinforced concrete structures.

CHANGES IN THE CODE OF RULES
 FOR THE DESIGN OF STRUCTURES
 MADE OF CONCRETE WITH POLYMER
 COMPOSITE REINFORCEMENT

The article presents information about the
 changes made to the code of rules for the
 design of concrete structures reinforced with
 polymer composite reinforcement. New rules
 for the formation of relative deformations at
 the base point of a two-line diagram of concrete
 under axial tension, used to calculate
 reinforced concrete elements for the second
 group of limit states, are described. The rules
 for calculating re-reinforced structures of
 T-or I-beam cross-sections with a shelf in a
 compressed zone by the method of limiting
 forces are presented. The rules for taking into
 account the inelastic properties of concrete of
 the stretched zone when determining the elastic-
 plastic moment of resistance for the extreme
 stretched fibre of concrete with a rectangular
 cross-section and a T-shape with a shelf
 located in the compressed zone are clarified.

STRENGTHENING OF THE FOUNDATIONS OF RENOVATED BUILDINGS WITH INJECTION PILES

The paper describes the technique of strengthening shallow foundations of reconstructed buildings using injection piles. First, the constructive solution of the existing foundations, the structural scheme of the building, as well as the loads transferred to the building structures before and after its reconstruction are established. At the same time, an assessment of the soil conditions of the construction site of the reconstructed building is carried out; a bearing soil layer is revealed for deepening the lower ends of injection piles. Based on the data obtained, the loading of the base of the foundations of the reconstructed building is assessed and the need for their reinforcement (or further operation without reinforcement) is established. In the case of strengthening the foundations of the building, the method of transferring the additional load to the injection piles is selected. Then their bearing capacity and design loads allowed on the piles are substantiated. The construction of foundations is carried out, taking into account their reinforcement with injection piles, which are hereinafter called combined. Verificationcalculations of the base of the combined foundations are performed for the firstand second groups of limit states. In accordance with the regulatory documents, strength calculations of the main structural elements of foundations are carried out, which are necessary to ensure their full operation, taking into account the reinforcement. At the finalstage, working documentation is developed to strengthen the foundations of the reconstructed building. The stages of the design of strengthening the foundations of reconstructed buildings using injection piles presented in the work allow to properly and consistently organize the work of specialists.

ASSESSMENT OF FEM CALCULATION RESULTS ACCURACY WITH THE USE OF SYMMETRY PLANES IN GEOTECHNICAL PROBLEMS

To calculate different geotechnical objects (foundations of unique buildings, pile foundation grillages, slab foundations, embankments, etc.) we often need to simplify geometric model. Use of symmetry principles allows to simplify the model without reducing its detail. A relevant issue with the use of symmetry planes is the assessment of calculation results accuracy. The article considers the research of such an assessment for two traditional geotechnical problems: a shallow foundation slab and a shoring of deep excavation. Calculations have been performed with various options of symmetry planes usage: a full-scale model, using one symmetry plane, using two symmetry planes; differences in the ways of representing building structures: 2D and 3D continuum elements. Generating the finite element mesh, we took into account the fact, that the degree of mesh grinding significantly influence the accuracy of the calculation results. That’s why the finite element mesh was divided into two areas: the area of «interest» represented by small sized elements, and the «peripheral» area created by larger elements. We also considered the sequence of technological processes: the initial stage of the calculation is the creation of the initial stress state, the next stage is the creation of the structure (foundation slab, shoring of excavation), the final stage is the force application. The calculation results were evaluated according to the criteria related to the second group of limit states. The controlled parameters for the shallow foundation slab were: maximum and average settlement, relative difference of settlements; for the shoring of deep excavation - the maximum displacement of the enclosure structure. It was found out that to present the construction in the form of 2D-elements it is necessary to introduce additional boundary conditions. They prevent rotation in nodes located on the plane of symmetry. The authors make conclusions and recommendations for creating design models with the use of symmetry planes.