About the choice of the test method to clarify the actual working scheme of the beam

. This article discusses the problem of determining the actual scheme of a precast reinforced concrete frame of a building when performing engineering surveys. The method of connecting precast reinforced concrete structures in industrial buildings with heavy equipment plays an important role in assessing the stress state of building structures. It is not always possible to perform the opening of nodes in the conditions of existing production, since this requires stopping technological processes or dismantling equipment. The article discusses the possibilities of static and dynamic tests for the study of support units in precast reinforced concrete structures during engineering surveys. The possibilities of static and dynamic test methods are considered, as well as the advantages and disadvantages of these methods when evaluating support units in precast reinforced concrete structures of buildings and structures during surveys. As an example for the study, the case of determining the current structural scheme of a precast reinforced concrete floor using dynamic tests is considered. The article analyzes previous studies on the use of the dynamic method in determining the actual scheme of existing reinforced concrete structures of an industrial building. The results of theoretical studies and their comparison with experimental results are presented. The general conclusions based on the results of the study are formulated, and recommendations are given on the choice of a method of testing structures for cases of establishing a structural scheme of precast reinforced concrete crossbars in operated industrial buildings during survey work.


Introduction
When operating the erected building structures of any capital building, the main condition is to ensure its reliability.This condition ensures that the risks of an emergency situation are reduced.The main role in this task is assigned to specialists who perform survey work of existing building structures of buildings and structures [1].These specialists are called upon to assess the reliability of building structures of functioning buildings, as well as to predict the impact of impacts or loads that have changed over time on these structures.As the practice of performing engineering and survey work shows, the erected structures of buildings do not always correspond to design solutions [2].In combination with the resulting design errors and deficiencies at the stage of construction and installation work, emergency situations may occur, leading not only to unforeseen financial costs, but also to human casualties.
Ensuring the specified quality and reliability of building structures, buildings and structures is a fundamental task of the construction industry.To solve this problem in existing buildings and structures, it is necessary, in particular, to ensure the control of the work of building structures in accordance with the project or the established design scheme [3].One of the main criteria for the work of a building structure according to a given design scheme is the mandatory control of the connection points of structures.This statement is especially true for precast reinforced concrete structures.In cases of precast reinforced concrete structures, non-compliance with the structural connection of adjacent structures, for example, a crossbar and a column, can lead to overloads of individual elements of the frame and the occurrence of emergency situations.In addition, precast reinforced concrete structures are manufactured in factory conditions and were originally intended for a special type of their work in the construction scheme.For example, a precast reinforced concrete beam designed to rest on a column with a rigid butt joint cannot perceive the design load if its support on the columns is actually hinged.
When performing surveys of buildings and structures made of precast reinforced concrete frame, the condition of load-bearing structures is usually determined by opening the nodes [4].The nature of the location of the reinforcement in the nodes of the support zones of the interface of adjacent building structures allows you to determine exactly what type of connection the elements have -hinged or rigid.But opening the support nodes when performing a building survey is often impossible to do, since the building may be in operation at this moment or for other reasons of limited access.For example, if functionally necessary equipment is installed, the dismantling of which is impossible [5].Therefore, for these purposes, it is possible to resort to tests of a building structure to determine its permissible design scheme.Static tests are often used to determine the actual working scheme of the structure.During static tests, the structure is loaded with a stepped load, which in the end does not exceed 0.2-0.4 of its design load.When performing static tests, various sensors and devices must be installed on the structure that can detect deformations when the load changes.But the cost of such tests is quite high, there is also a risk of damage to the structure or its components, so it is necessary to look for alternative and safe ways to conduct tests to clarify the design scheme of structures.The analysis and experience of research conducted in this matter have shown that in some cases it is useful to study the dynamic characteristics that are determined as a result of dynamic tests, which is cheaper and less time-consuming than conducting static studies [6][7][8][9].
In single-span beams, the dynamic characteristics are well studied and widely used, in particular, to clarify the nature of the fastening of the support units, which are affected by bending moments.It is known that at small amplitudes characteristic of the vibration test method, an elastic shape appears in the element corresponding to the vibration tone and the degree of pinching in the support nodes.For single-span beams, the technique is known, with the help of which it is possible to determine the parameter λ depending on the vibration tone and boundary conditions.
This dependence (Fig. 3,4) is expressed in terms of the deformation ε or displacement A measured on the structure, Where A1, A2, ε1, ε2 are amplitudes and deformations measured in 1/8 span; The value of λ varies from 4.73 (two clamps) to 3.14 (two hinges).Thus, for single-span beams there is a criterion by which it is possible to determine the conditions of their support.The shape of the curved axis under forced vibrations in the resonant mode can be used to clarify the support conditions not only in the case of single-span beams, but also for continuous multi-span beams.
Within the framework of this study, it is proposed to evaluate the load-bearing capacity of a precast reinforced concrete floor of an industrial three-storey building built according to an individual project.During the commissioning of the equipment, it became necessary to assess the load-bearing capacity taking into account the actual operation of structures and their components.The overlap is structurally made of prefabricated reinforced concrete slabs and prefabricated main beams.The main beam is a five-span continuous, with spans 6+6+12+6+6 m.
The cross sections in the spans, symmetrical with respect to the average, are the same.The twelve-meter span is made of monolithic reinforced concrete.In the places available for inspection, it was revealed that during the concreting of the floor, the load-bearing reinforcement was missed.
Despite the fact that the beam was subsequently reinforced on the support, the assessment of its ability to perceive negative moments on the support was of particular interest.In addition, there were other deviations from the project: the connection of beams with columns was partially changed, fittings were replaced, etc.
The overlap from the outside and from the inside, for technological reasons, was covered with a metal screen, bulky equipment was installed on it.
Before commissioning, there was a need to install additional equipment with a total weight of 100 tons.It was necessary to give an objective assessment of the possibility of increasing the load on the overlap.Thus, one of the tasks of the engineers was to clarify the actual scheme of the building frame.

Methods
Dynamic tests of the structure were carried out to clarify and obtain the actual characteristics.In particular, the frequency of natural vibrations of a reinforced concrete beam was measured.
For measurements, a portable signal analyzer "SCADAS Mobile-I" and vibroaccelerometers of the ВД06A type were used.When performing dynamic tests, vibrations were excited as follows: in the middle of a twenty-meter span, an asynchronous electric motor was installed, on the shaft of which eccentrics (imbalances) were attached.At a speed of 2920 rpm, an inertia force of 1200 kgf was created.After the engine gained full speed, it was turned off, and vibrograms of forced vibrations were recorded with a decreasing frequency from 48.6 to 0 Hz.

Results
If we consider the average span of the beam as a free-standing beam, then theoretically the frequency of natural oscillations should be equal to:  f1= 8.5 Hz, f2= 75.8 Hz (hinged support),  f1= 19.2 Hz, f2= 104 Hz (pinching).
When processing the data of the dynamic tests carried out, two natural oscillation frequencies were identified: f1 = 39 Hz, f2 = 67 Hz.Then the ratio of the sum of the dynamic deformations in 1/8 and 7/8 of the span to the deformation in the middle of the span (α) was calculated.The measured value α = -1.9, while for a single-span beam α varies approximately linearly from -1.39 with pinched supports and up to 0.766 with hinged supports.There is no coincidence with the measured value of α, as well as the coincidence of the measured and calculated frequencies for a single-span reinforced concrete beam.
Such a discrepancy suggests that it is impossible to judge the degree of pinching of a multi-span structure by the previously obtained dependencies for single-span reinforced concrete beams.
To determine the actual stress state, it is proposed to consider a three-span pinched beam (Fig. 6).The influence of the extreme spans can be neglected, since their rigidity is small compared to the rigidity of the middle spans.This assumption greatly simplifies the further calculation.
Canonical equations for such a reinforced concrete beam: Solving this system of equations, we get:  1 3 ,  2 3 ,  3 3 ,  4 3 .For elastic pinching, it is assumed that the section on the support can rotate by some angle Q (Fig. 7), then the canonical equations will be written as follows: Hence, x1, x2, x3, x4 are defined as functions of the angle Q.

EJ
The following are the ratios of moments with elastic pinching to moments with rigid pinching: Multiplying the graph of moments (rigid support) by the corresponding coefficients β, we get an adjusted graph of moments.
The correctness of the calculation according to this scheme is checked with respect to the measured and calculated natural oscillation frequencies.

Discussion
The results of the research conducted in this paper allow us to assert that dynamic tests are in many cases more profitable than static ones.They are less costly in terms of costs and require less time compared to static tests..One of the important advantages of dynamic testing is the ability to test elements of an industrial building without stopping the technological process.For further research in this area, metal frames of operated buildings for which engineering surveys are carried out are of interest.

Conclusions
The study analyzes the possibilities of modern methods of testing building structures to determine the conditions of support (articulated or rigid support) of horizontal precast reinforced concrete building structures.The possibilities of static and dynamic tests for solving these problems are considered.To analyze the work of building structures and the conditions of their support on neighboring structures, the method of dynamic tests was chosen.The results of previous studies are analyzed.In the work, full-scale studies of reinforced concrete floors made of precast reinforced concrete elements were carried out using the dynamic method and the applicability of this method for determining the working conditions of contact sections of structures was evaluated.Analytical formulas for estimating the measured and calculated parameters of the natural oscillation frequencies of the studied structure are given.The methodology considered in this paper demonstrates the possibilities of dynamic tests for analyzing the work of building structures.To clarify the conditions for the support of precast concrete elements when performing surveys of buildings and structures, it is recommended to use dynamic tests as less labor-intensive, less dangerous and cheaper than static tests.

Fig. 1 .
Fig. 1.Open beam support assembly during inspection (a) general view of the autopsy, (b) during the opening, the connection of the beam with the column was established using reinforcement.This indicates pinching.

Fig. 2 .
Fig. 2. The presence of reinforcement in the upper zone of the beam indicates pinching.View of the beam from below.


is the stiffness of the pinching.It is convenient to make a replacement:The value of the coefficient c' is determined by the results of dynamic tests.Using the integral of the vibration equation of a reinforced concrete beam it is possible to determine the coefficients A, B, C, D from the boundary conditions.The characteristic λ is determined by the formula of the natural oscillation frequency, which is measured as a result of dynamic tests:The stiffness coefficient c' is determined from the ratio of the second and first derivatives of the equation (3).