Keywords

1 Development and Structure of Regulatory Framework for Inspection of Existing Constructions

The modern history of the regulatory construction framework in Ukraine began in 1991 with the attainment of independence. For a long period in Ukraine, Soviet construction norms and standards were in effect. Currently, Ukraine distinguishes between documents of two levels - state construction norms (DBN) and national standards (DSTU).

It is worth noting that as of 1991, the system of Soviet construction norms lacked documents on inspecting the technical condition of buildings and structures. Various industry-specific documents were used for this purpose. For instance, VSN 57-88 [1] was used to inspect residential buildings and different documents were used for industrial buildings. Inspection of steel structures of industrial buildings could be carried out according to ORD 00 000 89 [2] or the Guide for the Design of Strengthening of Steel Structures [3] based on the applicable norm at that time. Similar recommendations for the inspection and strengthening of reinforced concrete structures can be found in [4], and this is a partial list of various recommended, methodological, and auxiliary documents. In the list of recommended sources for [5], 34 documents issued in the 1970s-80s dedicated to the issues of technical inspection, assessment of the technical condition, and strengthening of damaged structures could be found. Logically, many guides led to different groups of engineers using different documents, resulting in works on assessing the technical condition of buildings and structures by professionals working within the same city being carried out in different ways, yielding different results.

In 1995, Ukraine introduced DBN 362-92, “Assessment of the Technical Condition of Steel Structures of Industrial Buildings and Structures in Operation” [6] - the first state construction normative document developed by a group of scientists under the leadership of A. Perelmuter. Although it was a progressive document, it included parts from the abovementioned documents [2, 3]. Its main advantage was the clear and logical structure – the document contained information on the procedure for conducting technical inspections and assessing the technical condition by various methods: analytical, which is based on tables of typical defects and damages, and recommendations for determining the technical condition based on the conducted inspection. The most exciting provisions were recommendations for refining steel properties in structures and refining the actual loads and impacts. One of the weaknesses of this norm was that, despite its status, it remained an industry-specific document, as it was intended only for the assessment of steel frames of industrial structures. Another weakness was that the classification of technical conditions differed in names from other documents, including those for inspecting industrial buildings and structures. In 1997, the Cabinet of Ministers of Ukraine adopted a resolution requiring all industrial enterprises to certify buildings and structures. Each building was supposed to have a passport with drawings, a description of structures, loads that these structures can withstand, established technical conditions, and recommendations for further safe operation. The passport was compiled based on a technical inspection. To determine the technical condition (there were a total of 4 – from normal to emergency), a document [7] was created, a collection of various legislative acts, including those regulating the technical condition of buildings. Unlike [6], the classification of the technical condition was envisaged only based on visual parameters. At the same time, this document introduced the calculation of the term of the next planned inspection, which was based on the term of the previous one and coefficients that depended on the technical condition of the structure and the aggressiveness of the surrounding environment.

The schedule for planned inspections of buildings (structures) was recommended to be determined based on the safety coefficient using the formula:

$$ T = \, T_{b} \cdot \, K_{b} ,years $$
(1)

where Тb is the term until the first planned inspection for buildings (structures) operating under average conditions for a given industry.

The safety level of buildings (structures) is assessed by the safety coefficient Kb, which is the product of three coefficients: the purpose reliability coefficient (Yn), the coefficient characterizing the environmental hazard of production resulting from the failure of building structures (Kek), and the coefficient of the influence of the aggressiveness of the production environment (Kag):

$$ K_{b} = \, Y_{n} \cdot \, K_{{{\text{ek}}}} \cdot \, K_{{{\text{ag}}}} $$
(2)

The primary purpose of the technical condition passport of a building was to contain information about the main technical parameters of the building (a precursor to BIM models) and answer whether it is possible to operate this building until the next inspection.

In the next 15 years, the situation related to the norms for assessing the technical condition of buildings and structures remained the same. In 2016–2017, two national standards were introduced - DSTU-N B V.1.2-18:2016 [8] and DSTU B V.2.6-210:2016 [9]. Standard [9] is the second edition of the normative document [6], which practically retained its structure with partially expanded calculation methods for assessing the load-bearing capacity of damaged steel elements. Standard [8] is a new document based on [5, 7]. Ideologically, the document is based on a visual assessment of the technical condition of building structures. Table 1 are presented for masonry structures, containing coefficients for reducing the load-bearing capacity of stone structures in the presence of damage, γt.

Table 1. Coefficient for reducing load-bearing capacity when forming force cracks from compressive forces (Adapted from Table V.3.2 DSTU-N B V.1.2-18:2016 [8]).
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This table is borrowed from the Recommendations for Strengthening Masonry Structures in 1984 [10]. Given that information about the origin of these coefficient data was not found, using these coefficients without additional verification is not advisable. At the same time, the approach itself is exciting and may have further development in the Eurocode system.

The weakness of these two standards is the simultaneous application of both. It is explained by the fact that different developers worked on the standards. A positive aspect is that the two groups of developers agreed on specific points – such as the number of technical conditions now fixed at four technical conditions and their names. Standard [8] regarding the inspection of steel structures contains a direct reference to [9]. At the same time, the scope of application of the standard [9] has been expanded to include steel structures of buildings and structures of any purpose.

2 Comparison with the International Standard for the Evaluation of Operating Structures.

The main provisions of Ukrainian regulatory documents were compared with the international standard ISO 13822:2010 [11]. The general procedure for assessing the condition of existing structures is quite similar, and it does not make sense to pay attention to minor differences. Let us focus on significant differences. According to [9], the assessment of the technical condition is the determination of the level of damage to steel structures and the establishment of the category of its technical condition. According to Sect. 4.1 [8] the investigation of the object is the element of supervision that determines the technical condition of the object. Even though standard [8] in Sect. 1.2 regulates the inspection of objects to ensure standards [12, 13], which essentially corresponds to EN1990 [14] and ISO 2394 [15], this document does not contain information on determining the reliability level of existing structures. Thus, the assessment of the condition of existing structures comes down to the need for an expert to classify the structure into one of four technical conditions: 1 - normal, 2 - satisfactory, 3 - not suitable for regular use and 4 - emergency. A significant part of the document is a description of procedures for the scope and procedure of inspection work, depending on various situations, which does not cover all possible situations and sometimes contains information more typical of textbooks than regulatory documents. Also, tables are compiled for the main types of load-bearing structures (foundations, reinforced concrete, masonry and timber structures) to establish the category of the technical condition of the structure. A similar table is included in [9] to establish the technical condition of steel structures. Similar tables are absent in [11], while in section C.1, the expert is suggested to visually assess structures as “without defects”, “minor”, “severe”, etc.

The practice of using similar tables at the stage of preliminary assessment of existing structures in new European documents would allow for a reduction in decision-making time. Perhaps it is not necessary to introduce a division into technical conditions, but having materials for a preliminary assessment that would allow classifying defects into groups could allow more qualitative decision-making regarding the need for a detailed assessment.

For example, a deflection in the form of a separate element from the plane of the truss (Fig. 1), with values of fy ≤ 15 mm or fy/L ≤ 1/750, this structure can be used without a detailed analysis; if not, additional calculations should be performed, taking into account the geometric nonlinearity of the elements. The limit values of defects can be set in EN, with adjustments in National Annexes. or may be presented in reference documents to Eurocodes. In general, the use of the method of comparing geometric parameters of a defect with limit values (or relative geometric parameters of a defect, for example, the depth of the notch in the flange to the total width of the flange) to determine further actions in the study of a damaged structure is another feature that distinguishes Ukrainian standards from [11]. At the same time, the described “tabular” method of determining the technical condition of building structures sometimes turns into the final part of the assessment. The condition of a building or structure is assessed by the presence of a crack in the brickwork or the presence of horizontal bending of a steel beam, not by the influence of this defect on the reliability of the damaged structure. The authors of this article have seen conclusions where a house was recognized as an emergency due to frost damage to a wall to a depth of 30% of its total width without performing additional calculations, etc.

Fig. 1.
figure 1

(Adapted from Table V.1 DSTU B V.2.6–210:2016 [9]).

Excerpt from the table “Limit values of defects and damage”

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According to [8], permanent loads gp, in Pascals (Pa), from the weight of the coverings (overlays) are recommended to be determined, taking into account the results of uncovering the roof (enclosure) and the actual composition of its layers. The characteristic values of these loads are determined by weighing samples and processing the weighing results using the formula:

$${g}_{p}={p}_{p}\pm {\text{K}}\cdot \frac{{S}_{g}}{\sqrt{m}},$$
(3)

where \({p}_{p}=\frac{1}{m}\cdot \sum_{i=1}^{m}{p}_{i}\) is the arithmetic mean value of the weight of samples in Pascals (Pa);

\({S}_{g}=\sqrt{\frac{1}{m-1}\cdot \sum_{i=1}^{m}{\left({p}_{i}-{p}_{n}\right)}^{2}}\) is the root mean square deviation of weighing results in Pascals (Pa);

pi is the total weight of all layers of the enclosing structure in layer number i, Pa;

m is the number of samples (no less than 5);

K is the coefficient that considers the sample size, determined by Table 2.

Table 2. Coefficient K (Adapted from Table 6.2 DSTU-N B V.1.2-18:2016 [8]).
Full size table

The “plus” sign in formula (3) is considered for the unfavourable effect of increased load, and the “minus” sign is considered for the favourable effect.

It is permissible to determine gp taking into account the non-uniform distribution of the permanent load over the surface of the enclosing structure using the formula:

$${g}_{p}={p}_{p}\pm \frac{{\mathrm{1,64}\cdot S}_{g}}{\sqrt{1+\mathrm{0,1}\cdot \left(L+B\right)+\mathrm{0,006}\cdot L\cdot B}},$$
(4)

L and B are the length and width of the load area of the calculated structure, measured in meters.

Obviously, not all accumulated experience of Ukrainian scientists in assessing the technical condition of existing structures is presented in regulatory documents. It is challenging to cover all research conducted over the past 30 years within one article. Therefore, we will focus on the activities of scientific groups and publications issued 10–30 years ago. Some provisions described in the State Standards for inspecting steel structures were developed by A. Perelmuter and published in the monograph [16]. Methods for refining cranes and atmospheric loads were developed, presumably with S. Pichugin, V. Pashynskyi, and other researchers [20]. The methodology for checking the load-bearing capacity of beams with horizontal deflection is based on the methodology of A. Rzhanitsyn [17] was additionally verified based on experimental tests by scientists from Poltava – S. Pichugin, V. Semko, and S. Hudz [18, 19]. The Poltava group also developed a methodology for calculating steel beams with one-sided notches in the flanges [21, 22]. According to the research, it was established that in steel beams with a symmetrical cross-section and one-sided notches in the flanges, additional stresses from torsion occur, and the magnitude of these stresses depends on the length of this notch. The formula will determine the maximum normal stresses in the section with a notch

$${\sigma }_{max}={\sigma }_{bend}+\left[\mathrm{0,12}\cdot \mathit{ln}\left(\frac{{{\ell}}_{c}}{{\ell}}\right)+1\right]\cdot {\sigma }_{\varpi },$$
(5)

where ℓc is the length of the notch; ℓ is the span of the beam; σbend is the normal bending stress determined by considering the actual geometric characteristics of the damaged section; σω is the normal stress from warp torsion of the beam.

Subsequently, this methodology was refined in the works of O. Voskobiinyk [23]. The research on the behaviour of damaged steel-reinforced concrete structures was also conducted [23], and tables of allowable values for defect and damage parameters of bent and compressed steel-reinforced concrete elements were proposed, similar to the tables in DSTU [8].

Works by Z. Blikharskyy [24], V. Klymenko [25], O. Semko [26], and many others are dedicated to the assessment of the technical conditions of existing building structures. Research on corrosive damage to steel structures is the focus of studies by V. Korolov and O. Gibalenko [27]. Numerous researchers have contributed to investigating the stress-strain state of existing structures, including structures with damage or imperfections. However, these studies have yet to be incorporated into Ukraine’s regulatory documents for various reasons.

An exciting direction related to the reliability of existing enclosing structures was initiated in the works of V. Pashynskyi and G. Farenyuk [28, 29]. A new research direction, thermal reliability of enclosing structures, was proposed. Thermal reliability refers to the ability of the structure to maintain its performance over a specified service life continuously. It was established [29] that the number of days per year during which thermal failure of the enclosing structure occurs under specific criteria is a convenient and illustrative indicator of thermal reliability.

The initiated direction has been continued in the works of V. Semko and M. Leshchenko, who investigated the thermal reliability of structures combining load-bearing and enclosing properties-walls with a frame made of thin-walled cold-formed steel elements. For this purpose, statistical characteristics of materials used in these structures were established [30]. The study [31] investigated the changes in strength and thermal conductivity of polystyrene concrete (which can be used as insulation in this technology) depending on its density. Using the obtained statistical data, methods for assessing the reliability of enclosing structures with cold-formed steel elements were developed according to various criteria [32,33,34]. Applying probabilistic approaches and risk theory also allowed the formulation of a method for standardizing the thermal resistance of enclosing structures based on a specified failure probability or estimating the probability of thermal failure for a particular design solution [35]. Determining the probability of failure-free operation of load-bearing elements made of cold-formed thin-walled steel profiles, including existing ones, is addressed in the eighth section of the work [36].

Due to the hostilities in Ukraine, the issue of assessing the reliability of damaged structures returns to the consideration of Ukrainian scientists. As already mentioned, A. Perelmuter and S. Pichugin published a paper dedicated to some peculiarities of calculating the reliability of damaged structures [37]. The article describes methodologies for considering the influence of the structure’s previous operational history on its reliability level. It also emphasizes the importance of refining material properties and loads, providing illustrative examples.

3 Conclusions

Based on the conducted analytical study of various normative documents and scientific works, the following recommendations can be made:

  1. 1.

    The procedure for assessing existing structures presented in [11] could be enhanced with additional materials regarding types of damage to building structures. Additional information on the geometric parameters of these damages and their maximum dimensions could allow for quicker decision-making during the inspection stage. The availability of such tables would enable a broader range of experts to participate in the assessment process, and only defects and damages exceeding the sizes presented in reference tables would require the involvement of experts and additional investigations, if necessary, to implement measures to enhance the reliability of the structures.

  2. 2.

    It would be advisable to provide calculation methodologies for reliability coefficients under loads for permanent loads, including the weight of existing structures and finishing materials. The same applies to determining reliability coefficients based on material properties. This would allow for a more accurate assessment of the reliability of existing structures.

  3. 3.

    A large group of structures can perform a load-bearing function and an enclosing one—such as walls with frames made of cold-formed steel elements (or wooden elements), walls made of lightweight concrete blocks and others. The reliability of such structures needs to be considered from the perspective of failure probability based on the criteria of the first (or second) group of limit states and the criteria of thermal failure. Thermal failure can be the first step toward failure for these structures based on strength criteria. Since it may cause corrosion for steel elements, decay for wooden elements, or alter the strength properties of lightweight concretes due to significant moisture accumulation.