Nakov, Darko
Preferred name
Nakov, Darko
Official Name
Nakov, Darko
Main Affiliation
Email
nakov@gf.ukim.edu.mk
17 results
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Item type:Publication, Influence of the residual tensile strength on the factor for quasi-permanent value of a variable action ψ2(Springer, Cham, 2021); ; ; Mark, PeterSteel fibres are known to aid in deflection control, control the process of cracking and mainly improve the toughness of structural elements and the whole structure. Large experimental program was performed at the Faculty of Civil Engineering-Skopje to find out how steel fibres and the residual tensile strength affect the time-dependent deformation properties and deflections of concrete. Specific realistic loading with permanent and repeated variable loads in loading interval of 8 h per day was applied on full scale beams that were monitored up to an age of concrete of 400 days. The beams were with cross section dimensions 15/28 cm and total length of 300 cm, manufactured from concrete class C30/37. They were reinforced with same percentage of longitudinal and shear reinforcement, but with different amount of steel fibres (0, 30 kg/m3 and 60 kg/m3). Using the experimental results, detailed analysis of the time-dependent deformation properties of concrete and their effect on the time-dependent behaviour was done. A value for the factor for quasi-permanent value of variable action ψ2 is proposed for each type of concrete. It was concluded that the factor for quasi-permanent value of a variable action ψ2 depends linearly on the residual tensile strength. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Application of different methods for determination of the dynamic amplification factor (DAF) from moving loads on roadway RC bridges(Springer Nature Switzerland AG 2022, 2022); ; The dynamic effect of traffic loads on roadway bridges is usually introduced through the so-called dynamic amplification factor - DAF. This parameter shows how many times the impacts obtained from the load's static action are increased. One of the main goals of the dynamic tests of reinforced concrete bridges, with proof load, is to determine the DAF's actual value and compare it with the value taken into consideration in the design project. This research focuses on the possible methods for the determination of the DAF on roadway reinforced concrete bridges. For this purpose, results of dynamic tests performed on newly built bridges, with proof loading on various motorway sections in the Republic of North Macedonia, were used. Therefore, semi-prefabricated buildings with pre-stressed concrete beam girders and a static system - simple beam were analyzed. During the test, the main girders' vertical displacements were measured, through which the DAF was determined. For processing, analysis, and filtering of the measured signals' dynamic component, appropriate techniques have been used. After processing the signals, the DAF's value is determined by applying two proposed methods. With the processed results, a comparative analysis is performed between the DAF's determined values obtained from applying different methods. These analyses allow us to study the impact of using different measurement techniques and methods for processing the signals for calculating the dynamic effect of the moving loads. In this way, an attempt is made to suggest which methods for determining the DAF obtain more reliable results for its value. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Design principles of Eurocode 2(Macedonian Association of Structural Engineers (MASE), 2022)The adoption and implementation of new standards for design and execution of structures in one country is complex and long term process, which requires active participation of all available experts, institutes and organizations. If the same standards are expected to be implemented in more countries, than the process is even more complex, since there should be a lot of compromises, denials or adjustments with regards to the current design and execution praxis. The need for harmonization of technical standards within the European Community for the past few decades has led to the development of new technical standards in the field of civil engineering, so called Structural Eurocodes, or just Eurocodes. The use of these common standards is intended to lower trade barriers for construction products and enable companies and engineers to compete and work on a more equitable basis throughout Europe. Further, they are meant to lead to more uniform levels of safety in construction in Europe. The Eurocodes have been adopted, or there is an interest for adoption in many other countries in Europe and on other continents, as well. Еurocodes are set of 10 standards: EN 1990-Basis of structural design, EN 1991-Actions on structures, EN 1992, EN 1993, ЕN 1994, EN 1995, EN 1996, EN 1999-Design of concrete, steel, composite, timber, masonry and aluminum structures, accordingly, EN 1997-Geotechnical design and EN 1998-Design of structures for earthquake resistance. This paper refers primarily to the first part of Eurocode 2, named EN 1992-1-1: General rules and rules for buildings. Since Eurocodes are based on the limit states design, design principles and procedures for reinforced concrete members at the ultimate limit states will be explained and discussed (bending without and with axial loading, shear, torsion and punching). - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Overview of standards for static and dynamic proof load testing of RC bridges(Macedonian Association of Structural Engineers (MASE), 2022); ; The primary purpose of the testing of structures with proof load is to obtain data on their actual load-bearing capacity, stability, serviceability, as well as on other parameters of their behaviour, ie., on the compliance of the constructed structure with the used numerical model. In addition, such testing can be a final control (confirmation) of the quality of the newly built buildings. Bridges are one of the structures that, depending on their purpose and span, are subject to proof load testing, which depending on the way the load is applied, can be static and dynamic. As a prerequisite for performing a quality test, it is necessary to follow and apply standardized procedures, including a quality test program and the application of precise, calibrated measuring equipment. The neglect of these procedures leads to doubts about the measured results' reliability and usability. This paper first focuses on reinforced concrete bridges' static and dynamic proof load testing procedures. The specific aspects of the proof load testing are then presented, with an overview of the necessary previous work to be done before starting the test, determining the intensity of the proof load, the procedures that follow during the test process, and the post-processing of the measured results and their analysis. Based on the performed review and systematization of the procedures for examination with proof load, as well as the experiences gained from examination of a large number of real scale objects, in the end, specific procedures and practical recommendations for conducting this type of examination are proposed. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Monitoring based structural identification of two railway bridges(Faculty of Civil Engineering Skopje, 2020-07); ; Data-driven tools applied on measured structural responses enable extraction of valuable information on the behavior of in-service structures and identification of the actual system in its “natural habitat”. An output-only testing campaign on two railway bridges is in the focus of this study. The specifics of the dynamic behavior of the structures are studied under three different scenarios. More precisely, the measured accelerations at relevant points of the structures, for unloaded and loaded conditions, are herein employed with the following objectives: I) Structural dynamics identification via estimation of the first natural bending frequency of the structures during ambient unloaded conditions using the AutoRegressive Moving Average (ARMA) method and the time-domain Subspace System Identification (SSI) method; II) Assessment of traffic safety aspects based on the maximal amplitudes of vibrations during loaded operating conditions of the structure, for two separate train induced vibrations (train velocity of 40 km/h and 60 km/h); III) Evaluation of the dynamic effects through a dynamic amplification factor estimated from the recorded responses during dynamically loaded structure (train velocity of 40 km/h and 60 km/h). By utilization of the response-based data analysis methods the first natural frequencies of the steel structures are successfully identified and compared with FEM numerical results. The acceleration data is further assessed by preprocessing filtering techniques and extracted features are assessed according to EN 1990:2002 or UIC (International Union of Railways) code 776-2 (2009). - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Comparison of Code for Design and Analysis of Composite Columns(Macedonian Association of Structural Engineers (MASE), 2023); ; This paper presents the construction regulations and regulations for the design and analysis of coupled columns. Composite columns are composite materials made up of several components such as concrete, steel and reinforcement, while using the rationally positive aspects of concrete as a pressure load and having a part in increasing the fire resistance of the section and steel as reinforcement or a rigid core, which accept loads of pressure and tension also has a great role in increasing the fire resistance of the section and also with adhesion between concrete and steel in the section, a rational solution for the design of modern structures is possible. The trend of innovations in construction requires modern solutions, one of them is the coupled construction system, which also requires greater analysis. In terms of the practice of application and performance of coupled structures, due to the problem that is not sufficiently researched, the largest number of designers claim either reinforced concrete or steel sections in appropriate classical reinforced concrete or steel structural systems. Available literature and practical applications are very few. They are mostly studied in Eurocode 4, and their expressions and explanations are the basis of other regulations from Japan, China, Russia, USA, India, Australia, United Kingdom, Germany, Italy, Ethiopia, Canada, etc. which have an almost similar concept of analysis according to Eurocode 4. The paper shows comparisons of regulations and building regulations for the design of coupled columns as part of the coupled construction system with pictures, diagrams and empirical beams and shows the most applied concept of analysis of coupled columns as well as the simplified method of analysis according to Eurocode 4. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Crack width control in RC beams: experimental and analytical results(Macedonian Association of Structural Engineers (MASE), 2022); ; In order to satisfy the required level of reliability, the structure has to be designed so none of the ultimate and serviceability limit states will be reached within the expected service life. By undertaking measures for ensuring the required durability, its serviceability is directly affected, as well as its load-bearing capacity, which may be endangered due to the dilapidation of the structure. Limitation of crack width and the adequately provided concrete cover, which primarily lead to corrosion protection of steel reinforcement, are one of the basic and maybe the most important preconditions for ensuring the required durability. Despite the wide range of knowledge gained through numerous experimental and theoretical studies, there are still certain dilemmas regarding the implementation of models given in the codes for crack control. In order to determine the influence of long-term sustained load on the serviceability limit state-crack control and to estimate the level of accuracy of the considered analytical models, an experimental program on RC beams was realized. Eight beams with dimensions 15/28/300cm were made and monitored in a laboratory environment with constant ambient conditions. This paper presents formation and development of the cracks in the considered time period of one year. An overview of the obtained results using several modern analytical models for crack prediction is also provided. In this paper, the results show that the modern analytical models given in the standards with satisfactory accuracy can predict the serviceability limit state-crack control due to long-term load. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Reliability assessment of the superstructure of precast prestressed bridges(Macedonian Association of Structural Engineers (MASE), 2022); ; Given that all buildings are designed for a certain "lifespan", there comes a time when the question must be asked whether they are still safe and serviceable? The need to ask this question arises from the fact that over time, each object is exposed to external influences and suffers certain geometric and material degradations, mechanical damage, increased intensity of moving load (weight and frequency of vehicles), and also the production and control of materials was then at a lower level. The answer to this question can only be revealed through their research and reliability assessment, and the final answer can be: demolition and reconstruction, repairing and / or strengthening, no interventions required. Reliability is the probability that a system (construction) will perform the functions for which it is designed during a certain time interval ("lifespan") under specific operating conditions. In order to determine the reliability index of the structure (βsystem), the reliability indices at the level of the structural element (βi) should be determined first, and then through their connection (depending on the structural system and the limit states) the reliability of structure is determined. Due to the lack of means and conditions to examine the necessary geometric, material and other data for an old bridge, a reliability assessment was performed on a relatively new bridge, i.e. reinforced concrete pre-stressed girder overpass on the highway section Miladinovci - Stip. For that purpose, the project of the same was used, as well as all available reports on the embedded materials that are of particular use for modeling the real model. Reliability is examined in terms of ultimate limit state (flexural moments) using FORM (First Order Reliability Method) and MonteCarlo simulation technique. At the end of the paper, a comparative analysis is performed between the results of the two methods with the recommended minimum values in EN1990 [1] for a 1 year reference period. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Cracks in reinforced concrete structures due to restrained imposed deformations – case studies(Macedonian Association of Structural Engineers (MASE), 2022); ; ; - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Comparison of Completely and Partially Concreted Composite Columns According to Eurocode 4(2023); ; The paper provides a review of the analysis of fully and partially concreted sections of coupled columns with a comparative analysis of the obtained results. The purpose of this paper is to perform an analysis of fully concreted composite columns with steel profile core and additional soft reinforcement and partially concreted steel profile columns and additional soft reinforcement. In the analysis, the same and similar cross-sections are taken to see how effective it is in carrying axial forces and biaxial bending for comparing fully coupled columns with partially coupled columns. The entire analysis is carried out according to Eurocode 4 and theoretical assumptions are introduced in the analysis and biaxial bending and shearing of the cross sections of the two types of coupled columns are calculated. The analyzed sections are examples from practice loaded with real loads. The materials of the spliced columns are rigid steel profiles with steel grades S235 – S460, concrete grades C20/25 – C50/60, soft reinforcement quality S460 and B500 with reinforcement percentages of 1-6% and an assumed fire-resistance scenario. from R30 – R240. Combinations of calculations for different brands of concrete, different qualities of steel and reinforcement for different cross-sections with percentages of reinforcement, as well as combinations of moment ratios and comparisons of cross-sections, have been made. From the given static influences, the cross section for combined pressure and biaxial action is determined and interaction N–M diagrams are determined along both axes with percentages of reinforcement shown as a final output that can be used in future simplified calculations. Results and conclusions were obtained from the comparison, with which one can see the differences of coupled columns of fully and partially coupled sections.