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Oštećenje čelika može značajno utjecati na performanse čeličnih elemenata, utječući na druge konstrukcijske i nekonstrukcijske dijelove zgrade i ugrožavajući sigurnost i funkcioniranje. Razumijevanje procesa i mehanizama oštećenja čelične konstrukcije ključno je za poboljšanje njezine trajnosti. U ovom su radu prikazane numeričke i eksperimentalne analize ponašanja “Y“ priključaka rešetkastih nosača u ravnini opterećenih uzdužnom silom, s posebnim naglaskom na učinke oštećenja. U ovom su radu prikazani rezultati numeričkih i eksperimentalnih istraživanja izravno zavarenog Y priključka SHS pojasnog štapa i dijagonalnih elemenata s oštećenjem priključka u obliku uske vertikalne pravokutne pukotine. Odstupanje rezultata dobivenih numeričkim i eksperimentalnim postupcima iznosilo je u prosjeku 3 % do 5 %., Steel damage can significantly impact steel elements’ performance, affecting other structural and non-structural building parts and compromising safety and functioning. Understanding the processes and mechanisms of steel structure damage is crucial for improving its durability. This paper presents numerical and experimental analyses of type “Y” joint behaviour of truss beams in a plane loaded by an axial force, with a special accent on the damage effects. This paper presents the results of numerical and experimental research on the directly welded Y joint of the SHS chord and diagonal members with damage in the joint in the form of a narrow vertical rectangular crack. The deviation of the results obtained by the numerical and experimental procedures was, on average 3 % to 5 %.

U radu je prikazan novi pristup koji omogućuje aktivno praćenje stanja konstrukcija i otkrivanje oštećenja na masivnim armiranobetonskim konstrukcijama, a utemeljen je na primjeni piezoelektričnih pametnih agregata. U matričnom je obliku izveden inovativni trodimenzionalni indeks oštećenja baziran na valićnoj dekompoziciji signala i energiji prostiranja valova. Iako se predloženi trodimenzionalni indeks oštećenja može koristiti za sve vrste armiranobetonskih konstrukcija, prije svega se ipak preporučuje za masivne infrastrukturne građevine. Pristup predložen u ovom radu teoretski je izveden za proizvoljni oblik armiranobetonskog elementa, te je numerički provjeren u raznim scenarijima variranjem geometrije armiranobetonskih elemenata, isto kao i položaja, veličine i rasprostranjenosti oštećenja. Kvazistatička analiza piezoelektričnih pametnih agregata modelirana je pomoću standardne metode konačnih elemenata, a u ovom je istraživanju uspješno primijenjena eksplicitna metoda konačnih elemenata za modeliranje prostiranja ultrazvučnih valova. Rezultati bazirani na numerički generiranim simulacijama upućuju na perspektivnost novog pristupa za nerazorno otkrivanje oštećenja pomoću trodimenzionalnih indeksa oštećenja., A novel approach to active structural health monitoring and damage detection of massive reinforced concrete structures using piezoelectric smart aggregates is presented in this paper. An innovative three-dimensional damage index, based on wavelet signal decomposition and energy of wave propagation, is derived in matrix form. Although the proposed three-dimensional damage index can be used for all types of reinforced concrete structures, it is primarily recommended for massive infrastructure buildings. The approach proposed in this paper is theoretically considered for an arbitrary shape of a reinforced concrete element, and it is numerically verified for various scenarios by varying the geometry of reinforced concrete elements, as well as the position, size and quantity of damage. Quasi-static analysis of piezoelectric smart aggregates is modelled using a standard finite element method, and the explicit finite element method is successfully applied in this research for modelling propagation of ultrasonic waves. The results based on numerically generated simulations indicate that the new approach to non-destructive damage detection using three-dimensional damage indexes is quite promising.

This paper presents determination of the Mohr-Coulomb material parameters for concrete under indirect tensile testing, design of experiments and numerical simulations. Experiments are designed to control input variables associated with inhomogeneous starting material, sample preparation and testing equipment. Statistical analysis of the experimental data is applied in order to correlate material's deformational behaviour with change in cement content. Presented paper discusses a simple method, based on experimental and theoretical framework, to determine two material parameters for concrete: cohesion (c) and angle of internal friction (φ) from indirect tensile test. Cohesion is obtained from experimental results using elastic and plastic theory of concrete deformation. Prandtl's theory is applied to obtain the angle of internal friction from measured bearing capacity and cohesion. Numerical analysis by finite-element method is used for verification of experimental results, employing step-by-step integration procedure for solving the boundary value problem.

Timber-concrete composite structures represent a composite system that is formed by coupling of timber and concrete elements using different types of connectors. They are applied in the rehabilitation and construction of new facilities. In order to enable their wider application, it is necessary to ensure the general availability of relevant technical regulations in relation to their design and construction. The main goal of the doctoral dissertation is to contribute to the improvement of the design process of timber-concrete composite structures and their maintenance during the service life. Thus, certain multi-parametric probabilistic analyzes and optimizations should be carried out which would result in certain recommendations that would have practical application in the certain conditions of design, construction and maintenance of these composite systems. In order to determine the most important design parameters, a stochastic sensitivity analysis of the maximum equivalent stress and maximum deflection of the timber-concrete composite beam was performed. A multiobjective optimization was also conducted in order to choose an optimal solution for the dimensions of the observed composite beam. A new probabilistic deterioration model has been developed that describes the true nature of the behavior of the timber-concrete composite system due to the long-term load.

Point loads can act on membrane structures; nevertheless, they are rarely checked in the structural analysis. This is a consequence of the fact that there are no precise instructions that would oblige and direct membrane structures designers how to conduct this analysis. The work to create Eurocode for the Structural Design of Tensile Membrane Structures is underway. When this standard comes into force, it will require following Eurocode 1 with regard to actions on membrane structures. This means that point loads will be included in this structural analysis. However, the consequences of point load actions have yet to be investigated. The subject of this doctoral dissertation research is the change of geometry of membrane structures under point load action. The research for this doctoral dissertation is conducted by testing numerical models of membrane structures in software package Sofistik 2016. Parametric analysis is used to find out how much and in which way varying the values of selected parameters changes the geometry of membrane structures under point load action. Nine parameters divided into groups of those which are related to load, membrane material and the supports have been analyzed. A large number of models differing by parameter values has been created. The goal of the research is to find out how the change of values of analyzed parameters affects the changes of geometry, how big is the influence of these parameters, and what is the importance of point loads for changes of geometry of membrane structures. Based on the obtained results, the dependence of changes of geometry under point load on the analized parameters is determined. The results of the research show that the point load can cause larger changes of geometry compared to the area load whose value is taken to be the usual snow load. Since the point load actions are currently almost completely neglected, this result was not expected. Based on this, it can be concluded that the point load actions

This paper presents an experimental-numerical analysis of damage localization of concrete plate-like elements on the basis of hybrid approach. The proposed hybrid approach uses the fast discrete wavelet transform, energy approach, and time of flight criterion for the purpose of localization of single and multidamage problems inside or on the periphery of concrete elements. Verification of the proposed damage localization approach has been performed under laboratory conditions using a laser scanning-based system with piezoelectric excitation of the wave propagation. Numerical simulation of the wave propagation is performed using the explicit finite element method using 3D models with linear-elastic material model of concrete with Rayleigh damping. The Rayleigh damping coefficients are determined on the basis of experimental data and implemented in numerical models. Validation of the numerical model is conducted, based on the comparison with sensor output signals obtained through experimental measuring and a very good agreement of results is obtained. The proposed hybrid approach to damage localization is verified using 15 different models/specimens, varying the number, shape (circular or notched), and position of damage, as well as the number and placement of actuators/sensors. For all the analyzed scenarios, the hybrid approach successfully localized the damage even for the least number of used sensor positions. In the models with the circular damage, the damage image created on the basis of the hybrid approach is almost identical to the actual shape of the damage, indicating a good potential of the method for damage localization.

The subject of the scientific research are composite structures type timber-concrete with the special attention to the mechanical beha-viour, of both, composite system as a whole and all the containing elements: concrete plate, timber beam and adequate connecting sys-tems. The essence of the work is in the comparative analytical-nume-rical-experimental analysis of two series of composite beams of the timber-concrete type by different connecting systems. The conclu-sions have been formed on the base of the experimental testing of beams up to the collapse in the adequate laboratory conditions, nu-merical modelling using the software ABAQUS and analytical nu-merical method according to the postulates of the theory of elasticity, defined in Annex B, Eurocode 5. Besides testing the beams according to static loading bending to the collapse, the work provides the data about the huge correlated research in order to determine mechanical constants of the adequate strength and stiffnesses, as well as the physical characteristics of the used timber and concrete material for making experimental samples. The special attention has been dedica-ted to determining the sliping parameters of connecting systems which were used while making composite beams. All the experiments have been carried out in accordance with the modern procedures contained in the suitable codes.

This paper contains a proposed numerical procedure for determination of stress and deformations state due to creep and shrinkage of concrete and relaxation of prestressed steel in composite structures with apply of the finite element method. Stiffness matrix of linear finite-element is performed by use of layers technique, but rheological properties of concrete and prestressed reinforcement are introduced by fictive loading. For procedure development, generalized incremental stress-deformation links are used for viscous materials derived by application of displacement method. The manner is analysed in more details to determine fictive forces due to viscous deformations, especially the axial forces part as a contribution from the pure member bending. Improved procedure is formulated which provides considerable correction of results accuracy without the need of structure discretization on the bigger number of finite elements. Here developed procedure for calculation of composite structures is applicable in various technological procedures of construction-work with enabled gradual inclusion of some individual parts of composite section into a stress active section. It has been shown that time - dependent (rheological) changes considerably change state of stress and deformations of CS and that in their design negative impacts cannot be left out of the analysis. To that aim, the author has developed a program package with whose application, it is simple to get the needed results.

Adhesive joints offer many advantages compared to the traditional joining techniques in metallic structures. However, the lack of unified design standards and procedures limit their wider use in civil engineering industry. For the purpose of standardization, extensive experimental work has to be done on investigating their behavior under the service conditions. Besides the influence of the joint geometry, the manufacturing process and the temperature and humidity effects, type of the loading has the greatest influence on the behavior of adhesive joints. Main goal of this dissertation was to investigate behavior of adhesive joints in metallic structures under static and cyclic loading, using modern experimental and numerical methods. In order to achieve this the selection of proper adherent surface preparation procedure was necessary. Experimental investigation of the behavior of adhesive joints subjected to static and cyclic loading was conducted on two types of adhesive joints: galvanized steel façade joint and double lap shear joint. Based on the results of the experimental investigation, as well as the results from the literature, new models and methods were developed for modeling of the behavior and predicting the fatigue life of adhesive joints subjected to static and cyclic loading.

The doctoral dissertation deals with an extremely complex and very topical issue of bridge management. Basic imperatives of correct formation and design of contemporary population of bridge structures are safety and serviceability as fundamental properties of bridges; it is tended to retain serviceability in certain time period (service life) providing durability and reliability. The task of a builder is to conceive, design and build bridges in such a way that they can withstand all the impacts they will be exposed to. Bridge structures must be reliable and serviceable for an optimal number of years, so for that reason, all requirements concerning bearing capacity, serviceability and durability should exceed the minimum permissible level. It is impossible to achieve absolute safety, infinite durability or perfect serviceability, but we must strive to an optimum condition. On the other hand, there is a large number of bridges around the world, serving different functions, which were built in various environments ,various times, from various materials, of various forms, systems, spans and dimensions. During service, due to permanent material fatigue and effects of other diverse parameters and processes, damage and progressive deterioration of bridge structures are inevitable, which has a negative effects on bearing capacity, serviceability and durability of bridge structures, and therefore on their reliability and safety. However, a prolonged downtime of bridges because of repairs, reconstruction or replacement can cause very inconvenient disruptions of road and railway traffic. Preventive maintenance of bridges, as an optimal solution from the technical, organizational and economic aspect was not paid due attention, especially in our country. A permanent increase of bridge structure damage was observed, with the enormous decrease of bearing capacity and safety of bridges, and even some very important bridges in the world collapsed. Such a problem required an efficien

In this doctoral thesis, a number of researches of arches and vaults of Orthodox churches in the medieval Serbia has been conducted in order to comprehensively determine the historical factors that have influenced the course of development of arch and vault structures, define the specific proportions and their implementation in construction of arches and vaults, analyze the structural characteristics and performances of arches and vaults, form a typological classification of arches and vaults and to determine their value in relation to the arch and vault forms in the region. As a part of the dissertation, the materials used for building arches and vaults have been analyzed as well as the masonry techniques, and also the partial or complete reconstructions of arch and vault assemblies in the temples have been determined. On the basis of these segments in the research, which are systematized, some of the previous interpretations of arches and vaults inside the churches are reviewed and corrected in this dissertation, and in some cases completely new assertions have been made. The research covers the subject of arches and vaults of church buildings which were being erected in the medieval Serbia, in the period from the mid 12th to mid-14th century, that is, during the reign of the Nemanjiš dynasty. А complete scientific apparatus has been implemented, which included historical method, analytical method, modeling method, comparative method, descriptive method, typological method, the method of synthesis, with 354 footnotes, 230 bibliographic units, with 115 pictures and 15 tables. Based on structural analysis of arches and vaults, the influence of proportion within the specific forms of arches on the intensity of internal forces that are present in these arches has been particularly emphasized. In this thesis, the particularities in the masonry techniques for above mentioned vaults that are of importance for the stability of these structures have been pointed out. As on

U ovom radu ispitano je dinamičko ponašanje spregnutih međuspratnih konstrukcija tipa drvo-laki beton usled svakodnevnih ljudskih aktivnosti. Pri sanaciji, nadogradnji i seizmičkom ojačanju objekata na postojeće drvene međuspratne konstrukcije najčešće se izlivaju betonske ploče koje se sprežu sa postojećom konstrukcijom. Beton u gornjoj ploči međuspratne konstrukcije povećava njenu krutost i time smanjuje setljivost na dinamičku pobudu. Potreba da se smanji dodatno opterećenje na postojeće temelje konstrukcije, dovodi do sve češće upotrebe lakog betona pri sanaciji i rekonstrukciji objekata. NJegovom primenom povoljni efekti sprezanja ostaju očuvani, a prednosti u vidu smanjenja dodatne težine dolaze do izražaja. Međutim, u dostupnoj literaturi, podataka o dinamičkom ponašanju spregnutih međuspratnih konstrukcija izvedenih sa lakim betonima malih zapreminskih masa nema, pa je cilj ovog rada bio da se ispitaju osnovne dinamičke karakteristike drvenih međuspratnih konstrukcija saniranih lakim betonom raznih zapreminskih težina. Analizirano je dinamičko ponašanje, odnosno sračunate su prve tri sopstvene frekvencije oscilovanja četiri međuspratne spregnute konstrukcije sa identičnim poprečnim presecima i spojnim sredstvima, ali sa različitom klasom zapreminske mase lakog betona u gornjoj betonskoj ploči. Preliminarnim laboratorijskim ispitivanjem relevantnih svojstava četiri vrste lakog betona, monolitnog drveta i spojnih sredstava, koja su se koristila pri sprezanju drveta i lakog betona, dobijeni su potrebni ulazni podaci za numerička istraživanja. Za potrebe analitičkog istraživanja predložena je jednostavna metoda dinamičkog proračuna sopstvenih frekvencija oscilovanja elastično spregnutih konstrukcija. Tačnost rezultata predloženog dinamičkog proračuna potvđena je eksperimentalno, i poređenjem sa rezultatima programskog paketa Ansys., This work has examines dynamic behavior of composite wood-lightweight concrete floor structures exposed to normal everyday human activities. In a case of restoration, renovation or seismic reinforcement, the most common approach is to pour concrete slabs onto existing wooden floor structures. Concrete in the upper zone of floor structure is increasing its stiffness and thus reducing the dynamic excitation. In the attempt to reduce the weight on the existing foundations, lightweight concrete is used for restoration and renovation. Coupling effect remains undisturbed but added weight is significantly reduced when lightweight concrete is used. However, the available literature does not provide data about dynamic behavior of the composite wood-lightweight concrete floor structures; therefore the goal of this research work is to examine the characteristics of the restored wooden structures coupled with lightweight concrete of the different specific weights. This research work analyzes dynamic behavior that included first three frequencies of four composite floor structures with identical cross sections and connections, but different specific weights of the lightweight concrete in the upper layer of the concrete slab. Preliminary laboratory testing of four different types of the lightweight concrete, wood and connections, yielded necessary data for the further numeric analysis. Simple calculation method for obtaining mode frequencies of the partial composite structures has been proposed, for the purpose of this analytical research. The accuracy of the proposed dynamic model has been confirmed through experimental research and compared up against the results from the Ansys