Damir Lazarević

Experimental and Numerical Analysis of Traditional Column Connections with the Possible Retrofit Concept

Several historical constructions in Croatia have permanent column problems, particularly at connections with capitals and bases, characterized by specific fractures. Experimental and numerical analysis of traditional column connections were conducted. Distinct fractures concentrated around the connections, as well as the traces of previous restorations noted by the chronicles, intrigued us to conduct research of the possible fundamental deficiency of the structural system. Traditional connections can be described as carefully smoothed stone contact areas, joined with centrally placed iron dowel fixed with a lead infill in a slightly larger hole. The comprehensive numerical and experimental efforts have shown that even a small imperfection in the construction process or a disturbance of bearing system have consequence, the contact leans to one side. As the joint has a very limited ability to compensate rotations, the relative rotation between column elements causes high local stresses at the edge of a column, which eventually results in numerous fractures. Ignoring or not fully appreciating the real nature of connections leads to significantly different stress distributions and orientations of thrust lines which may grossly overestimate the safety factor. This usually leads to inadequate rehabilitation, but also if no action is taken continuous fracture propagation can endanger local or global stability of the structure. Laboratory tests were performed on stone samples provided from traditional quarries and we tried to restore a rather similar level of stress and stress distribution caused by eccentric forces. Although some results will be presented, final objective will be to obtain the value of rotational stiffness and stress – strain diagram for connections, which is crucial for understanding of the load carrying mechanism, numerical treatment of connections and adequate retrofit strategies. In order to ensure serviceability and durability, we have started to test the possible retrofit concept with a layer of lead placed between two connected areas. The material such as lead enables small rotations and protects connected areas from large stress concentrations. Lead has already been used to fill the dowel holes and the additional horizontal thin layer appears to be an appropriate and almost invisible intervention.

Decision support model for seismic strengthening technology selection of masonry buildings

When the decision has been made to seismically strengthen a building, in order to mitigate possible damaging effects of an earthquake, choice of seismic strengthening is entirely left to the expert. The choice is often made without any specific analyses of the need and implementation possibilities, but depends completely on the expert’s knowledge and experience. The article presents the development of the innovative approach to strengthening technique selection for existing buildings that incorporates all important elements that influence the strengthening selection process. The presented model is simple, quick, precise allowing analytically supported decision making. Although the developed model is designed particularly for masonry buildings, decision support model for strengthening technique selection is adaptable to any building type. technique selection for existing buildings that incorporates all important elements that influence the strengthening selection process. The presented model is simple, quick, precise, allowing analytically supported decision making. Although the developed model is designed particularly for masonry buildings, decision support model for strengthening technique selection is adaptable to any building type.

Influence of spatial variability of ground motion on seismic response of bridges

U radu je prikazan pristup seizmickoj analizi mostova pri prostorno promjenjivoj potresnoj pobudi. Objasnjen je fenomen prostorne promjenjivosti potresa, utjecaj na odziv mostova i razlike u odnosu na istodobnu pobudu oslonaca. Detaljno je opisan model takve pobude, s osvrtom na postupke tvorbe prostorno promjenjivih zapisa. Prikazane su i numericke metode proracuna, ucinkovite za rjesenje ovoga problema. Opisana metodologija je primijenjena na seizmicku analizu lucnog mosta. Rezultati analize su pokazali nepovoljan ucinak prostorne promjenjivosti pobude na odziv vecine razmatranih velicina mosta.U radu je prikazan pristup seizmickoj analizi mostova pri prostorno promjenjivoj potresnoj pobudi. Objasnjen je fenomen prostorne promjenjivosti potresa, utjecaj na odziv mostova i razlike u odnosu na istodobnu pobudu oslonaca. Detaljno je opisan model takve pobude, s osvrtom na postupke tvorbe prostorno promjenjivih zapisa. Prikazane su i numericke metode proracuna, ucinkovite za rjesenje ovoga problema. Opisana metodologija je primijenjena na seizmicku analizu lucnog mosta. Rezultati analize su pokazali nepovoljan ucinak prostorne promjenjivosti pobude na odziv vecine razmatranih velicina mosta.

Reconstruction of Rector’ s Palace atrium in Dubrovnik: a key role of column connections

Rector’ s Palace is one of the most valuable heritage buildings in the historic Old town of Dubrovnik and the world famous setting for concerts and exhibitions. The Palace Atrium is surrounded by two stories structure comprising quadripartite masonry vaults supported by masonry walls and stone columns. Historically, this structure has suffered numerous collapses and damages and restorations were frequently made. Despite this, many distinct fractures are concentrated around the traditionally made column connections with capitals and bases. Carefully smoothed stone contact areas are joined with iron dowel, which is centrally placed in a slightly larger hole, with a lead infill. These contacts do not have sufficient rotational stiffness and the ability to transmit eccentric loads. Therefore, high edge stresses and fractures at connections are unavoidable. With time, the small rotational stiffness of joints becomes even smaller and the behaviour is approaching one of the hinge joint. This effect may endanger the global stability of this structure, as only three aligned hinges are sufficient to render the system unstable, i.e. the system is “ almost” statically determinate and has only a small reserve in bearing capacity. Various retrofit strategies for column connections can cause considerably different stress distribution and by ignoring or underrating these effects, the safety factor can be seriously overestimated. This paper confirms the above assertions using a range of computational models: from simple frame and shell models to complex 3D solid models. This work also points out the significance of connections in historical buildings and attempts to investigate possible retrofit concepts. Numerous retrofit solutions were considered and results show a significant influence of this small detail of the bearing system on the global behaviour and stability of structure.

Modelling of Tram-Track Interaction

The paper deals with numerical model of a tram moving on the track. The rail is modelled as elastically supported continuous beam with lumped masses. A vehicle is approximated as a discrete system consisting of three rigid masses connected with linear elastic springs. Dissipation mechanisms in the system are modelled with the linear dashpots. Two basic assumptions that are used are: a) the Euler-Bernoulli beam theory and b) the separation of the wheel from the rail is not permitted. The system of differential equations of motion is solved by an implicit version of step by step method based on the direct integration technique. The algorithm is conceived on the first order predictor - (re)corrector method. The contact condition is modelled by the force method. Results of the calculations are compared with the results of measurement on the tram track. Numerical predictions for displacements and accelerations are in good agreement with the experimental findings

Numerical simulations of the railway ballast

Degradation of the railway ballast occurs in the classical railway structure after a certain time of use. Several factors influence the life span of the ballast, such as quality of the stone material, quality of subgrade, sleepers, loadings of the structure, etc. In order to determine and predict the behaviour of the ballast, we need to define the load transfer path and interaction of the ballast particles. The numerical simulations are usually carried out in order to determine the characteristics of the loaded structure. Discrete element method (DEM) has several advantages in use for research of the granular material such as railway ballast. Several simplifications of the railway system in the numerical models affect the accuracy of the simulation results in different ways. This review paper of the numerical simulations on ballast behaviour, presents that results of the simulations are in a good agreement with experimental investigation results, therefore the use of numerical models for prediction of railway ballast behaviour is justified.

Iterated Ritz Method for solving systems of linear algebraic equations

U radu je opisano stanje razvoja nove iteracijske metode za rjesavanje sustava linearnih algebarskih jednadžbi. Metoda je pogodna za izrazito velike sustave slabo popunjenih matrica. Osim vlastitih obilježja, posjeduje i svojstvo opcenitosti, jer su mnogi iteracijski postupci samo poseban slucaj ovoga pristupa. Algoritam je realiziran samostalno, a potom je pridružen programu otvorena koda FEAP. Provedene su i raznolike provjere, posebice na prakticnim modelima. Premda je postupak tek djelomice istražen, vec pokazuje dobre rezultate.

Učinak prostorne promjenjivosti potresne pobude na seizmički odziv mostova

U radu je prikazan pristup seizmickoj analizi mostova pri prostorno promjenjivoj potresnoj pobudi. Objasnjen je fenomen prostorne promjenjivosti potresa, utjecaj na odziv mostova i razlike u odnosu na istodobnu pobudu oslonaca. Detaljno je opisan model takve pobude, s osvrtom na postupke tvorbe prostorno promjenjivih zapisa. Prikazane su i numericke metode proracuna, ucinkovite za rjesenje ovoga problema. Opisana metodologija je primijenjena na seizmicku analizu lucnog mosta. Rezultati analize su pokazali nepovoljan ucinak prostorne promjenjivosti pobude na odziv vecine razmatranih velicina mosta.U radu je prikazan pristup seizmickoj analizi mostova pri prostorno promjenjivoj potresnoj pobudi. Objasnjen je fenomen prostorne promjenjivosti potresa, utjecaj na odziv mostova i razlike u odnosu na istodobnu pobudu oslonaca. Detaljno je opisan model takve pobude, s osvrtom na postupke tvorbe prostorno promjenjivih zapisa. Prikazane su i numericke metode proracuna, ucinkovite za rjesenje ovoga problema. Opisana metodologija je primijenjena na seizmicku analizu lucnog mosta. Rezultati analize su pokazali nepovoljan ucinak prostorne promjenjivosti pobude na odziv vecine razmatranih velicina mosta.

NUMERICAL MODEL OF TRAM-TRACK INTERACTION WITH FAILURES ON RAIL SURFACE

The paper deals with numerical model of a tram moving on the track. The rail is modelled as an elastically supported continuous beam with lumped masses. A vehicle is approximated as a discrete system consisting of three rigid masses connected with linear elastic springs. Dissipation mechanisms in the system are modelled with the linear dashpots. The input parameters are obtained from the manufacturer and from measurements in the laboratory and in situ. Two basic assumptions that are used are: a) the Euler-Bernoulli beam theory and b) the separation of the wheel from the rail is not permitted. The system of differential equations of motion is solved by an implicit version of step by step method based on the direct integration technique. The algorithm is conceived on the first order predictor – (re) corrector method. The contact condition is modelled by the force method. Results of the calculations are compared with the results of measurement on the tram track. Numerical predictions for displacements and accelerations are in good agreement with the experimental findings.