Papers by Author: Robert Jankowski

Abstract: During severe earthquakes, pounding between adjacent superstructure segments of highway elevated bridges was often observed. It is usually caused by the seismic wave propagation effect and may lead to significant damage. The aim of the present paper is to show the results of the numerical analysis focused on damage-involved pounding between neighbouring decks of an elevated bridge under seismic excitation. The analysis was carried out using a lumped mass structural model with every deck element discretized as a SDOF system. Pounding was simulated by the use of impact elements which become active when contact is detected. The linear viscoelastic model of collision was applied allowing for dissipation of energy due to damage at the contact points of colliding deck elements. The results show that pounding may substantially modify the behaviour of the analysed elevated bridge. It may increase the structural response or play a positive role, and the response depends on pattern of collisions between deck elements. The results also indicate that a number of impacts for a small in-between gap size is large, whereas the value of peak pounding force is low. On the other hand, the pounding force time history for large gap values shows only a few collisions, but the value of peak pounding force is substantially large, what may intensify structural damage.

Abstract: The present study aims to determine the effectiveness of a nonlinear mathematical model in simulating complex mechanical behaviour of a seismic isolation system to protect structures during strong and damaging earthquakes. In order to construct the Polymeric Bearings considered in this research, a specially prepared flexible polyurethane elastomer with increased damping properties has been used. The usefulness of the proposed mathematical model has been verified by comparing the dynamic response of a base-isolated single-storey structure model under various seismic excitations with the results obtained from the detailed numerical analyses where the previously examined experimental model has been considered as a multi-degree-of-freedom system. The results obtained from the numerical investigation using lumped-mass models have confirmed that the proposed nonlinear mathematical model can be successfully used to simulate the complex mechanical behaviour of the Polymeric Bearings in numerical analyses.

Abstract: A large number of accidents involving damage of temporary grandstands during different types of events were observed in the past. The aim of the paper is to analyse numerically the effectiveness of a polymer damper in damage reduction of a temporary steel scaffolding grandstand exposed to dynamic load due to jumping. The element has been installed as a diagonal one at the back part of the structure. The method has been compared with the typical solution of strengthening the grandstand with the diagonal stiffener of tubular cross section. The results of the study clearly show that the response of a temporary steel grandstand equipped with a diagonal damper as well as with the diagonal stiffener is substantially different. The application of the polymer damper leads to considerable reduction in the level of measured accelerations due to the increased structural damping preventing from damage.

Abstract: Earthquakes are the most unpredictable damaging loads which can affect civil engineering structures. Due to insufficient separation distance between adjacent structures with different dynamic properties, structural collisions may occur during ground motions. Although the research on structural pounding has recently been much advanced, the studies have mainly been conducted for concrete structures. The aim of this paper is to show the results of experimental investigation, focused on dynamic behaviour of closely-separated three models of steel structures which have been subjected to damaging earthquake excitations. The study was performed using three models of steel towers with different dynamic parameters and various distances between the structures. The acceleration time histories of the Kobe and the Northridge earthquakes were applied as the seismic excitation. The unidirectional shaking table, located at the Gdansk University of Technology (Poland), was used in the experimental study. The results have confirmed that collisions may lead to the increase in the structural response, although they may also play a positive role, depending on the size of the separation gap between the structures.

Abstract: Structural interactions between adjacent, insufficiently separated buildings have been repeatedly observed during damaging ground motions. This phenomenon, known as the structural pounding, may result in substantial damage or even total collapse of structures. The aim of the present paper is to show the results of the nonlinear numerical analysis focused on pounding between inelastic three-storey buildings under seismic excitations. The discrete lumped-mass numerical models of two building have been used in the analysis. The results of the study indicate that the response of the lighter and more flexible inelastic building can be substantially influenced by structural interactions, and collisions may even lead to the permanent deformation of the structure. On the other hand, the behaviour of the heavier and stiffer building does not really change considerably during the earthquake. The results of the study also indicate that incorporation of the inelastic behaviour of colliding buildings with different dynamic characteristics is very important for the purposes of accurate numerical modelling of pounding-involved structural response under damaging seismic excitations.

Abstract: Mutual pounding between structures during earthquakes may cause serious structural damage. The aim of this paper is to show the results of a shaking table experimental study focused on pounding between structures in series under several earthquake excitations. The experiments were performed using three tower models with different configurations and different gap distances between them. In the first stage of the study, one rigid tower was installed between two flexible structures, while later on, the flexible tower was located between two rigid structures. The results of the study show that pounding observed during damaging earthquakes might affect the behaviour of structures significantly. It was observed that the rigid towers are more influenced by pounding than the flexible structures. Moreover, the optimal gap size was found to be either the distance which prevents pounding (and therefore prevents from damage) or the zero gap.

Abstract: In this paper, the results of two experiments, focused on testing the effectiveness of a method of enhancing the seismic (dynamic) resistance of masonry columns with the use of flexible polymer adhesive, are shown. The first experiment was devoted to investigate the damping properties of a polymer working between two stiff layers, whereas the aim of the second one was to verify if the identified damping properties of the polymer can improve the dynamic behaviour of short masonry columns (prisms) strengthened by confinement made of GFRP grid bonded by the polymer adhesive. The results of the first stage of experimental investigation indicate that bonding two stiff elements with the analyzed polymer adhesive leads to the significant increase in overall damping properties. The results of the second experiment show that the GFRP grid confinement with flexible polymer adhesive is more effective than stiff epoxy and mineral adhesives.

Abstract: Pounding between insufficiently separated buildings, which may result in considerable damage or may even lead to the total collapse of colliding structures, has been repeatedly observed during earthquakes. Earthquake-induced collisions of buildings has been intensively studied applying various structural models. It was assumed in the analyses, however, that the seismic excitation is identical for all structural supports; whereas, in the reality, the ground motion differs from place to place due to spatial seismic effects connected with propagation of the seismic wave. The aim of the present paper is to conduct a detailed non-linear damage-involved analysis of pounding between two structures under non-uniform earthquake loading. A case of pounding between the Olive View Hospital main building and one of its stairway towers, observed during the San Fernando earthquake of 1971, has been considered in the study. In the numerical FEM analysis, non-linear material properties have been simulated using stiffness degradation (due to damage under cyclic loading) model of concrete and elastoplastic damage model of reinforcing steel. A method of conditional stochastic modelling has been used to generate the input ground motion records. The results of the study indicate that the incorporation of the non-uniform ground motion excitation may lead to substantial change of pounding-involved response of the structures. The difference between the uniform and non-uniform responses has been found to be relatively large considering the fact that the variation in the simulated input ground motion records was rather small. This shows the importance of incorporation in the damage-involved numerical analysis the effects connected with propagation of the seismic wave.

Abstract: The safety of civil engineering structures is one of the most important issues of building industry. That is why the assessment of the damage-involved structural response has recently become of major concern to engineers. Among a number of different approaches to diagnosis of damage, the method of measuring the changes in natural frequencies is considered to be one of the most effective indicators of global damage. From the practical point of view, the method has been successfully applied to relatively small structures, while in-situ tests on large structures, such as bridges, tanks or dams, are very difficult. The aim of the present paper is to show the results of the numerical analysis concerning the diagnosis of damage in a cylindrical steel tank with self-supported roof which is filled with liquid. The tests have been conducted for various stages of damage, introduced in the numerical model by reducing the stiffness of tank-soil system as well as by cutting the connection between the shell and roof of the tank as well as between roof elements. The results of the numerical analysis have shown the characteristic decrease in the natural frequencies for the case of tank-soil system with reduced stiffness, which is the global type of damage. On the other hand, cutting the welds, which can be considered as the local type of damage, has not lead to the differences in the natural frequency values, although differences in local deformations of shell in the vicinity of cuts have been observed in vibration modes.

Abstract: Base isolators, like lead rubber bearings, high damping rubber bearings or friction pendulum systems are extensively used in practice in many seismically active regions to protect structures against earthquake forces. The present paper reports the results obtained from the experimental study aimed to determine the effectiveness of the Polymeric Bearings in suppressing structural vibrations during dynamic excitations and therefore to minimize structural damage. The response of the analyzed two-storey structure model both fixed and supported by the Polymeric Bearings under different earthquake excitations was studied. The reduction in lateral response due to seismic events was measured by comparing the peak accelerations of the two-storey steel structure models – with and without base isolation system. The results of the study have shown that the use of the Polymeric Bearings leads to significant improvement in dynamic properties by suppressing structural vibrations. The results have indicated that the Polymeric Bearings can be successfully used to reduce damage of structures during seismic events.