Andrzej Świercz’s research while affiliated with Polish Academy of Sciences and other places

The subject of research is a steel arch-tied bridge at a high-speed railway line in Poland. After the construction was completed, aresonance phenomenon was observed at the bridge, consisting of the occurrence of intense (visible to the unaided eye) undamped vibrationsof some vertical hangers in the horizontal direction, transverse to the track axis. These vibrations occurred without the presence of a railwayload on the bridge. Before the bridge was put into operation, an acceptance static and dynamic load test was performed, and then the bridgedeck vibrations were monitored for a year. The research during dynamic loads testing included both quasi-static (10 km/h) and high-speed(200 km/h) testing train passages. The vertical displacement measurements were carried out in three cross sections of the span, and the ac-celeration of vibrations on girders and selected hangers was also measured. Next, an innovative system for determining displacements indi-rectly using inertial sensors (inclinometers and accelerometers) was used for bridge deck vibration monitoring. The primary aim of theresearch was to investigate the possibility of assessing the safe operation of the bridge using a monitoring system consisting of a limitednumber of inertial sensors. The second aim was to verify the feasibility of calibrating the numerical model based on the results of dynamicload testing. Numerical analyses of the behavior of the bridge during the passage of trains with speeds up to 200 km/h were carried out. Thedeveloped and calibrated numerical model provides additional information about the overall structural vibrations, facilitating the interpreta-tion of outcomes of the monitoring system. No significant impact of hanger vibrations on the monitored displacements and accelerations ofthe bridge deck vibrations during the passage of trains was found.

In this contribution, we propose two novel concepts of an adaptive ultra-light high-altitude aerostats. Both proposed concepts enable changes of aerostat volume and shape during the flight, which results in the possibility of vertical motion control. The first proposed construction is telescopic aerostat with multi-segmented construction and controllable segments’ couplings. In turn, the second construction is based on a self-deployable tensegrity structure equipped with pre-stressed and tensioned elements of controllable lengths. Basic advantages and features of the proposed design are shown using dedicated numerical examples.KeywordsHelium-filled aerostatsTelescopic constructionTensegrity structuresFlight control

Helium-filled aerostats can be lifted by the traditional hot-air balloon (SKYLAB), in the compact form, and then, after reaching some starting level ${H}_{1}$, helium can be pumped into the aerostat (using storage tank with a compressed gas), starting its autonomous mission. Optimal design process of aerostats dedicated to perform determined mission (between the altitudes ${H}_{1}$ and ${H}_{2}$) will be discussed with preliminary experimental verification process, based on scaling procedure of aerostat mini-model, so-called CANDY, and fly-tests on SKYLAB will be proposed.KeywordsHelium-filled aerostatsFlight-testsMini-modelsScaling procedure

In this paper, the authors propose, investigate, and discuss a concept of novel type of deployable helium-filled aerostat as a low-cost mean of transport. Internal construction of the aerostat is based on ultra-light tensegrity structure equipped with prestressed tensioned elements of controllable lengths. Such tensegrity structure allows for adaptive morphing of the aerostat understood as simultaneous controllable modifications of aerostat volume and shape during the flight. The controlled volume changes enable influencing buoyancy force and obtaining desired vertical motion during the ascending and descending process. In turn, external shape changes allow for lowering the aerodynamic drag and energy usage needed to uphold stable horizontal position or maintain the desired flight path. Moreover, such internal structure allows for convenient storage, transportation and deployment of the aerostat construction on the ground or in required point at the atmosphere. The article presents an analysis of the exemplary operational mission of the aerostat. The authors introduce the mechanical model capturing interaction of the internal tensegrity structure and aerostat envelope based on the finite-element method, as well as dynamic model allowing for simulation of the aerostat’s vertical and horizontal motion influenced by buoyancy and drag forces. Both these models are used to positively verify the feasibility of the proposed concept of deployable tensegrity-based aerostat with adaptive morphing and its efficiency in realization of the assumed flight mission.

Measurements of displacements of bridges under dynamic load are particularly difficult in the case of structures where access to the area under the tested structure is impossible. Then, remote measurement methods are preferred, such as interferometric radar. Interferometric radar has high accuracy when measuring displacement in the direction of its target axis. The problems appear when a bridge vibrates in two directions: horizontal (lateral or longitudinal) and vertical. The use of one radar to measure those vibrations may be impossible. This paper presents the application of a set of two interferometric radars to measure vertical vibration and horizontal longitudinal vibration with high accuracy. The method was positively verified by experimental tests on two railway bridges characterized by different levels of horizontal displacement. The accuracy of the radar measurements was tested by the direct measurement of vertical displacements using inductive gauges. In conclusion, in the case of vertical displacement measurements using one interferometric radar, the influence of horizontal displacements should be excluded. In the case of locating radars at the area of bridge supports, it is necessary to either use a set of two radars or first investigate the magnitude of possible horizontal displacements in relation to vertical displacements.

Performance of any Structural Health Monitoring (SHM) system strongly depends on a set of sensors which are distributed over the structure under investigation. Optimal deployment of sensors on large scale structures such as tied-arch bridges is quite a challenging problem. Moreover, deployment of a sensor network consisting of different types of sensors (accelerometers, inclinometers or strain gauges) over a large scale bridge renders the task of optimization even more demanding. In the present study, a conventional sensor placement method for distribution of a homogenous sensor network is expanded to the heterogeneous case. First, the basic equations governing the estimation error will be recalled. Then, the Fisher information matrix is assembled using normalized translational and rotational mode shapes. Finally, a computational procedure is proposed which allows optimal sensor positions to be selected among thousands candidate locations. The effectiveness of the proposed strategy is demonstrated using a realistic example of a tied-arch bridge located in Poland.

This study presents and tests a method for semi-active control of vibrations in sandwich-type beam structures. This method adapts a strategy called prestress accumulation release. The prestress accumulation release strategy is based on structural reconfiguration: it uses short time, impulsive and localised changes of actuator properties (such as stiffness or damping), which are applied to a part of the system in the moments, when its strain energy attains a local maximum. The method has been earlier applied as a global control scheme to mitigate the fundamental vibration mode of a cantilever beam (by stiffness control) and in the task of mitigating the first four modes of a frame structure (by damping control). This study proposes a prestress accumulation release strategy and tests its effectiveness for the case of a three-layered sandwich structure, with the internal layer fabricated from a material with dissipative characteristic locally controllable through the material damping coefficient. In contrast to the earlier research, the control is applied thus at the level of material characteristics instead of a discrete set of dedicated actuators. Based on the finite element method, a numerical experiment involving a passively damped, as well as prestress accumulation release-controlled, three-layered cantilever beam excited by initial displacements was performed. The effectiveness of the approach was studied for a broad range of internal layer damping parameters. The presented results revealed a high potential of the prestress accumulation release strategy in semi-active damping of vibrations of sandwich-type structures.

The success of virtually all structural health monitoring (SHM) methods depends on the information content of the measurements, and thus on the placement of the available sensors. This paper presents an efficient method for finding optimal sensor distribution over structural system with many degrees of freedom (DOFs). The objective function is based on the classical Fisher information matrix. Originally, this yields a computationally hard discrete optimization problem. However, the proposed numerical solution method is based on a concept taken from structural topology optimization, where a discrete optimization problem is replaced with a continuous one. Two numerical examples demonstrate the effectiveness of the proposed methodology. These are a 5-bay truss with 24 DOFs and a two-story frame structure whose finite element model has been condensed to 14 DOFs.