Miroš Pirner

Actual behaviour of a ball vibration absorber

This paper describes the theory, experiments and practical application of the ball vibration absorber for horizontal movement, as well as its efficiency in comparison with that of the pendulum absorber. Two prototypes of such absorbers were recently installed on small TV towers in the Czech Republic and their dynamic behaviour examined. The laboratory test of an absorber of actual dimensions confirmed its efficiency.

Wind pressure fluctuations on a cooling tower

The results of model and in situ measurements of the fluctuating wind pressures over a cooling tower are presented. On the model, wind pressures were detected by means of two Bruel-Kjaer microphones at Re = 5.04 × 105; the pressure transducers used in situ were identical to those used in measurements on the Post Office Tower, London (Gt. Britain), at Re = 3.48–5.22 × 107. The power spectral densities obtained from the model and in situ measurements are in good agreement. Great attention was given to the decay of the coherence function above a certain reduced frequency the value of coherence ceases to diminish. Values of normalised spectral densities and coherence functions are recommended for theoretical calculations of the dynamic response of a cooling tower.

Long-time observation of wind and temperature effects on TV towers

Abstract This paper is concerned with the application of available data to the determination of service life of antennae towers and masts, the important part of which is the GRP cylindrical extension. In some cases, they have already been in service for nearly 30 years, so the question of their remaining lifetime is important. Long-term observations were made on one of these extensions in 1993–94, the measured data being registered and classified according to their magnitude, using the rain-flow method. The application of the Palmgren-Miner criterion has given the possibility of estimating the remaining service life of the structure.

Aeroelastic characteristics of a stressed ribbon pedestrian bridge spanning 252 m

This paper describes experiments with an aeroelastic model of a suspended footbridge. Its sensitivity to the effect of vortex separation at wind velocities from 15 to 17.5 ms−1 was ascertained. Instability of the flutter type was not observed. To reduce the amplitude of flexural vibrations dampers were designed to be installed in the railing posts.

Non-linear Model of a Ball Vibration Absorber

Wind excited vibrations of slender structures such as towers, masts or certain types of bridges can be reduced using passive or active vibration absorbers. If there is available only a limited vertical space to install such a device, a ball type of absorber can be recommended. In general, it is a semi-spherical horizontal dish in which a ball of a smaller diameter is rolling. Ratio of both diameters, mass of the rolling ball, quality of contact surfaces and other parameters should correspond with characteristics of the structure. The ball absorber is modelled as a holonomous system. Using Lagrange equations of the second type, governing non-linear differential system is derived. The solution procedure combines analytical and numerical processes. As the main tool for dynamic stability investigation the 2nd Lyapunov method is used. The function and effectiveness of the absorber identical with those installed at the existing TV towers was examined in the laboratory of the Institute of Theoretical and Applied Mechanics. The response spectrum demonstrates a strongly non-linear character of the absorber. The response amplitudes at the top of a TV tower with ball absorber were reduced to 15–40 % of their original values.

The Response of Masonry Barrel Vaults to Repetitive Static and Dynamic Loads and Example of Rehabilitation of Historic Barrel Vaults

The article will present the main results of experimental and theoretical research into non-reinforced barrel vault constructions and barrel vault construction reinforced with composites based on high-strength fibres and epoxy resin, or special polymer cement mortar. Vaulted constructions of historic and heritage buildings are extremely sensitive to deformations of the supporting construction, and their response to seismic effects may often be accompanied by cracking and mechanical vault failures. The results of research and study into the dynamic behaviour of barrel vaults brings new knowledge applicable in the prevention of vault failures in regions with an increased intensity of natural or technical seismicity, for the identification and localisation of failures using e.g. MAC or COMAC criteria. It can also be valuable in their serviceability assessment and service life extension. The second part of the article presents the results of theoretical and experimental analysis and a practical example of the stabilisation and reinforcement of 16 extensively damaged barrel vaults (with a 3.05 m span) with large lunettes situated over the cloister in the Premonstrate Monastery in Tepla (built in the 16th century), located in a seismically active region of West Bohemia.

The Influence of Bell’s Swaying on Neo-Gothic Cathedral

One of the negative aspects of technological progress is the threat to historical heritage by the impacts of technical seismicity. Dynamic load represents a significant threat to the safety and durability of historic buildings. The long-term effects of these types of loads lead to degradation of materials and connections, cause the occurrence of cracks and can increase the size of these cracks, and potentially lead to large-scale damage. Loads which would be safe for undamaged structures could become critical. This risk is present for various types of historic buildings; differences may occur due to the condition of walls, the amount of dynamic load and maintenance quality – which also includes the timely identification of damage (building diagnostics) and subsequent timely (and hence not too expensive) repairs. The aim of this paper is minimizing the effect of the bell’s ringing on the Church.

Vortex-Induced Vibration of Stay Cables, Verification on the Footbridge

Long-span footbridges rank among engineering structures with a high social significance. Their architecture design must in general satisfy matters such as attractive look, reliability and light. These requirements predestinate the bridge to be sensitive to the wind effects and the assessment against the wind becomes crucial. One of the several wind phenomena occurring on such a type of structures is termed as the vortex shedding. This flow induced excitation lying in a periodic vortices separation may arise at low wind velocity on both the main parts of a bridge structure including bridge girder with pylons and additional members such as stay cables, hand railing, etc. According to the character and intensity of the excitations, the adequate treatment to reduce or avoid the oscillations should be given. It is also performed when the vortex induced oscillations do not influence the reliability of a footbridge, but the look at the amplitudes could start panic and could start vandal’s action – increasing the amplitudes.

Wind Tunnel Experimental Study of Coupled Rocking-Swivelling Model of Guyed Mast Shaft

Systematic monitoring of rotational vibration (both torsion and swivelling) of guyed mast shafts has been performed in ITAM since 2005. The occurrence of this phenomenon is conditioned by the fact that the guy ropes are attached to the construction of the shaft, i.e. out of its axis. The simple static calculation model serves for making the proof of the occurrence of the torsional moment, affecting the shaft, which is guyed by three ropes. The exact theoretical solution of the real phenomenon assumes the introduction of dynamics of guy ropes, which vibrate in 3D shapes during the shaft’s movement along the orbit and it ́s torsion (swivelling).

Experimental Study of Torsion of Guyed Mast Shaft

The torsion of the mast shaft has been monitored on several masts in the Institute of Theoretical and Applied Mechanics (ITAM) since 2005. The occurrence of this phenomenon is conditioned by the fact that the guy ropes are attached out of the axis of the mast shaft. An experimental mast-shaft model of 15 m height guyed in 3 directions was used for the more detailed study of its rotational motion. The excitation was realized by transversal auxiliary rope drawn by harmonic human force in resonance. The displacement of the shaft top results in translation caused by the guy force and the rotation, caused by the eccentricities of the guy-ropes, attached out of the shaft axis. The resulting movement of the shaft support is an elliptical orbit. The shaft of our experimental mast was equipped in two levels with accelerometers for registration of horizontal x, y motions. The two x, y signals in two levels were monitored and analysed.