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Dynamics of a vibration damperworking on a principle of a heavy ball rolling inside a spherical dish
Náprstek, Jiří ; Fischer, Cyril ; Pirner, Miroš
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 semispherical 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 modeled as a holonomous system. Using Lagrange equations of the second type, governing non-linear differential system is carried out. 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.
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Experimentální studie dynamiky chůze ve vertikálním a horizontálním směru
Pirner, Miroš ; Urushadze, Shota
The authors have noticed the newest observations and few analysis of excitation mechanism. They suppose that the import knowledge of the forces frequencies of step or strides for different walking velocities is the most important for the further analysis and analysis of the mechanisms. The authors received the vertical force dependence on the walking velocity, on stride length and on the weight of pedestrian. The new research step is focused onlateral horizontal forces.
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Basic information serving to developed qualitatively new codes corresponding to probabilistic assessment exposed to dynamic loaded structures
Pirner, Miroš ; Fischer, Ondřej ; Marek, Pavel ; Urushadze, Shota
The first condition for a reliable structure is a well-judged design, correctly and carefully calculated, and based on authentic input values. So fare we were used to take these values from the Codes and in the case that they can very accidentally, this variability can be respected by means of a coefficient. A more realistic approach is to take into account directly the random nature of the property in question, and to describe it in probabilistic terms (by a histogram). This can be done with the use of SBRA (Simulation Based Reliability Assessment) method. Here some considerations are shown, how the actual Czech Codes should be reorganized for probabilistic design, in the case of dynamic civil engineering problems. In this paper the problems of loading are treated; the problems of structural response and some special examples are shown in the full text on CD.
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Influence of aging of concrete on the dynamic response of structures
Fischer, Ondřej ; Pirner, Miroš
Downtrend of the fundamental natural frequency of a structure with time, detected by periodical diagnostic in situ measurements, can signal damage, starting anywhere in the structure, or the progressing degradation of its material. The influence of aging (creep) of concrete on E-modulus is examined in the paper, using the data found on a real chimney during the first 7 years of its existence. An attempt to take the uncertainty of the measured input data for the prognosis of the future behaviour of the structure into account with the use of the SBRA method, is given.
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Experimentally obtained values and their elaboration using SBRA-method
Pirner, Miroš
The article is aimed to inform about the knowledge accumulated in the field of dynamic experiments and identification of condition of concrete bridges, attention is primarily given to the method of Simulation Based Reliability Assessment (SBRA-method). The second part informs about application of this method in the field of aerodynamics of civil engineering structures.
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