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Approximate calculation of eigen-values of linear viscously damped system with passive damping element
Hračov, Stanislav
The paper presents an approximative method for eigen-solution of non-classically damped linear system representing classically damped structure equipped with passive damping element (viscous damper). The proposed procedure avoids using a numerically demanding state-space approach. It operates in the original dimension of the problem and utilizes the real eigen-modes for its further size reduction. The method is based on the dividing of the damping matrix to classical and non-classical part and the application of the perturbation strategy. The accuracy of the procedure is demonstrated by considering numerical examples.
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Dynamics of beam pair coupled by visco-elastic interlayer
Náprstek, Jiří ; Hračov, Stanislav
Two parallel slender beams with visco-elastic interlayer can serve as a relevant mathematical model of a number of engineering systems. A typical example is a double skin facade of tall buildings. The outer skin can be considered as a dynamic damper and thus contribute to comfort inside the building. Many applications emerge in power piping when using two coaxial pipes with an interlayer in order to suppress vibration due to flow and structure interaction. Similar inconvenient behavior can exhibit panels which fall into the flutter post-critical state. Further applications can be expected at industrial chimneys, towers, etc. In principle the idea of vibration damping using such a formation is inspired by very well known tuned mass damper. However its internal structure, function, limitations and singular states are incomparably more complicated and therefore adequate applications should be cautious.
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Dynamic analysis of tall slender structure equipped with passive absorber and founded on various subsoils using simplified model
Hračov, Stanislav
The paper is focused on the investigation of dynamic behavior of tall slender structures taking into account the soil-structure interaction effect (SSI) and a presence of a tuned mass damper (absorber). The main aim is to qualitatively and quantitatively assess effects of both phenomena on changes in basic dynamic properties and response of the structure. The real tower equipped with absorber is chosen and analyzed. Analytical and numerical analysis of modal properties and of the response of a simple discrete model are performed. The structure and absorber are represented by one degree of freedom, the interaction interface by spring-dashpot system. This simplified model together with multi degree of freedom discrete model is analyzed parametrically for different dynamic properties of subsoil to cover all possible dynamic effects. The influence of neglecting of both phenomena is quantified and suggestions for assessments of such a type of structures are proposed.
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Modal analysis of tall slender structures using software GMAST
Hračov, Stanislav ; Pospíšil, Stanislav ; Brůna, Tomáš
The paper is concerned with a verification of modal properties of tall slender structures computed by recently developed software GMAST. Software GMAST determines eigen-frequencies, eigen-modes and corresponding modal quantities of stacks, towers, guyed masts and similar civil engineering structures using slope-deflection method. A numerical model is based on an idealization of the structure as a continuous 2D beam with continuously distributed parameters on elastic supports. Shaft of structure is divided into sections which could be considered as prismatic i.e. with constant unit mass, bending stiffness and static axial force. The effect of guy ropes is introduced in a form of elastic supports with a frequency dependent stiffness. The applicability of the software in praxis is presented on an example of existing structures, for which the numerically and experimentally identified modal properties are compared.
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