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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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Experimental and numerical verification of vortex-induced vibration of hangers on the footbridge
Urushadze, Shota ; Pirner, Miroš ; Pospíšil, Stanislav ; Král, Radomil
The paper deals with vortex-induced oscillations of footbridge hangers at a very low wind speed. The two-spans (2x50m) reinforced concrete structure with the central pylon (25m height) is designed as the harp system of hangers with circular cross-section. The oscillations were observed at the low wind speed and the high double amplitudes has been measured. The wind-excited deformation shape corresponded to the 6th, 7th and 8th natural modes of a hanger. The individual hanger as well as thein group were analysed to estimate the dynamic response, possible aeroelastic vibration and special interference or coupling effects. It also includes proposal for the elimination of the extensive vibration. The analysis is complemented by numerical investigation using Computational Fluid Dynamics methods.
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Applicability of existing indexes of non-proportionality of damping in case of theoretical model of slender structure with installed TMD
Hračov, Stanislav ; Pospíšil, Stanislav ; Náprstek, Jiří
The paper analyzes an applicability of to date published indexes of non-proportionality in the case of a linear viscously damped numerical model of slender structure equipped with tuned mass damper (TMD). The installation of TMD into the structure not only reduces the level of undesired vibration, but it can also cause due to damping element of TMD a significant increase of damping nonproportionality. The paper recommends the most suitable indexes for such a type of structure and points out to impropriety of the others. The point of view of is also focus on the validity of the existing criterions for neglecting of non-diagonal terms of a modal damping matrix. Only indexes and criterions based on the properties of the modal damping matrix were taking into account.
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Analysis of eigen-values of viscously damped two degrees of freedom system due to changes of parameters of secondary subsystem
Hračov, Stanislav ; Náprstek, Jiří ; Pospíšil, Stanislav
The paper deals with an analysis of non-classically damped two degrees of freedom system from the point of view of eigen-values. The theoretical relations between the system parameters for the case of multiple complex eigen-values were derived. The parametric problem was carried out analytically and graphically. The influence of the relative frequency tuning on the overdamping of individual eigen-values with increasing of the secondary damping was studied. The overdamping intervals were defined with respect to the mass ratio and the frequency tuning. The boundaries between respective overdamping intervals represent among others the frequency tuning of system with multiple complex eigen-values. The phase between components of eigen-modes and its relation to the eigen-values were examined with respect to the secondary damping increase.
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