 

Influence of ice accretion at bridge rope on Strouhal number
Pospíšil, Stanislav ; Gorski, P. ; Kuznetsov, Sergeii
The paper is concerned with the method and results of wind tunnel investigations of the/nStrouhal number (St) of a stationary iced cable model of cablesupported bridges with/nrespect to different angles of wind attack. The methodology leading to the experimental/nicing of the inclined cable model was prepared in a climatic section of the laboratory./nThe St values were determined within the range of the Reynolds number (Re) between/n2.4·104 and 16.5·104, based on the dominant vortex shedding frequencies measured in/nthe wake of the model.

 

Breathinginduced fatigue in thinwalled steel bridge construction
Škaloud, Miroslav ; Zörnerová, Marie ; Urushadze, Shota
Several years ago, the authors started an experimental investigation into the breathing phenomenon of the webs of thinwalled steel girders , which to date comprised more than two hundred tests. The cumulative damage behaviour generated by web breathing was analysed, the process of the initiation and propagation of fatigue cracks induced thereby was studied, and their impact on the failure mechanism and the fatigue limit state of the girders was determined. And, to conclude, a recommendation for design, taking account of the aforesaid, was established.


Aeroelastic divergence modeled by means of the stochastic resonance
Náprstek, Jiří ; Pospíšil, Stanislav
The divergence is one of the most important and dangerous phenomenon of aeroelastic postcritical states occurring at a prismatic slender beam in a crossflow. This phenomenon manifests by stable periodic hopping between two nearly constant limits perturbed by random noises. Experimental observation and numerical simulation motivates an idea to model this process as the effect of the stochastic resonance. Being observed and practically used in a number of disciplines in physics (optics, plasma physics, atd.) its mathematical basis follows in the most simple case from properties of the Duffing equation with negative linear part of the stiffness. The occurrence of this phenomenon depends on certain combinations of input parameters, which can be determined theoretically and verified experimentally in the wind tunnel.


Postcritical multimodal vibration of a continuous inverse pendulum type system
Náprstek, Jiří ; Fischer, Cyril
Autoparametric effects are processes endangering structures during a seismic attack. Tall structures exposed to a strong vertical component of an earthquake excitation nearby the epicenter can collapse due to autoparametric resonance effect. Vertical and horizontal response components being independent in the linear regime get into a complex interaction due to nonlinear terms in postcritical regime. Two generally different types of the postcritical regimes are presented: (i) postcritical state with possible recovery; (ii) exponentially rising horizontal response leading to a collapse. A special attention is paid to processes of transition from semitrivial to postcritical state in case of time limited excitation period as it concerns the seismic processes. Solution method combining analytical and numerical approaches is developed and used. Its applicability and shortcomings are commented. A few hints for engineering applications are given.


Stochastic resonance in a simple duffing equation
Náprstek, Jiří
The response of a dynamic nonlinear single degree of freedom system with cubic characteristic to a combination of additive random Gaussian white noise and external deterministic periodic force is studied. It is coming to light that under certain circumstances the jumping between wells of the bistable elastic potential can occur leading to interwell hopping. The frequency of this hopping can be for certain combination of input parameters nearly constant and corresponding to external periodic force frequency. This state leads to high ratio of periodic component amplitude and intensity of the random component of the response process. In such a case the system can be used as a harmonic selective resonator detecting the useful weak harmonic signal in the high noisy background.
