Michal Polák

An Experiment Focused on Fans Behaviour and Induced Grandstand Vibrations during a Football Match

The sport fans behaviour has changed in recent decades, especially on football and ice hockey stadiums in the Czech Republic. Currently the fans more support their sport teams with more intensive cheering like as jumping or swaying. The main results of an experimental analysis that was focused on fans behaviour and simultaneously on the induced vibrations of a selected cantilever grandstand sector during a football match are mentioned in the paper. The actual types of fans cheering, the changes in fans behaviour during the match, relative proportion of active and passive spectators, the features and the levels of grandstand vibrations were investigated in detail.

An Experimental Analysis of Grandstand Vibrations Caused by Crowd of Spectators during Two Football Matches

A significant dynamic load may be induced by synchronize movement of larger spectators group (like as jumping, bouncing, jouncing, swaying or abruptly rising) on a sport stadium during a sport event. This load could generate discomforting grandstand vibrations and in some extreme cases, the large grandstand vibrations are detectable by spectators even visually. A dynamic experiment is described in the paper which was realized on a steel cantilever grandstand during two football matches in the place where the most active spectators were concentrated. Vertical grandstand accelerations were observed in three points that were located at the ends of three grandstand main beams. The results of the experiment had shown predictably that the most excessive grandstand vibrations were induced by fans in the particular situations when the home team scored a goal and during the fans celebration after the matches especially. In these particular situations, the detected vibrations could be perceived by spectators as uncomfortable.

Experimental Evaluation of Tensile Forces in Short Steel Rods

There are a lot of structures in building and civil engineering where the significant structural elements are loaded by large tensile forces (e.g. tension bars of building structures). In many practical cases it is important to know the actual value of tensile forces in tensile structural elements for assessment of their reliability. The four experimental techniques are used for determination of tensile forces in practice most often. The vibration frequency method, which is one of them, is very suitable for experiments done only one time or sometimes, especially in cases when the examined structural elements are already activated and the application of an experimental method is necessary in this situation. The experiment described in this paper was focused on the tensile force determination in steel rods, which were very short and relatively stiff. The evaluated tensile forces of the investigated short steel rods were affected by a significant error when only the simplest models (the string model, the simply supported beam and the fixed beam) and measured natural frequencies were applied. In order to precise the determined tensile forces, the theoretical beam model supported by simple supports with torsion springs (“the elastically fixed beam”) and the measured natural modes of the rods had to be necessarily taken into account.

The Experimental Observation of the Prestress Forces in the Structural Elements of a Tension Fabric Structure

The periodic experimental observation of the prestress forces in some selected structural elements of the tension fabric structure is mentioned in this paper. The described experiment was focused to the tension fabric roof which covers the tram stop K Barrandovu in Prague. So far the observation was performed three times and it was every time aimed on three types of basic structural elements support cables anchored to the ground, edge cables placed in a fabric pocket and fabric membranes. The used non-destructive experimental procedures are based on the geometrical nonlinearity and the frequency method, the applied measurement system was developed by authors. The dependence of cable forces on temperature changes can be found out from the obtained results.

Detection of Cracks on Small Specimens Based on Modal Analysis

The paper presents the detection and localization of the induced cracks on small specimens based on a modal analysis. The cracks were induced to cement specimens of dimensions 40 × 40 × 160 mm in three different positions. Their position was detected using different techniques based on natural mode shape changes between virgin and damage states of the specimen. The sensitivity of these techniques to the damage level and position was investigated, compared and then discussed in the conclusion.

An Experimental Analysis of the Real Dynamic Behavior and the Reliability Verification of an Existing Footbridge

The investigated existing footbridge, on which the dynamic experimental analysis was concentrated, was built across Vltava River in Prague in 1984. Since putting into operation, the footbridge was apparently sensitive to dynamic excitation by passing pedestrians. Above that in the course of its existence the considerable part of the footbridge deck was overflowed by two great floods that happened in August 2002 and in June 2013. After the second flood, a visible damage on the footbridge deck was detected by a visual inspection. The described dynamic experiment was divided into two basic parts. The experimental modal analysis was carried out first and then the dynamic load test was realized. It was focused to the deck vibrations induced by different pedestrians groups. It was found out the footbridge deck vertical accelerations exceed the pedestrian comfort limit in some normal situations. On the basis of comparison between modes of natural vibrations, which were evaluated by the experimental modal analysis and the equivalent ones from previous dynamic experiments, it was validated that the discovered deck damage caused the partial variation of fundamental dynamic footbridge behavior.

An Experimental Analysis of Large Vibrations of Selected Cable-Stays on a Cable-Stayed Bridge

The cable-stayed bridge at the inner ring road in Prague in the Czech Republic was put into operation in 1997. The longest bridge span is supported by fourteen cable-stay pairs and next fourteen ones are symmetrically situated on the other side of the pylon. The cable-stays in the ten longest cable-stay pairs are tied together since the bridge construction to prevent vibrations of individual cables caused by wind. The experiment described in the paper was carried out because the large amplitudes of vibrations (about fifteen centimetres) were observed visually on some medium-length cable-stays in April 2015. The experimental analysis was focused on the nine day long continual observation of the selected six cable-stays vibrations during the standard bridge operational state. The analysis of probable causes and the recommended measures to reduce observed large vibrations are mentioned in the paper.

Diagnostics of an Existing Cable-Stayed Bridge by Means of the Dynamic Load Test

The cable-stayed bridge at the inner ring road in Prague in the Czech Republic was put into operation in 1997. In 2008 a crack was found in the bridge structure area where the end beam was connected with the upper deck of the box section and where the bridge expansion joint system was anchored. The basic objective of diagnostic works, which were started immediately after the finding of the crack, was obtaining the basis for bridge structure modifications, among other things, to prevent formation of similar damages in the future. A series of diagnostic methods was used for determination of the bridge actual state and the real static and dynamic bridge behavior. One of these methods was the dynamic load test. It was focused on an examination of the bridge forced vibration which was caused by dynamic effects of a usual traffic flow. An arrangement of the test was not quite usual because the experiment was concentrated on dynamic behaviour of bridge support areas especially. The abutment area, the pylon area and the area of a pillar with the bearing with a tensile structural element were observed on the investigated bridge. The dynamic load test was performed in two stages. The first stage was focused on investigation of bridge dynamic behaviour in original structural conditions and on obtaining the basis for design of bridge adjustments. The second stage was realized after bridge reconstruction in December 2012 and verified the effectiveness of bridge modifications.

Damage Detection and Localization on Cement Specimens

This paper is focused on cement specimen testing by impact excitation non-destructive technique. The impulse excitation method was used for measuring of the natural frequencies and modes of longitudinal, transversal and torsional vibration of the specimens. The objective was to find dynamic properties of the specimens without a crack, with a crack and with a healed crack by cement paste and based on their comparison detect and localize the crack.

Human-Induced Vertical Vibration of the Footbridge across Opatovicka Street

The presented article is focused on a theoretical dynamic analysis of the footbridge across the Opatovicka street in Prague, which acts as a simply supported beam. The structure was loaded by an ordinary pedestrian traffic, synchronous runners and vandals. The ordinary traffic was simulated by the Monte Carlo method as a stream of moving periodic forces with stochastic parameters. The synchronous runners and vandals were modeled as a combination of biomechanical models of human body, which influenced the structure vibration only passively, and driving forces, which loaded the structure in the contact points between the human body models and the structure. The driving force was the time – dependent function based on decomposition to the Fourier’s Series and periodic triangle function. The obtained theoretical results are compared with experimental data.