Filtern
Erscheinungsjahr
- 2015 (17) (entfernen)
Dokumenttyp
Schlagworte
- Elastic track elements (1)
- Force transfer (1)
- Geometric trackbed irregularities (1)
- Ground vibration (1)
- Multi-beam-on-support model (1)
- Parametric excitation (1)
- Rail pad (1)
- Railway (1)
- Reduction (1)
- Sleeper pad (1)
Eingeladener Vortrag
- nein (5)
Ground vibrations due to different technical sources are analysed in theory and experiment for the dispersion of Rayleigh waves and the admittance spectra. Both tasks are theoretically based on the same concept: The admittance function in frequencywavenumber domain yields the dispersion as its maxima, and the admittance function in space domain is obtained by integrating it over the wavenumbers. On the experimental side, many signal processing methods have been applied to many sites and have been developed by the authors in the last 35 years, i.e., time-domain methods, including the cross-correlation method, and frequency-domain methods such as the spectral analysis of surface waves with two or multiple sensors, the wavenumber-transform method, and the spatial autocorrelation method. All methods are presented by their basic formula and by at least one example site. Different sensor arrays and deterministic and stochastic sources have been tested for the spatial autocorrelation method and the wavenumber-transform method at several sites. In addition, all frequency-domain methods are presented for a specific layered site comparing their quality. The evaluated dispersion curves are very similar, but a somewhat higher frequency range has been found for the fastest method, i.e., the multi-sensor spectral-analysis-of-surface-waves method. The theoretical solutions have been used for the inversion of the measured dispersion to the soil profile of the specific layered soil. The theoretical soil model has subsequently been used to predict the ground vibration spectra of hammer and railway excitation that exhibit a good agreement with the corresponding measurements. Thus, the contribution shows the benefit of active and passive seismic methods for the prediction of railway vibration, including a new version of the spatial autocorrelation method for technical vibrations. On the other hand, technical and namely railway vibrations are considered a seismic source for the exploration of near surface soils.
Deckenschwingungen stellen in der Regel das größte Problem bei Erschütterungsgutachten dar. In diesem Übersichtsbeitrag werden folgende Aspekte der Deckenschwingungen mit einfachen bis komplexen Rechenmodellen dargestellt.
Die Resonanzanregung der Decken hängt von der Phasenlage der Auflagerschwingungen ab. Der Wellenlauf der anregenden Freifeldbodenschwingungen kann die Resonanzstärke deutlich verringern. - Der Wellenlauf kann bei Pfahl- oder Plattengründungen zu Abminderungen der Freifeldamplituden führen (kinematische Bauwerk-Boden-Wechselwirkung). - Die Steifigkeit des Untergrundes hat einen starken Einfluss auf die Resonanzstärke. Dies hat einerseits mit der Abstrahlungsdämpfung zu tun, andererseits auch mit einer Schwingungstilgung. Die Tilgung kann in einem vereinfachten Wand-Decken-Modell mit einer einheitlichen Deckeneigenfrequenz oder realistischer mit einem Eigenfrequenzband berechnet werden. In einem schmalen Frequenzband vor der Deckeneigenfrequenz können weitere (Wand-) Eigenfrequenzen und nach der Deckeneigenfrequenz eine Frequenzlücke auftreten. Normalerweise nehmen die Deckenamplituden mit der Höhe im Gebäude zu. In der Frequenzlücke hingegen nehmen die Amplituden mit der Höhe ab. - Benachbarte Deckenfelder können gemeinsame Deckeneigenfrequenzen besitzen, vor allem wenn sie die gleichen Abmessungen haben. Man bekommt dann ein Band von Deckeneigenfrequenzen. - Bei einer Schwingungsanregung im Gebäude auf einer Decke schwingen auch die benachbarten Deckenfelder mit. Im Frequenzbereich lassen sich Übertragungsfunktionen darstellen, im Zeitbereich können Amplituden-Abstandsgesetze betrachtet werden. - Bei großen Stützenbauwerken treten häufig Kopplungen der Decken verschiedener Stockwerke auf. - Mit den verschiedenen Methoden lassen sich typische Gesetzmäßigkeiten für die Amplituden und Frequenzen der Deckenschwingungen ableiten.
An drei Straßenbrücken unterschiedlicher Länge und Bauart wurden vor Inbetriebnahme ambiente Schwingungsmessungen mit bis zu 350 Messpunkten durchgeführt, um auch höhere Eigenformen detaillierter zu ermitteln. An einer Eisenbahnbrücke der Strecke Hannover-Würzburg wurden bei Testzugfahrten mit definierten Geschwindigkeiten verschiedene Resonanzanregungen beobachtet. Dies wird mit dem Achsfolgespektrum des ganzen Zuges erklärt, wobei eher spezifische Frequenzauslöschungen als Resonanzanregungen maßgeblich sind.
Train-induced ground vibration can be excited by wheel and track irregularities and by two kinds of irregularities of the soil, by geometric irregularities or by the spatially varying soil stiffness. For both types of irregularities, the effective track irregularity on top of the track is calculated in wavenumber domain and with wavenumber integrals. For a general multi-beam track model, the wavenumber integrals are solved numerically. The irregularities of the soil are filtered by the track when transferred from the bottom to the top of the track. The high-wavenumber irregularities are strongly reduced due to the bending stiffness of the track and the compliance of the support. In addition, soft track elements reduce directly the stiffness variation of the support. Therefore, the mitigation effect of elastic track elements for these excitation components seems to be important. For under-sleeper pads and slab tracks, calculation and measurements are presented including additional excitation components and the dynamic vehicle–track interaction, and the relevance of the excitation mechanisms is discussed based on the dynamic forces which are acting on the ground. Due to the restricted amplitudes, the parametric excitation by the stiffness variation seems to be less important than the geometric irregularities. The calculations yield the correct trends of the measurements and many details of the measured ballast, slab, and under-sleeper-pad tracks.
The computation of the wave propagation in homogeneous and layered soils can be performed by a numerical integration in wavenumber domain. The numerical difficulties of an infinite integral and an integrand with poles can be solved. But if this computation must be repeated for many distances, many frequencies, many loads, or many soil models, it becomes a time consuming task which is not acceptable for a user-friendly prediction tool for railway induced ground vibration. Therefore, an approximate method for the computation of the wave field has been developed. The computation consists of several steps. At first, an approximate dispersion profile is calculated according to rules which have been derived from exact solutions. Secondly, the dispersion is used to achieve the amplitude for a certain frequency and a certain distance by calculating the approximate solution of a corresponding homogeneous half-space. Thirdly, three layer corrections are added which include lowfrequency near-field effects, high-frequency far-field effects, and a resonance amplification around the layer frequency. This procedure yields the wave field due to a point load. For a train load, many of these point-load responses have to be summed up, and a frequencydependant reduction factor has to be multiplied to incorporate the effect of the load distribution along and across the track. - The prediction method is applied to real sites, and the appropriate soil models are identified by approximating the measured transfer functions (frequency-dependant amplitudes) which is presented as an alternative to the approximation of the dispersion (frequency-dependant wave velocities). These examples demonstrate the general behavior of layered soils: the low amplitudes of the stiff half-space at low frequencies, the high amplitudes of the softer layer at high frequencies, the strong increase of amplitudes and a possible resonance amplification at mid frequencies. The material damping of the layer yields a strong attenuation of the amplitudes with the distance for high frequencies. The response depends strongly on the resonance or layer frequency which is shown for different layer depths and velocities always in good agreement with measurements. The layer frequency can be of immense influence if train-speed effects are analysed in a layered soil. The good agreement with many measurements in this contribution as well as in the references validates the prediction of ground vibration based on the theory of a layered half-space.
The reduction of train-induced ground vibration by elastic elements such as rail pads and sleeper pads has been analyzed by a combined finite-element boundary-element method. The dynamic compliance of the track, the transfer function of the total force on the ground and the ground vibration ratios have been calculated for a variety of isolated and un-isolated track systems. It has been found that the soil force transfer, which describes the excitation force of the soil, is an appropriate quantity to predict the reduction of the ground vibration and the effectiveness of isolated tracks. All force transfer functions of isolated tracks display a vehicletrack resonance where the wheelset on the compliant track is excited by wheel and track irregularities. At higher frequencies, considerable reductions of the amplitudes are observed as the benefit of the resilient element. The influence of the stiffness of the rail or sleeper pads, the ballast and the soil, and the mass of the sleeper and the wheelset on the resonance frequency and the reduction has been investigated. Sleeper pads are advantageous due to the higher mass that is elastically supported compared to the rail-pad track system. The combination of elastic rail and sleeper pads has been found to be disadvantageous, as the second resonance occurs in the frequency range of intended reduction.