Dynamic vibration analysis of ballastless track

Abstract

Additional dynamic loads and friction emerge on the railroad superstructure due to irregularity and roughness that occur between the rails and the wheels. In particular, when the vibration frequencies of the railroad vehicle and natural frequencies of the system match, a dynamic phenomenon called reso-nance occurs. Therefore, numeric analyses that use finite element models need to be performed, and res-onating dynamic effects need to be determined. Fur-thermore, field vibration surveys should be conduct-ed, and the natural frequencies of the railroad should be measured to support and verify the dy-namic analysis. In this study, a ballastless superstructure with a steel baseplate of light metro track, which lies between Aksaray and the Airport in Istanbul, was analyzed with a numerical method using ANSYS 9.0 finite el-ement software. Since the railroad superstructure has a symmetric structure with respect to the track axis, a rail system that consists of a single rail and a steel baseplate was modeled to simplify the compu-tations in this analysis. In the discrete supported track model, the rail was modeled as an Euler-Bernoulli beam and steel baseplate, and it was as-sumed to be a rigid mass. Dynamic computation re-sults and graphs were determined by performing harmonic analysis with the aid of ANSYS software. Harmonic receptance analysis enables the continu-ous dynamic behavior of the vehicle to be deter-mined; as a result, the ability of the design to suc-cessfully withstand resonance, fatigue, and other harmless effects of constrained vibration is revealed. Harmonic receptance analysis is a technique that is used to determine steady-state receptance against sinusoidal (harmonically) changing loads of a linear structure. The purpose of this analysis is to estimate the receptance of the structure at various frequen-cies and plot it. In the numerical analysis, the effect of various track parameters for the 0-1500 Hz interval on the dynam-ic behavior was calculated. When modeling with the finite elements method, the wheel load was applied both above the support and in the middle of two supports. A field vibration survey was conducted to determine the natural frequencies of the railroad and the dynamic receptance behavior, and to vali-date the finite element model according to the meas-urement results. The frequency receptance behavior of the rail and the support was measured by apply-ing a hammer impact load on the rail head in the field, and the finite element model of the ballastless track with steel baseplate was verified. A single peak value is observed on the dynamic receptance graph based on measurement results, and this value indi-cates the track natural frequency. The natural fre-quency of the ballastless track varies depending on baseplate distance and the type of elastic layer un-der the baseplate. But it is difficult to identify the natural frequency of the rail. Survey results for the ballastless track were remarkably consistent with the finite elements model, and the natural frequency and dynamic receptance values for both of them were found to be very close to each other. This is because there are not any granular materials that have unknown dynamic properties, like the ballast and soil in this superstructure; only elastic layers under rails and steel baseplate are present. It is pos-sible to establish dynamic properties fully consistent with measurements, particularly for ballastless track models. Consequently, there is a good chance of precisely determining the dynamic behavior of the track for vehicle and track parameters. This study is the first in Turkey that combines rail-road dynamic analysis and modeling and surveying, and it can be considered as a significant develop-ment for urban railroad systems and high speed railroad projects, which have grown in the last sev-eral years. It is crucial to analyze the effect of vari-ous track parameters (rail type, elastic layer, sup-port distance, etc.) on the dynamic behavior of the railroad, as the ballastless superstructure is pre-ferred for urban rail systems (subway and tramway systems). It is possible to explicate with finite ele-ment models validated by field surveys. Deciding on the most fitting design in terms of vibration and de-termining the natural frequencies of the track during the planning stage is necessary to reduce the nega-tive environmental effects of urban rail systems, which are located very close to areas sensitive to vibration and noise, such as historical structures, residences, hospitals, and schools. Furthermore, future studies should focus on determining the exci-tation frequencies caused by trains and natural fre-quencies of nearby buildings.

Özet

Demiryolu üstyapısında ray ile tekerlek arasındaki temas sırasında düzensizlik ve pürüzlülükler ne-deniyle ek dinamik yükler ve titreşim ortaya çıkmaktadır. Özellikle, demiryolu taşıtının neden oldu-ğu titreşimlerin frekansları ile sistemin doğal frekansları çakıştığında rezonans denilen dinamik fe-nomen ortaya çıktığı için, sonlu eleman modellerini kullanan nümerik analizlerin yapılması ve re-zonans durumundaki dinamik etkilerin belirlenmesi gereklidir. Bu çalışmada, İstanbul’da bulunan Aksaray-Havaalanı arasında kalan hafif metro hattında çelik seletli balastsız (beton döşemeli) üst-yapı, nümerik yöntemle ANSYS 9.0 sonlu eleman programı kullanılarak analiz edilmiştir. Bu ana-lizde, demiryolu üstyapısı yol eksenine göre simetrik bir yapıya sahip olduğundan hesaplamada ko-laylık sağlamak için tek ray ve çelik seletten oluşan yol sistemi modellenmiştir. ANSYS programı ile harmonik analiz yapılarak dinamik hesap sonuçları ve grafikleri elde edilmiştir. Harmonik tepki analizi, aracın devamlı dinamik davranışını, dolayısıyla, tasarımın rezonans, yorulma ve zorlanmış titreşimin diğer zararlı etkilerine başarıyla karşı koyup koyamayacağını belirleyebilmeyi sağlar. Harmonik tepki analizi lineer bir yapının zamanla sinüzoidal (harmonik) olarak değişen yüklere karşı sürekli durum tepkisini belirlemekte kullanılan bir tekniktir. Nümerik analizde, 0-1500 Hz fre-kans aralığı için değişik yol parametrelerinin dinamik davranışa etkisi hesaplanmıştır. Demiryolu-nun öz frekanslarını, dinamik tepki davranışını tespit etmek ve ölçüm sonuçlarına göre sonlu ele-man modelini doğrulamak amacıyla arazi titreşim ölçümü yapılmıştır. Arazide ray mantarına çekiç darbe yükü uygulanarak, rayın ve mesnetin (çelik selet) frekans tepki davranışı ölçülmüş, çelik se-letli balastsız hattın sonlu elemanlar modeli doğrulanmıştır.