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Chenab River Bridge – Track Vibration & Control Methods

Guided by- Dr. Sarvesh Chandra

Hitesh Sahu Junior Undergraduate

Department of Mechanical Engineering

Indian Institute of Technology Kanpur

Kanpur-208016

Email: hiteshs@iitk.ac.in

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TABLE OF CONTENTS

1. CHENAB RIVER BRIDGE – AN INTRODUCTION .............................................................................................. 1

2. FORCE DISTRIBUTION DIAGRAM .................................................................................................................. 2

3. SOURCES OF RAILWAY NOISE AND VIBRATION ............................................................................................ 3

3.1 ROLLING NOISE .................................................................................................................................................. 3 3.2 CURVED SQUEAL ................................................................................................................................................ 3 3.3 BRIDGE NOISE ................................................................................................................................................... 3 3.4 AERODYNAMIC NOISE ......................................................................................................................................... 3 3.5 GROUND VIBRATION AND NOISE ........................................................................................................................... 3

3.5.1 Low Frequency Vibration ...................................................................................................................... 3 3.5.2 High Frequency Vibration ..................................................................................................................... 3

4. BRIDGE VIBRATION ...................................................................................................................................... 4

5. EXCITATION OF BRIDGE NOISE ................................................................................................................... 5

5.1 WHEEL-RAIL INTERACTION .................................................................................................................................. 6 5.2 TRACK STIFFNESS ............................................................................................................................................... 7

5.2.1 Static Stiffness ....................................................................................................................................... 7 5.2.2 Dynamic Stiffness ................................................................................................................................. 7

6. REDUCING BRIDGE NOISE ........................................................................................................................... 9

6.1 FASTENER STIFFNESS .......................................................................................................................................... 9 6.2 RAIL DAMPING .................................................................................................................................................. 9 6.3 BALLAST MATS .................................................................................................................................................. 9 6.4 DAMPING OF BRIDGE STRUCTURES ....................................................................................................................... 9 6.5 PLATE THICKNESS ............................................................................................................................................... 9 6.6 CLOSED STRUCTURES ........................................................................................................................................ 10

REFRENCES

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1. CHENAB RAIL BRIDGE – AN INTRODUCTION

Chenab Bridge, currently under construction is an arc type rail bridge, located between Bakkal and Kauri in the Reasi district of Jammu and Kashmir (J&K), India. The 1,315m-long bridge is being built at a height of 359m. The bridge will be 1,315 m (4,144 ft) long, with a 485 m (1,570 Ft.) trussed arch span 359 m (1,178 Ft.) above the river Chenab and a 650 m (2,130 Ft.) long viaduct on the Kauri side1. The bridge is part of several bridges and tunnels which makes up the Katra-Laole Section of the USBRL (Jammu Udhampur Srinagar Baramulla Railway Link) Project in Jammu and Kashmir. Another, smaller, arch bridge in the link will be the 657 m (2,156 ft) long, 189 m (620 ft) high Anji Khad bridge between Katra and Reasi. Once construction is completed in March 2016, it will be the tallest rail bridge in the world. It will have a lifespan of 120 years 2.

Figure 1

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2. FORCE DISTRIBUTION DIAGRAM The weight on the bridge may vary from just one person to maybe the weight of a train. This weight is called live load. The live load is the reason the bridge is built; to allow people and objects to cross over. Besides the live load, the bridge also has to be engineered to be able to support its own weight as well. This is called the dead load of the bridge. The use of the arch is just one way that engineers are able to support the dead load and the live load of a bridge.

The weight of the dead load and the live load combined has the effect of a downward force, the

gravitational force. In an arch bridge, the downward forces are for the most part dissipated down to the abutments, the two base supports of an arch that are securely anchored in the ground (Shown in Figures 3, 4 & 5). Because the bridge is at rest, the abutments therefore have an equal and opposite force to the downward forces. The dissipation of the forces is a distribution and a changing of the direction of the downward forces. For example, the downward forces at the center of an arch bridge don't have any supports directly below them to keep the bridge at rest. Instead, these forces are dissipated, or redirected down to the abutments. Instead of a single downward force, the form of the arch redirects part of the force in a horizontal direction. The addition of these two perpendicular forces results in an angled resultant force. This resultant force is now directed at the abutment and can be properly supported.

Figure 3

Figure 4 Figure 5

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3. SOURCES OF RAILWAY NOISE AND VIBRATION There are several sources of railway noise and vibrations. The noise from rail traffic is caused by the locomotive engines and fans, by the rolling noise generated by wheel-rail contact, and at high speeds also by the noise caused by air resistance. The main causes of railway noise and vibration that affect the system summarized below.

3.1 Rolling Noise This broad-band noise is caused by vibrations of the wheel and rail which are excited at their contact by irregularities of the running surfaces. This is the dominant source of noise from railway operations. 3.2 Curved Squeal These are also caused by wheel-rail interaction but has a quite different character. These occurs while traversing tight radii due to unsteady transverse forces at contact during curving.

3.3 Bridge Noise When a train runs over a bridge the noise emitted can increase considerably, depending on the type of bridge. The bridge is excited by dynamic forces acting on it from the track.

3.4 Aerodynamic Noise This type of noise is caused by the unsteady air flow over the train. Aerodynamic sources of noise generally increases rapidly with speed, typically between 60 log10 V and 80 log10 V, where ‘V’ is the velocity of the train.

3.5 Ground Vibration and Noise Vibrations transmitted through ground can be experienced in two ways. 3.5.1 Low frequency vibration Low frequency vibration between about 2 and 80 Hz is perceived as feel able ‘whole body’ vibration. This tends to be associated mostly with heavy freight trains at particular sites. 3.5.2 High frequency vibration High frequency ground-borne vibration from about 30 to 250 Hz causes the walls, floors and ceiling of rooms to vibrate and radiate low frequency noise. This is associated with trains in tunnels like metros. We will be focusing on Bridge Noise hereafter.

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4. BRIDGE VIBRATION When a train traverses a bridge, it usually produces more noise than when it runs on plane track at grade. This increase in noise varies considerably from one bridge to another but it can typically be 10dB or more. Bridges, therefore, often form ‘hot spot’ in noise maps. There are two main reasons for amplification of noise when a train crosses a bridge. The first is that vibrations are transmitted from the rails into the bridge structure which then also radiates noise. Due to the large surface area of bridge, it acts as a ‘sounding board’; the noise radiated can therefore be considerable even though the vibration amplitude is attenuated by the track fastening system. The second reason is that, depending on the method of the rail fastening used on a particular bridge, the noise radiated by vibration of the rail itself may be considerably more than for track at grade. A number of attempts have been made to quantify the noise from different bridge types on the basis of measured data. Figure 6 (based on the data from Ungar and Wittig3), shows a wider collection of measured data of bridge noise divided into seven categories. These results are shown as the increase in A-weighted noise level relative to the same train on plain track, and contains values from -6 dB to +20 dB. It is clear from this that steel bridges are generally nosier than concrete ones, and bridges with direct rail fastening are mostly nosier than those with ballasted track.

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5. EXCITATION OF BRIDGE NOISE The way in which a train on a bridge generates noise is shown schematically in Figure 74.

Figure 7

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5.1 Wheel-Rail Interaction The main source of excitation of the bridge is the surface roughness at the wheel/rail interface. The combination of wheel and rail roughnesses causes a relative displacement between wheel and rail at contact patch, according to their motilities. The coupling with the vibration in the lateral direction can usually be ignored, on the basis that the vertical vibration contributes most to the energy transfer into the bridge, although in some situations, particularly on curves, the lateral direction should be included5. As well as roughness excitation, the presence of rail joints can lead to impact excitation on the bridge. Bridges usually require expansion joints in the rails and is normally more convenient to locate these on the bridge itself.

Figure 8 Wheel-Rail Interface; Generation of Vibration and Noise

Track decay rate will be, in general, lower on bridges with direct fasteners than on ballasted track. This means that the component of noise radiated by rail itself will also be higher on such a bridge than on ballasted track.

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5.2 Track Stiffness Track stiffness is a significant parameter from the aspect of design, construction and maintenance of the railway superstructure and substructure. This parameter represents the basis for calculating stresses in the elements of the track and track foundation. There are multiple ways to define track stiffness. The most common definition is that the stiffness (D) represents the proportion between vertical load (Q) and track deflection (y) at a given moment (t)6:

The function of a sleeper depends on the support conditions and these are directly related to the track stiffness. Thus, track stiffness is an important track property. Relatively high track stiffness is desired to provide adequate track resistance to the applied loads and to limit the track deflection. This will in turn, reduce the track deterioration7. Too high track stiffness and especially variations in stiffness on a stiff track can cause increased dynamic forces on sleepers, ballast and in the wheel-rail interface. This can lead to wear and fatigue damage on track components. Low track stiffness leads to large rail displacements and high bending moments in the rails. On the other hand, low track stiffness leads to better load distribution between sleepers and lowers train/track interaction forces8. 5.2.1 Static Stiffness Figure 9 shows a typical load-deflection curve9 for an elastomeric rail pad. Under static loading, elastomers have a non-linear load-deflection curve. In general the stiffness of an elastomer increases with increasing load. This means that the static stiffness of a resilient rail fastening must be defined at a particular load. This load will depend on factors such as axle load of expected traffic. Also shown in Figure 9 are two definitions of the static stiffness of a fastener, tangent stiffness and secant stiffness. The secant stiffness is measured as the static stiffness between clip load and a static wheel load. For small deflections about a mean load, the tangent stiffness is more appropriate. Thus for vibrational loading, this is the appropriate definition.

5.2.2 Dynamic Stiffness Under static loading an elastomer normally acts as a Hookean elastic material, where the deflected shape will return to its original shape when the load is removed. When the material is subjected to stresses and strains that change with time, such as the excitation due to wheel-rail roughness, the material exhibits viscoelastic behavior. The viscoelastic nature of elastomer fastenings produces lower deflections under dynamics loads compared to static loads, meaning that the dynamic stiffness is much higher than the static stiffness. The deflection also lags the applied load due to damping effect. The dynamic stiffness is the more important parameter in terms of vibration attenuation.

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Figure 9 A typical load deflection curve for an elastomeric rail pad

When selecting the ideal static and dynamic stiffness of a fastening the following three factors, in order of importance, are9

1. Minimizing the wheel impacts on the track supports (Safety Criteria).

2. Constraining the rail from excessive motion particularly gauge widening and vertical deflection (Safety Criteria).

3. Providing the correct level of vibration isolation from the rail and the support structure.

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6. REDUCING BRIDGE NOISE In this section we will try to understand the techniques by which we can control the noise and vibration of railway system. Here, some of the techniques discussed will not be applicable to existing bridges but only to the new bridges.

6.1 Fastener Stiffness At first sight it would always be beneficial to reduce the stiffness of the resilient track support in order to increase the vibration isolation between rail and bridge. However, in practice this does not necessarily produce as larger an effect as might be expected. This happens because of its certain limitations, some of them are mentioned below.

Wooden sleepers provide considerable resilience in series with the fasteners. In the case of stiffer fasteners, this will be greater than the resilience of the fasteners themselves.

Track decay rate may be reduced. This is in turn increase the average vibration of the rail and hence its noise radiation.

6.2 Rail Damping By applying a damping treatment to the rail, it is possible to reduce the average rail vibration by increasing the attenuation with distance along the rail. The main effect of this is to reduce the component of rolling noise on the bridge, which is usually greater with direct fasteners than on ballasted track. There are several methods of adding damping to the rail like using multi-material wheels, Resilient wheels etc.

6.3 Ballast Mats For ballasted track, it is possible to introduce ballast mats under the ballast in order to increase the vibration isolation. Such mats are generally used in for tracks in tunnels. Measurements of vibration isolation on bridges are much less common, and in any case the mats used on the bridges often stiffer than for tunnels. As with the ballast itself, internal resonances can occur at relatively low frequencies within a ballast mat. Unfortunately, most measurements of the effects of such mats are limited to frequencies below 400 Hz, and it is not clear whether the improvements found here are also always found at higher frequencies.

6.4 Damping of Bridge Structure Noise can be reduced if the damping of the bridge structure can be increased. There are several examples in the literature of the application of constrained layer damping treatment to steel box girder bridges.

6.5 Plate Thickness Thicker plate tends to have a lower vibration amplitude. However, the effects of acoustic short-circuiting are smaller for thicker plates, and these two effects tend to cancel each other out in much of the frequency range. It is instructive to consider the ratio Wrad/Win. For a single plate, this is given by,

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Where

0 = Density of Air c0 = Speed of Sound

= Radiation Ratio Wrad= Radiated Sound Power Win = Input Power

= Rotational Speed of Wheel

= Damping Loss Factor

= Density of Plate

h = Depth of Plate

Only quite extreme changes of thickness could lead to a significant noise reduction. It can therefore be concluded that it is more effective to minimize the input power. The same applies to the addition of stiffeners to the plates of a bridge.

6.6 Closed Structures Figure 10 compares two bridge designs. On the left is an open girder consisting of a deck plate and two webs, while on the right a box girder is shown which is closed by a bottom plate and also plates at either ends. For a closed structure, such as this box girder, the sound radiation from the inner surface is contained within the structure. This has the effect of reducing the effective radiating area by up to a factor 2, and hence the radiated sound power by 3 dB. The structure must be completely closed, however, as otherwise the noise from the inner surface could escape through any gaps.

Figure 10 Typical open and closed girder design

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REFERENCES

[1] Wikipedia [2] Efficient Infrastructure, The trade magazine on infrastructure technology, Volume I, June-July 2013. [3] E.E. Ungar and L.E. Wittig. Wayside noise of elevated rail transit structures: analysis of published data and supplementary measurement. US department of Transportation Report no. UMTAMA-06-0099-80-6, December 1980. [4] Thomas and Jones, 2002 [5] O.G. Bewes. The calculation of noise from railway bridges and viaducts. EngD thesis, University of Southampton, 2005. [6] Influence of Track Stiffness on Track Behavior Under Vertical Load, Traffic Infrastructure Review, Accepted: March 31, 2011, Approved: Oct 3, 2012. [7] Li and Berggren, 2010. [8] Berggren, 2009. [9] O.G. Bewes. The calculation of noise from railway bridges and viaducts. EngD thesis, University of Southampton, 2005. [10] TCRP 2005.