chapter-3 Track Stracture.pptx

railway engineering

1. DEBRE MARKOS UNIVERSITY COLLEGE OF TECHNOLOGY CIVIL ENGINEERING ACADAMIC PROGRAM RAILWAY ENGINEERING CHAPTER 3 RAILWAY TRACK STRUCTURES Biniyam A. January, 2022 1

2. Contents Component and function of track structure Track loads Track Analysis: Rail support; sleepers/ties; ballast and subballast Ballasted and Slab track Rail fastening system Track modeling

3. 3.1 Component and function of track structure In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on. Track usually consists of steel rails installed on sleepers/ties and ballast, on which the rolling stock, usually fitted with metal wheels, moves. The railway track has to fulfill two main functions: to guide the train with safety to carry the load of the train and to distribute the load to the subgrade over an area that is as large as possible

4. Components of track structure

5. TRACK LOADS The requirements for the bearing strength and quality of the track depend to a large extent on the load parameters: axle load: static vertical load per axle; tonnage borne: sum of the axle loads; running speed The static axle load level , to which the dynamic increment is added, in principle determines the required strength of the track. The accumulated tonnage is a measure that determines the deterioration of the track quality and as such provides an indication of when maintenance and renewal are necessary. The dynamic load component which depends on speed and horizontal and vertical track geometry also plays an essential part here.

6. Forces on the Track Those loads can be categorized into three main groups: Vertical loads Lateral loads (transverse), and Longitudinal loads (parallel) Depending on their nature, those loads can be divided into three groups: Static loads (normally caused by the vehicle body mass) Quasi-static loads (or dynamic ride loads), Dynamic ( dynamic wheel/rail) loads, which are associated with significant irregularities that may occur during the life of the track structure and vehicle Irregularities of the track geometry Discontinuities on the running surface (switches, joints) Wear of the running surface of the rails Wear of the wheels (out-of-round wheels) Vehicle suspension and vehicle asymmetries Dynamic wheel/rail forces are much higher in magnitude than quasi-static (dynamic ride) forces.

7. Quasi-static (Dynamic Ride) Forces Definition : the sum of the static load and the effect of the static load at speed and they are classified in the frequency range between 0.5 and 30 Hz . The load includes the effects of the geo m etri ca l rough n ess o f t h e track on vehicle response and unbalan c ed supe r e l e vation ( t h e e f f ect o f the train load no t being distributed evenly over both rails). The quasi-static force has been found to be typically between 1 . 4 and 1 . 6 times the static wheel load before unbalanced superelevation effects are included4. which are associated with vehicle movements: Vehicles running on the track apply certain forces on the track structure due to the behavior of the vehicle body, bogie and other masses in reaction to geometrical irregularities in the track.

8. Dynamic Forces The Dynamic Wheel / Rail Forces Dynamic forces come in two categories: P 1 Forces P 2 Forces The P 1 Force P 1 forces are classified in frequency range between 100 Hz and 2000 Hz . These forces are also called impact forces . They correspond to surface irregularities or defects in rails and wheels and produce strong impact to rail and wheel. P 1 is a very high frequency force occurring ¼ - ½ ms after crossing the angular discontinuity that occurs at the bottom of the dip and has a very short duration

9. The P 1 Force Effect of P 1 forces Rail hammering just after the joint gap and produces high stresses in the rail web. It contributes to bolt hole failures in bolted joints by increasing the stress range. contribute to the cracking of concrete sleepers . Its effects are largely filtered out by the rail and sleepers, do not directly affect ballast or subgrade settlement. they have a great influence on wheel/rail contact behavior .

10. The P 2 Force P 2 forces are classified in lower frequency range (30 Hz and 100 Hz) They correspond to the movement of unsprung masses of the vehicles. have a lower-amplitude and longer-duration than the P 1 forces. The peak force occurs in the area of the first running-off sleeper after the joint. The wheel set mass and the rail/sleeper mass move down together and compress the ballast beneath the sleeper. P 2 forces therefore increase contact stresses, contribute to the total stress range experienced by the rail web and at joint bolt holes Increase the loads on sleepers and ballast in the immediate neighborhood of the joint. P 2 forces are of great interest to the track design engineer. P 2 forces contribute primarily to the degradation of track geometry.

11. Force Limits Railway organizations around the world have set limits on the various forces at the wheel / rail contact area. Vertical Force Limits In the vertical direction high forces can cause damage to the rails and supporting structures and can cause rolling contact fatigue when combined with high tangential forces such as occur during traction, braking or curving. Eg. UIC limits a maximum static load of 112.5 kN per wheel and a maximum dynamic vertical force per wheel of between 160 kN and 200 kN, depending on maximum speed (provided this values does not exceed the static wheel load plus 90 kN). In small radius curves (less than 600 m) a limit of 145 kN for the quasi- static vertical force.

12. Lateral Force Limits In the lateral direction high forces can cause distortion of the track on ballast-bed. This is normally protected against by using the simple but widely established Prud Homme limit for the track shifting force at one wheel set, which can be calculated from the static load (Po force) : Where, Y and P are in kN. Lateral forces of very short duration are less likely to shift the track and therefore only forces that act for more than 2m of track length are usually counted. In small radius curves (less than 600 m) UIC sets a limit of 60 kN for the quasi-static lateral force.

13. D e ra i lm e nt Possibility of wheel climb derailment is indicated by the ratio of the lateral force Y to vertical force V Nadal theory is used to establish limits for the Υ/V derailment ratio with 0,8 as the limiting value. Wheel Unloading Very low vertical forces at the contact patch can indicate that a vehicle is tending to derailment by rolling over or by failing to follow twists in the track. E.g. In the UK a lower limit of 60% of the static wheel load (i.e., unloading by over 40%) is set.

14. Track Components The Principle :- Track Components do not function independently ! Each component layer must protect the one below . Main design components Rail Sle e per Rail pad/plate Ballast

15. Deflection Profile Source: Selig and Waters, Track Geotechnology and Substructure Management, 1994

16. Static vs. Dynamic Loads Dynamic loads higher Acceleration from speed Downward rotation of wheel Smaller wheels, faster rotation, more acceleration Speed/wheel influence – Pv= P + θP ( AREMA) Where, P v = Vertical Dynamic Load (lbs) D = Wheel diameter (in) V = Speed (MPH) P = Static Load (lbs) – Larger wheels impose less influence Additional dynamic loads from impacts such as caused by wheel flat spots, rail discontinuities (e.g. frog flange ways), track transitions (e.g. bridge approaches), track condition, etc.

17. Typical Track Stiffness Values Winkler Model of Rail Deflection The deterioration process due to variation in track stiffness

18. - Rail, fasteners, tie and ballast Upper S tructures The upper part consists of two parallel steel rails, anchored perpendicular to members called ties (sleepers) of timber, concrete, steel, or plastic to maintain a consistent distance apart, or gauge. Rail Rails are the longitudinal steel members that directly guide the train wheels evenly and continuously. Rail guides the conical, flanged wheels , keeping the vehicles on the track without active steering and therefore allowing trains to be much longer than road vehicles .

19. 1. Rail a) Characteristics b) Functions: Supports the loads of train and guides their wheel movements The excellence of the track determines the permissible wheel loads, speeds, safety provide a surface with smaller resistance bear the force of the wheels and spread it to sleeper used as track circuit in electrified railways and automatic block segments Rigidity Tenacity Hardness Roughness of top surface Composition Unlike other uses of iron and steel, railway rails are subject to very high stresses and have to be made of very high quality steel . Minor flaws in the steel that pose no problems in reinforcing rods for buildings, can, however, lead to broken rails and dangerous derailments when used on railway tracks

20. C) Profile The rail profile is the cross sectional shape of a railway rail, perpendicular to the length of the rail. A rail is hot rolled steel of a specific cross sectional profile (an asymmetrical I-beam) designed for use as the fundamental component of railway track. Composed of rail head , rail web and rail base The rail head and base must be large and thick rail head rail web rail base 1. Rail weight of a rail per length (Kg/m) , such as 75, 60, 50, 43 kg/m. Standard rail length : 12.5m and 25m. d) Types and length

21. The following rail forms are in use at present : Vignoles rail (standard railway rail with head, web and foot), Double-head rails with head, web and foot (obsolete) Grooved rails for tram ways, Switch rails and Crane rails etc 1. Rail

22. Where, δ- size of rail gap(mm) L- length of track(m) δq- structural joint gap, track of 38kg/m, 43kg/m, 50kg/m, 60kg/m, 75kg/m are 18mm t - temperature of rail gap( o C) e) Rail gap To adapt to the needs of expanding with heat and contracting with cold , the rail gap can not too big or too small. 2 0 2 q   0.0118 L  t  t   1  1 2 t 2   t m a x  t m i n  1. Rail The 25 m rail are welded into 100-200m long rail in factory, and then be welded again into1000-2000m long rail in the laid place Advantages: smooth driving low maintenance cost long life f) Continuous welded track (CWR)

23. good weld ability, high degree of purity good surface quality evenness and observance of profile and low residual stress after manufacturing g) Rail requirement To be able to withstand manifold and high forces, the rails must meet the following requirements: high resistance to wear, high resistance to compression, high resistance to fatigue high yield strength, tensile strength and hardness high resistance to brittle fracture 1. Rail

24. 2. Sleeper (a) Function bear the force of track Act as elastic medium to absorb blows & vibrations Longitudinal & lateral stability spread the force to ballast bed and roadbed keep the direction, position and gauge of track Supporting wheels and/or jacks direct ( in a derailment situation ). Acting as transverse beams when sitting on temporary ‘way beams’. Supporting signal engineering and other safety related equipment such as trip cocks and point motors. Supporting conductor rails, electrical bonds and feeder cables. Reducing noise and vibration on non-ballasted bridge decks

25. (b) Characteristic It is solid, flexible, reasonably cost, convenient for manufacturing and maintenance. 2. Sleeper

26. 2. Sleeper According to production material : reinforced concreted sleeper, wooden sleeper, steel sleeper According to their usage : regular sleeper, switch sleeper and bridge sleeper. Requirements Moderate weight- easy to handle Fixing and removing of fastening should be easy Sufficient bearing area and Able to resist shocks and vibrations Easy maintenance and gauge adjustment Track circuiting must be possible Minimum maintenance and initial cost D ） types

27. 1. Wooden Sleeper Timber ties are usually of a variety of hardwoods, oak being a popular material. They have the advantage of accepting treatment more readily, they are more susceptible to wear. They are often heavily creosoted. Creosote treating can reduce insect infestation and rot. However, creosote is also carcinogenic and environmentally damaging. Less often, ties are treated with other preservatives , although some timbers are durable enough that they can be used untreated. 2. Sleeper

28. 2. Concrete Sleeper Concrete ties have become more common mainly due to greater economy and better support of the rails under high speed and heavy traffic than wooden ties 2. Sleeper

29. 2. Sleeper

30. In past times steel sleepers have suffered from poor design and increased traffic loads over their normal long service life. The steel sleepers ’ cost benefits together with the ability to hold rail gauge, lower long-term maintenance costs, increase the life of other track components, reduce derailments and meet ever growing and stricter environment standards provide railroad companies with savings and capital to redirect to other areas of maintenance-of-way and business projects. 3. Steel sleeper Advantages : Long life, Better lateral rigidity Free from decay and fire hazards Good scrap value Lesser damage during handling Less maintenance problems Easy to maintain gauge Disadvantage s : Liable to corrosion Unsuitable to track circuiting Becomes center bound due to sloping ends Rail specific

31. Concrete monoblock sleepers have also been produced in a wider form (e.g. 57 cm (22 in)) there is no ballast between the sleepers ; wide sleeper increase lateral resistance and reduces ballast pressure . Wide sleeper 2. Sleeper

32. 2. Sleeper Y-shaped sleepers An unusual form (developed in 1983) r e d u ce d b a l la s t v o l u m e d u e t o t h e l o a d s p rea d i n g c h a ra c t e r i s tic s Compared to conventional sleepers. High Noise levels very good resistance to track movement . Three point contact curves means that an exact geometric fit cannot be observed with a fixed attachment point. Bi-block/Twin-block sleepers Bi-block sleepers consist of two concrete rail supports joined by steel Advantages include Increase lateral resistance lower weight than monoblock concrete sleepers eliminate damage from torsional forces on the sleeper centre due the more flexible steel connections Bi-block sleepers are also used in ballastless track systems

33. 2. Sleeper Y-shaped sleepers … Bi-block

34. Mono-block vs. twin block sleepers The advantages of the twin-block sleeper Well-defined bearing surfaces in the ballast bed; high lateral resistance in the ballast bed The advantages of the mono-block sleeper low price less susceptibility to cracking can be pre-stressed 2. Sleeper d) Arrangement Configuration number at each kilometer is decided by volume, speed and line level. The rule: wooden sleepers must no more than 1920 per 1km and no less than 1440 Reinforced concreted sleepers must no more than 1840 per 1km and no less than 1440

35. Ballast- it is a layer of broken stones, gravel , moorum or any other gritty material placed and packed below and around sleepers. for distributing the load from the sleepers to the formation and for providing drainage as well as giving longitudinal and lateral stability to the track . 3. Ballast and sub ballast A layer of loose, coarse grained material which, as a result of internal friction between the grains, can absorb considerable compressive stresses, but not tensile stresses.

36. a) Functions The six most important functions of ballast: To resist vertical & longitudinal forces and hold the track in position To provide energy absorption for the track To provide voids for storage and movement of fouling material in the ballast To facilitate the adjustment of track geometry To provide immediate drainage of water falling on to the track To reduce pressures on underlying materials by distributing loads 3. Ballast and sub ballast b) Requirements Tough and resist wear Hard enough Cubical with sharp edge Non porous, non-water absorbent Resist attrition Durable Good drainage Cheap and economical

37. Ballast Analysis and Design(depth determination) So u rce : -A R EMA 3. Ballast and sub ballast

38. Ballast depth determination (ballast and subballast combined) = f(applied stress, tie reaction, and allowable subgrade stress ) – Talbot Equation, h = (16.8p a /p c ) 4/5 Where, h = Support ballast depth p a = Stress at bottom of tie (top of ballast) p c = Allowable subgrade stress Note: Stress distribution independent of material – Japanese National Railways Equation p c = 50pa/(10+h 1.25 ) – Boussinesq Equation pc= 6P/2h 2 where P = wheel load (lbs) – Love’s Formula pc= pa{1-[1/(1+r 2 /h 2 )] 3/2 } Where, r = Radius of a loaded circle whose area equals the effective tie bearing area under one rail 3. Ballast and sub ballast

39. Minimum depth of ballast Stress distribution is assumed as 45 o -Consider stress overlap area -Provide adequate depth of ballast -Thickness of ballast is a function of sleeper spacing, sleeper size etc 3. Ballast and sub ballast

40. Sub ballast - Is a layer of material between the top ballast and sub grade with a gradation finer than the top ballast and coarser than the sub grade Function Used to reduce total ballast cost Provide a filter layer between the top ballast and a fine grained sub grade Application A sub ballast layer is recommended for most new construction . In addition to providing filter to keep sub grade particle from working up in to and fouling the ballast, it provides a good mat to distribute loads from the ballast and prevents ballast particles from being pushed in to the sub grade. 3. Ballast and sub ballast

41. A sub ballast layer is required whenever: The sub gr a de co n tains 85% or m ore (by we i ght) of s ilt and c lay sized particles or, The sub grade material has a plasticity index grater than 20 Material hard, angular, non - cementing material, d) Gradation To function as a filter layer, the sub ballast size range from the smallest ballast particles to the largest sub grade particles Depth D u ring structur a l an a ly sis, the sub ba l la s t la y er is con s i d ered as part of the total ballast depth A sub ba l l a st layer m ay co m pr i se up t o 40% of the t o tal bal l ast thickness on main running tracks and up to 50% on other tracks 3. Ballast and sub ballast

42. 4. Rail fastening system/ Union piece A Rail fastening system is a means of fixing rails to railroad sleepers . The terms rail anchors, tie plates, chairs and track fasteners are used to refer to parts or all of a rail fastening system. Various types of fastening have been used over the years. Union pieces are divided into => Rail joint fastenings => Middle joint fastenings . a) Function : The purpose of the rail fastenings: To maintain the track gauge Offer sufficient resistance in a vertical direction To transmit forces acting on and in the rails to the sleepers (cross, longitudinal, concrete plates etc.) Electrically insulate the sleeper against the remaining track grid, to minimize the loss of signals of the direct-current circuits

43. b) Types (1) rail joint fastenings Rail joint fastenings are used at the end of the two tracks. 4. Rail fastening system/ Union piece (2) Middle joint fastenings used to connect rail with the sleeper Based on sleeper type, (reinforced concreted joint fastenings and wooden joint fastenings)

44. 5. Ballasted and slab track Ballasted track Advantages : Lower cost, small noise emission scope, short construction period, easy repair when failure occurs, high efficiency of maintenance work due to its mechanization, easy to adjust its geometrical unevenness Disadvantages : The tendency to “float” ( longitudinal and lateral direction) Limited non-compensated lateral acceleration in curves caused by the limited lateral resistance provided by ballast Pulverization of the ballast grains in the ballast bed resulting in particles damaging the rail and wheels Problems with ballast churning with high speed Reduced permeability due to contamination, the wear of the ballast, and intrusion of fine particles from the sub grade

45. b) Slab track In slab track, ballast is replaced by another stable load distributing material such as concrete or asphalt. The necessary elasticity has to be provided by inserting elastic elements below the rail or the sleeper, as the concrete or asphalt layer is very stiff. In comparison to ballasted track, the advantages of slab track are in general reduction of maintenance and a higher stability of the track

46. Less or free maintenance, costs 20%~30% better line evenness Increased service life, and possibility of almost full replacement at the end of the service life Increased lateral resistance and stability Reduced structure height and weight The excess of super elevation and cant deficiency of the track with mixed used of freight- and passenger trains does not cause altering of the track position Track accessibility to road vehicles Preventing the release of dust from the ballast bed into the environment Slab track Advantages :

47. Higher construction costs and Higher airborne noise reflection Adaptability to larger sinkage in the embankment is relatively small Repair works take much more time and effort (in case of derailment) Transitions between ballasted track and slab track require attention Large attentions in track position and super elevation can only be made possible by substantial amounts of work The application of slab track may require extensive measures concerning the preparation of the foundation. The sub layers must be homogenous and capable of bearing the imposed loads without significant settlements. Slab track Metro, urban rail transit, Bridge Repair inconvenience, small space, traffic density Tunnels, subways, elevated crossing, stations & terminals High speed railway Practical uses of slab track Disadvantages

48. Main types of non-ballasted track: Embedded in concrete Prefabricated slabs Monolithic slab AC(asphalt concrete)-road bed Embedded rail Slab track (Japan)- prefabricated Floating Slab Track on springs (Gerb Company) LVT (Sonneville Company) Rheda (Germany)- sleepers embedded in concrete Slab Types

49. Comparison of ballasted and slab track

50. Track Modeling Classic Modeling theories Sprung/unsprung Mass model Dynamic Analysis ………… Track System Wheel-rail contact Vehicle System Using FEA software ABAQUS