Influence of asymmetrical topology on serviceperformance of railway prestressed concretesleepersKaewunruen, Sakdirat; Remennikov, Alex; Ishida, Tetsuya

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Citation for published version (Harvard):Kaewunruen, S, Remennikov, A & Ishida, T 2017, Influence of asymmetrical topology on service performance ofrailway prestressed concrete sleepers. in International Convention on Civil Engineering 2017 Proceedings:ICCE2017. 1 edn, vol. 1, Suranaree University of Technology, The Greenery Resort, Khao Yai, Pak Chong,Nakhon Ratchasima, Thailand.

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Influence of asymmetrical topology on service performance of railway prestressed 1

concrete sleepers 2

3

Sakdirat Kaewunruen1,*

, Alex M Remennikov2 and Tetsuya Ishida

3 4

1 Birmingham Centre for Railway Research and Education, School of Engineering, The 5

University of Birmingham, Birmingham, B15 2TT UK. 6

2 School of Civil, Mining and Environmental Engineering, University of Wollongong, 7

Wollongong, NSW 2502 Australia. 8

3 Concrete Laboratory, Department of Civil Engineering, The University of Tokyo, 113-8656, 9

7-3-1, Hongo, Bunkyo-ku, Tokyo, Japan. 10

*Corresponding author. Tel.: +44 (0) 1214 142 670; E-mail: s.kaewunruen@bham.ac.uk 11

12

13

Abstract 14

Railway networks are the catalyst for economic and societal growth of the cities, region and 15

country. Their physical assets consist of infrastructure, rolling stock, signaling systems and 16

electrification. By nature, the railway infrastructure is nonlinear, judging from its behaviors, 17

geometry and alignment, wheel-rail contact condition and operational parameters such as 18

tractive efforts. It is noted that degradation of ballast over time has not been considered in 19

most train-track interaction models. Indeed, the ballast degradation can cause differential 20

settlement along the track and induce dynamic impact forces acting on partial and 21

unsupported tracks. In addition, it is reportedly that ballast damages underneath a local 22

railseat can cause the risk of centre-bound cracks in concrete sleepers due to the unbalanced 23

support under sleepers. These cracks are initially vertical under bending mode and can be 24

further developed, resulting in unsecured spreading rail gauge. This paper presents nonlinear 25

finite element simulations of concrete sleepers in a track system. The simulations take into 26

account the tensionless nature of ballast support coupled with the asymmetric topology. The 27

finite element model was calibrated using static and dynamic responses in the past. In this 28

paper, the influences of topologic asymmetry on both static and dynamic behaviors of 29

sleepers are firstly highlighted. The topology asymmetry is often caused by on-site 30

modification for structural retrofit or local track timber-plating component arrangements. In 31

2

ballast bed

sleeper

rail

rail pad & fasteners

subgrade

wheel

axle

addition, it is the first to demonstrate the effects of sleeper length on the service performance 32

of concrete sleepers at risk. The insight into the influences of asymmetric topology will help 33

improve the rail construction criteria in order to adjust support profile and appropriately 34

mitigate sleeper/ballast interaction. 35

Keywords: Railway sleepers, crosstie, asymmetric support, performance based design, vulnerability. 36

37

1. Introduction 38

It is commonly known that railway sleepers (also called ‘railroad tie’ in North America) 39

are essential elements of railway track structures. Two of their major duties are: first, to 40

distribute train wheel loads from the rails to the underlying track bed and foundation; second, 41

to maintain and secure track gauge for safe and smooth train passages. Based on present 42

design practices, the design life span of the concrete sleepers is targeted at around 50 years in 43

Australia, Asia and North America [1-4]; and around 70 years in Europe [5]. Note that the 44

exact design principle in Europe has not been fully addressed due to the recent development 45

of Eurocode. Figure 1 shows the typical ballasted railway tracks and their key components. 46

There have been a number of recent investigations on the railway sleeper models [6-9]. For 47

fast computations, most of the models in practice adopted the concept of beam on elastic 48

foundation where a sleeper is laid on the elastic support, acting like a series of springs. It is 49

reported that only vertical stiffness is sufficient to simulate the ballast support condition 50

because the lateral stiffness seems to play an insignificant role in sleeper’s bending responses 51

and overall structural strength [10-11]. 52

53

54

55

56

57

58

59

60

61

62

63

Fig. 1. Typical ballasted railway track components. 64

3

In fact, many field measurements suggest a diverse range of sleeper flexural behaviors, 65

which are largely dependent to the support condition induced by ballast packing and tamping, 66

as well as to track components such as rail pads and fastening systems [12-18]. However, it is 67

found that the static and dynamic behaviors of railway sleepers with asymmetrical topology 68

(dimensional changes) have not been fully investigated [19]. Figure 2 shows a typical layout 69

of a track turnout system, where asymmetrical topology can often be observed [20]. A 70

railway turnout system have generally been analysed the using a grillage beam method [21]. 71

It is however noted that similar type of sleepers with the dimensional changes can also be 72

caused by on-site modification during construction and maintenance of railway tracks. 73

74 75

Fig. 2. Typical turnout system layout, adopted from [20]. 76 77

It is apparent that there are a number of railway sleepers that are topologically 78

asymmetrical. However, research and investigation into their reduced service performance 79

has not been adequately carried out. This paper presents an advanced railway concrete sleeper 80

modeling capable of static and dynamic analysis into the nonlinear effect of topological 81

asymmetry on the flexural responses and service performance of railway sleepers. In this 82

paper, the focus is placed on the flexural response and eigen behavior of the railway concrete 83

sleepers subjected to various spectra of ballast stiffness at the mid span, in comparison with 84

the current design method in accordance with the design standards. The insight obtained will 85

help track and rail engineers to enhance predictive track maintenance regime and condition 86

monitoring strategy that could improve reliability, availability, maintainability and safety 87

(RAMS) of a railway network. 88

4

2. Materials and finite element method 89

2.1 Finite element simulations 90

Extensive researches since 1990s have revealed that the two-dimensional Timoshenko 91

beam model is the most suitable option for modeling concrete sleepers under vertical loads 92

[6-8]. In this simulation, the finite element model of concrete sleeper (optimal length) has 93

been adopted from the previous development that has been calibrated against the numerical 94

and experimental modal parameters [11-15]. Figure 3 shows the extended version of the two-95

dimensional finite element model for an in-situ railway concrete sleeper. Using a general-96

purpose finite element package STRAND7 (G+D Computing, 2001), the numerical model 97

included the beam elements, which take into account shear and flexural deformations, for 98

modeling the concrete sleeper. The trapezoidal cross-section was assigned to the sleeper 99

elements. The rails and rail pads at railseats were simulated using a series of spring. In this 100

study, the sleeper behaviour is stressed so that very small stiffness values were assigned to 101

these springs. In reality, the ballast support is made of loose, coarse, granular materials with 102

high internal friction. It is often a mix of crushed stone, gravel, and crushed gravel through a 103

specific particle size distribution. It should be noted that the ballast provides resistance to 104

compression only. 105

As a result, the use of elastic foundation in the current standards in Australia and North 106

America [1, 22, 23] does not well represent the real uplift behaviour of sleepers in hogging 107

moment region (or mid span zone of railway sleeper). In this study, the support condition was 108

simulated using the tensionless beam support feature in Strand7 [24]. This attribute allows the 109

beam to lift or hover over the support while the tensile supporting stiffness is omitted. The 110

tensionless support option can correctly represent the ballast characteristics in real tracks 111

[24]. Table 1 shows the geometrical and material properties of the finite element model. It is 112

important to note that the parameters in Table 1 give a representation of a specific rail track. 113

These data have been validated and the verification results have been presented elsewhere 114

[25-26]. 115

Based on our critical literature review, the flexural influences on railway concrete sleepers 116

due to the variations of ballast support conditions together with the asymmetric topology of 117

sleeper has not yet addressed by the past researchers [21, 23, 27]. Especially when the uplift 118

behaviour due to ballast tensionless support in hogging region of sleepers is considered, a 119

5

finite element analysis is thus required to supersede the simple manual calculation. For this 120

study, the numerical simulations have been extended to conduct the performance analyses 121

using the nonlinear solver in STRAND7. The effects of asymmetric topology of concrete 122

sleepers on their flexural responses in a railway track system can be evaluated. The length of 123

sleepers varies from 2.5m to 4.0m, which is practically common in the 2 and 3 rail-seats 124

sections (see Figure 3). 125

126

127 a) symmetrical topology 128

129

130 b) asymmetrical topology (overhanging) 131

132 c) asymmetrical topology (fully supported) 133

134

Figure 3 STRAND7 finite element models of railway sleepers with asymmetrical topology 135

136

L

L

L

6

Table1 Engineering properties of the standard sleeper used in the model 137

Definition Parameter Unit

Flexural rigidity cEI = 4.60, rEI = 6.41 MN/m2

Shear rigidity cGA = 502, rGA = 628 MN

Ballast stiffness bk = 13 MN/m2

Rail pad stiffness pk = 17 MN/m

Sleeper density s = 2,750 kg/m3

Sleeper length L = 2.5 m

Rail track width gt = 1.5 m

138

3. Flexural responses 139

Using the design data in Table 1, the static bending moment envelops along the sleeper 140

when subjected to the equal wheel loads of 100kN at both railseats in comparison with the 141

standard design moments can be shown in Figures 4 and 5. Based on AS1085.14 (Standards 142

Australia, 2003), the design maximum positive bending moment at the rail seat = 12.50 kNm, 143

while the centre negative design bending moment = 6.95 kNm (if considered half support) or 144

=12.50 kNm (if considered full support). It is typical that the positive and negative moments 145

are associated with the railseat and mid-span sections, respectively. It shows that the standard 146

design moments provide the conservative results. The standard design moment at mid span is 147

about half between the other two cases (see Figure 3). 148

149

Figure 4 Maximum bending moment of overhanging sleeper 150

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.5 1

L/

L

M*/Mdesign

At railseat

At mid span

7

151

Figure 5 Maximum bending moment of fully-supported sleeper 152

153

Figures 4 and 5 exhibit clearly that the influence of the asymmetrical topology is 154

pronounced when a contact between bearer and ballast layer exists. Considering field 155

investigation, such the contact could occur when there is a differential settlement on the 156

mainline track (for example, run-through turnout road). Once the ballast-bearer contact 157

establishes, the bearer will take additional bending moment at the inner railseat. 158

159

160

Figure 6 Dynamic behaviour of overhanging sleeper 161

162

4. Natural Frequencies 163

The natural frequencies of the railway sleepers with asymmetrical topology can be 164

observed in Figures 6 and 7. It can be seen that the topology of bearer plays a key role in 165

dynamic natural frequencies and corresponding mode shapes of the bearers. Overhanging 166

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.5 1 1.5

L/

L

M*/Mdesign

At railseat

At mid span

0

0.2

0.4

0.6

0.8

1

0 0.5 1

Mode 1Mode 2Mode 3Mode 4

8

bearers tend to be relatively much affected by the topology aspect in comparison with the 167

dynamic behavior of fully supported bearers. Figures 6 and 7 clearly display the dynamic 168

softening behavior of the railway sleepers with asymmetrical topology. It is clear that the 169

dynamic softening is more pronounced at a higher frequency range. 170

171

172

Figure 7 Dynamic behaviour of fully-supported sleeper 173

174

5. Conclusions 175

Railway sleepers with asymmetric topology are very common in practice, evidenced by 176

on-site modification of railway sleepers. This paper presents numerical simulations of the 177

railway sleepers with asymmetric topology in order to investigate nonlinear mechanics and 178

dynamics induced by the critical structural effects of a variety of ballast conditions and 179

asymmetric topology. The flexural responses and free vibration behaviors of the railway 180

sleepers and bearers in a turnout system (switches and crossings) have been highlighted. The 181

finite element model of railway sleepers, which was established and calibrated earlier, has 182

been extended for investigations in this study. The influences of the variation of ballast 183

support conditions at sleeper end together with the asymmetric length of sleepers on the 184

bending of the railway sleeper are highlighted in comparison with the standard design. By 185

using the nonlinear solver in STRAND7, nonlinear sleeper/ballast contact mechanics can be 186

simulated. Under static and free vibration conditions for overhanging and supported bearers, 187

the numerical results exhibit that the bending moment resultants are barely affected by 188

topological aspects when the ballast-sleeper contact is not established. The standard design 189

bending moments tend to be overestimated for the overhanging sleeper, whilst they can be 190

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

Mode 1Mode 2Mode 3Mode 4

9

highly underestimated when sleeper end is laid on ballast. Generally, positive bending 191

moments at inner railseat of sleeper have generally high sensitivity to the spectrum of ballast 192

support conditions in comparison with the more pronounced influence of sleeper length. In 193

such case, the nominal bending moment at inner railseat could be larger than the structural 194

capacity of sleeper and resulted in structural cracks and failure. In contrast, such behavior is 195

insignificant and tolerable for overhanging sleepers. In addition, it is found that the 196

asymmetrical topology induces dynamic softening in the railway sleepers with asymmetrical 197

topology. This implies that the asymmetrical sleepers are prone to damage under high-198

intensity impact loading. 199

200

6. Acknowledgements 201

The first author wishes to gratefully acknowledge the Japan Society for Promotion of 202

Science (JSPS) for his JSPS Invitation Research Fellowship (Long-term), Grant No L15701, 203

at Track Dynamics Laboratory, Railway Technical Research Institute and at Concrete 204

Laboratory, the University of Tokyo, Tokyo, Japan. The JSPS financially supports this work 205

as part of the research project, entitled “Smart and reliable railway infrastructure”. Special 206

thanks to European Commission for H2020-MSCA-RISE Project No. 691135 “RISEN: Rail 207

Infrastructure Systems Engineering Network”, which enables a global research network that 208

tackles the grand challenge of railway infrastructure resilience and advanced sensing in 209

extreme environments (www.risen2rail.eu). In addition, the sponsorships and assistance from 210

CEMEX, Network Rail, RSSB (Rail Safety and Standard Board, UK) and G+D Computing 211

(Dr Erik Kostson) with respect to STRAND7 are highly appreciated. 212

213

7. References 214

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