Cwr Over Bridges

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PROBLEMS & SOLUTIONS

TO CONTINUE LWR/CWR

OVERBRIDGES


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S. Venkata KumarDy.CE/TS/SC Rly

&

V.Sridhar

DEN/CKP/SE Rly

Page 2

INDEX

Sl.No Item Page No

1. Introduction 4

2. Analysis of problems due to interaction 5


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The advantages ofLWR compared to fish plated track arewell known. However, a number ofrestrictions exist in permitting

uninterrupted length ofLWR/CWR over the bridges. The problem in

continuing LWR/CWR over bridges has been a long debated

subject.

The problems are due to the interaction ofthe forces in the rail

and the bridge as well as displacement ofthe various elements ofthe

bridge and track.

This paper attempts to understand the interaction between the

track and bridge laid with LWR/CWR over it and suggests measures

to keep this interaction under control for the safe and satisfactory

behaviour ofthe track and bridge. In general, safety should be

ensured in that:

a) The track structure has to be safe against buckling at the

highest temperatures.

b) The maximum rail stresses in the rail under the worst

condition including live loads should not exceed the yield

limit ofrail steel.

c) The gap arising from the fracture ofthe rail at the lowest

temperature should not exceed a pre-determined limit.

d) The stresses in the girder as well as in the substructure of

the bridge should not exceed safe limits.

In view ofabove, the LWR manual has laid down certain

restrictions in laying ofLWR over the bridges taking into

consideration the provisions ofthe Bridge codes and manuals.

The paper also attempts to suggest measures to over come the

limitations prescribed in the laying ofLWR over bridges, since the

Railway Engineers the world over have realized the advantages of

welded tracks vis--vis fish-plated tracks.


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2.0 ANALYSIS OF PROBLEMS DUE TO

INTERACTION

When an LWR is introduced over a bridge, it rests on a

surface subjected to deformation and movements and hence it results

in displacement oftrack. Assuming that both track and bridge are

able to move, any force or displacement that acts on any one of them

will induce forces in the other.

Interaction therefore takes place between the track and the

bridge as follows:

- Forces applied to LWR track induce additional forces into

the track and/or into the bearings supporting the deck and

movements ofthe track and ofthe deck.

- Any movement ofthe deck induces a movement ofthe

track and an additional force in the track and, indirectly, in

the bridge bearings.

The interaction offorces between track and bridge as

explained above are those that cause relative displacement between

the track and the deck.

These are,

1. the thermal expansion ofthe deck only, in the case of

LWR, or the thermal expansion ofthe deck and ofthe rail,whenever a rail expansion device is present.

2. horizontal braking and acceleration forces

3. rotation ofthe deck on its supports as a result ofthe deck

bending under vertical traffic loads

4. deformation ofthe concrete due to creep and shrinkage

5. longitudinal displacement ofthe supports under the

influence ofthe thermal gradient

6. deformation ofthe structure due to the vertical temperature


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gradient.

In most ofthe cases, the first three effects are ofmajor

importance and hence only they are analyzed in this paper.

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3.0 DETAILED ANALYSIS OF MAJOR FORCES

3.1 Changes in temperature

The following aspects oftemperature variation should be

considered:

1. Changes in the uniform component ofthe temperature

which causes a change in length in a free moving

structure.

2. Differences in temperature between the deck and the

rails, in the case oftrack with an expansion device.

The reference temperature for a bridge is the temperature of

the deck when the rail is fixed. The temperature of the bridge doesnot deviate from the reference temperature by more than +350C, and

the temperature ofthe rail does not deviate by more than +500C. The

difference in temperature between the deck and track does not

exceed +200C. (In case oftrack with an expansion device.)

In the case ofLWR, a variation in the temperature ofthe track

does not cause a displacement ofthe track and thus there is no

interaction effect due to the variation in the temperature ofthe track.


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3.2 Horizontal braking and acceleration forces

The braking and acceleration forces applied at the top ofthe

rail are assumed to be distributed evenly over the length under

consideration and the values ofthese forces are to be taken as per

Bridge Rules.

These values are used for all types oftrack, i.e., LWR or fish

plated with and without an expansion device. These longitudinal

forces are to be combined with the corresponding vertical loads.

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3.3 Bending ofthe deck

Vertical traffic loads on the bridge generate large track/bridgeinteraction forces as result ofdeck bending, which cause

longitudinal displacement ofthe upper edge ofthe deck end. The

interaction effects depend primarily on the flexibility ofthe deck and

on the position ofits neutral axis, but are also influenced by the

stiffness of the fixed elastic support and by the height ofthe deck.

Horizontal displacement ofthe deck due to the traffic loads

remains constant when considered along the neutral axis but varies


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when measured at the upper part ofthe slab supporting the track.The flexibility of the fixed support reduces the displacements

measured above by a constant amount equal to the backward

displacement ofthe support.

These displacements, which result in interaction between the

deck and the track, generate large forces in the track and the

supports.

4.0 PARAMETERS AFFECTING TH

PHENOMENON

The predominant forces generated due to interaction between

track and bridges are dependent on a number ofparameters ofbridge

and track or both:

4.1 Bridge parameters

4.1(1) Expansion length of the bridge (L):

For a single span simply supported bridge, the expansion

length is the span length. For a continuous bridge with a fixed

support at the end, it is the total length ofthe deck. Ifthe fixed

elastic support is located at some intermediate point, the deck is

considered to have two expansion lengths on either side offixed

elastic support.

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4.1(2) Support stiffness:

The resistance ofthe deck to horizontal displacement is a

fundamental parameter as it affects all interaction phenomena. This

factor is determined primarily by the total stiffness ofthe supports.

The total support stiffness is composed ofthe stiffness ofeach

support. The stiffness ofeach support is in turn composed ofthe

stiffness ofthe bearing, pier, base, foundation and soil.

The stiffness K ofthe support including its foundation to

displacement along the longitudinal axis ofthe bridge is given by

where, p= displacement at the head ofthe support due to decks

deformation (this could be calculated assuming the pier to be a

cantilever fixed at the base)

= displacement at the head ofthe support due to foundationrotation.

h = displacement due to horizontal movement of the foundation.a = relative displacement between upper and lower parts ofthebearing


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The value ofthe displacement component is determined at the

level ofthe bearing as shown in the above figure.

4.1(3) Bending stiffness ofthe Deck:

As a result ofbending ofthe deck, the upper edge ofthe deck

is displaced in the horizontal direction. This deformation also

generates interaction forces.

4.1. (4) Height of the Deck:

The distance ofthe upper surface ofthe ofthe deck slab from

the neutral axis ofthe deck and the distance ofthe neutral axis from

the center ofrotation ofthe bearing affect the interaction phenomena

due to the bending ofthe deck.

4.2 Track parameters:

4.2(1) Cross sectional area of the Rail :

The Cross sectional area ofthe Rail is also an important track

parameter.

4.2(2) Track resistance:

The resistance k ofthe track per unit length to longitudinal

displacement u is an important parameter. This parameter in turn

depends on a large number offactors such as whether the track is

loaded or unloaded, ballasted or caked, standard ofmaintenance etc.

The resistance to longitudinal displacement is higher on loaded track

than on unloaded track as can be seen from the figure below. The

value ofk has to be established by each railway system as per its

track structure.


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TRACK STIFFNESS PARAMETERS (FROZEN BALLAST)

Once the values ofK, the stiffness ofthe bridge structure and

k, the stiffness ofthe track have been evaluated, use can be made of

the interaction diagrams given in UIC774-3R for calculation ofthe

additional stresses in the rail and additional forces at the bridge

support due to each ofthe actions causing interaction effects: viz.,

(1) change oftemperature (2) acceleration and braking forces (3)

deck deformation.


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TRACK STIFFNESS PARAMETERS (NORMAL BALLAST)

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5.0 COMBINATIONS OF EFFECTS:

In view ofthe above, the consequence for the bridge laid withLWR track, the different criteria to be satisfied are as given below :

a) The permissible rail stresses in LWR should

be within limits.

b) Limits have to be placed on the absolute and

relative displacements ofthe deck and the

track

c) Limits are to be placed on the permissible end


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rotations ofthe bridge.d) The bridge elements should be designed for

the additional reactions due to the bridge-track

interaction.

Based on the above theoretical analysis ofthe bridge

and track, the LWR can be continued safely over the bridges.

But, for doing this, each individual bridge requires a detailed

analysis. Utilizing the interactive design graphs available in

UIC report 774-3R, this can be done. In this report, it has also

been indicted that a computer program has been developed for

track-bridge analysis and field tests have validated the results

ofthe theoretical analysis.

However, for the utilization ofthe above UIC report,

large number ofbridge and track parameters along with the

structural arrangement with load disposition and permitteddisplacements is required.

It is because ofthe difficulty in obtaining the above

data for each and every bridge and the rigorous analysis to be

done, that the LWR manual has prescribed the locations

where LWR can be provided with a simple consideration of

temperature variation alone.

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6.0 EFFECT OF TEMPERATURE VARIATION


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For a simple understanding ofthe problem let usconsider the effect ofthermal variation alone as the cause of

interaction between the girder and the LWR. As a result of

thermal variation the girder provided with bearings has a

tendency to expand or contract. On the other hand the central

portion ofthe LWR is fixed in position irrespective ofthe

temperature changes that occur. This results in interplay of

forces between the girder and the LWR, the magnitude oftheforce being dependent upon the nature offastenings being

provided between the rail and sleeper. To clarify this aspect of

interplay offorces between rail and girder, consider the case

ofa girder bridge provided with fastenings between the rail

and sleeper with a creep resistance equal to p kg per rail

seat. The bridge sleepers are rigidly fixed to the top flange of

the girder by means ofhook bolts. On variation of

temperature due to the creep resistance ofthe fastenings, free

expansion/contraction ofthe girder is prevented.Consequently additional forces are developed both in the

girder as well as in the rail. The magnitude of this force

developed depends upon the value ofp (the creep resistance)

and orientation/nature ofthe bearings provided in each span

ofthe bridge.

The following cases have been considered:

Single span bridge : 1. One end fixed, other end free.2. Both ends ofgirder with free bearings.

Multiple span bridge:1. One end fixed and the other free

with dissimilar bearings on a pier

2. One end fixed and the other free with

similar bearings on a pier

3. Free bearings at both ends.

The forces developed in the rail and girder for each ofthe five

cases mentioned are shown in Figs. 6.1 to 6.5:


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Fig.6.1

Fig.6.2


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Fig.6.3

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Fig.6.4


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Fig.6.5

These LWR force diagrams indicate that:

i) For sliding bearings at both ends ofthe girder, the increment of

force in the LWR is np/4, where n is the number ofsleepers per

span with creep resistant fastenings and p is the creep resistance

per rail seat (Fig.6.1). This increment offorce will remain the same

irrespective ofthe number ofspans ofthe bridge (Fig.6.4).

ii) In girders with one end fixed and the other end free the increment

offorce in the LWR at the roller end is np/2for a single span bridge,

where n = number ofsleepers in the span with creep resistance ofp

kg per rail seat (Fig.6.2). Ifit is a multiple span bridge with m

number ofspans, the increment offorce in the LWR at the roller end

will be mnp/2. The resultant LWR force diagram is shown in the

sketch (Fig.6.3). This is the case when on a pier, bearing for one

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girder is a fixed bearing while the bearing ofthe other girder is a

free bearing.

iii) There could be a situation where a pier supports similar nature

bearings i.e. the bearings ofthe two girders are either fixed or free.

In this case there will be no cumulative build up of force and theresultant LWR force diagram will be as indicated in Fig.6.5.

In order to avoid interplay offorces between the LWR and

girder a possible solution would be to provide rail free fastenings

between rail and sleeper on the girder bridge. It is with this

assumption that the provisions for laying LWR over bridges have

been framed in the LWR manual.


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Fastenings used to connect the rail to the sleeper could be oftwo types:

(1) Creep resistant fastenings and

(2) Rail free fastenings, which are now termed as zero

longitudinal restraint fastenings.

RDSO Report No. C-169 investigates the creep resistance

offered by different types ofrail sleeper fastenings. On the IndianRailways we have been traditionally using dog spikes and rail

screws as rail free fastenings although now Pandrol has come up

with a zero longitudinal restraint design as shown below.

Fig. 6.6

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Under normal circumstances there is a small gap between the

base plate (steel) and the top side ofthe rail foot. In case oflarge


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lateral forces, the base plate prevents the overturning ofthe rail. Thepad under the rail is made up oflow friction material like teflon,

which provides an almost zero friction movement between the rail

and sleeper.

Use ofrail free fastenings on bridges where LWR is proposed

to be used, is now mandatory due to requirement ofminimizing the

interaction offorces between the LWR and the girder. However, thisresults in another problem: enhanced gap at fracture, when the

fracture occurs on the approach ofbridge laid with LWR. Consider

an LWR laid on normal formation with the usual force diagram A B

C D. In the event offracture at location F the stress in the LWR is

released at that location and two new breathing lengths B1F and C1F

are formed on either side ofthe fracture locations as shown under.

Fig. 6.7

The gap g1 at the fracture location will be given by

[Assuming equal movement on either side ofF]

R| represents the longitudinal ballast resistance mobilised at the time

ofthe fracture, which is generally about 50% to 60% ofthe normal

R value, due to the sudden nature ofoccurrence ofa fracture.


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However, ifthe same fracture had occurred in the approach of

a bridge provided with LWR and rail free fastenings the

modification ofthe force diagram will be as given in the figure 6.8.

Fig. 6.8

In this figure, ABCDEFGH represents the altered force diagram.

Gap at fracture in this case will be

Where L0 is the span length ofthe bridge provided with rail free

fastenings.

Expressions (1) and (2) indicate that the gap at fracture is

enhanced by an amount equal to L0 t, when a girder bridge with rail

free fastenings is located in the central portion ofthe LWR. Indian

Railways have fixed the permissible gap at fracture as 50mm where

by expression (2) becomes

Over the years attempts have been made to increase the value

ofL0by adopting various techniques: -


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(1) One way could be to increase the value ofR, the longitudinal

ballast resistance mobilized at the fracture. This could be done

by: -

. Compacting the ballast in shoulders and cribs of the bridge

approach sleepers.

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Enhancing the sleeper density to 1660 Nos./km in the bridge

approach.

Heaping up ofballast in the bridge approach starting from the

foot ofthe rail.

Box anchoring sleepers wherever required.

These measures have to be taken in the bridge approaches

50m on either side. Table 1 ofthe LWR Manual 1996 gives

the maximum overall length ofgirder permitted on

LWR/CWR with the following stipulations:

1. Girder bridge should have sliding bearings on each end

with single span limited to 30.5m.

2. Rail should be provided with rail free fastenings throughout

the length ofthe bridge from abutment to abutment.

3. The approach track should be suitably upgraded as

mentioned above.

2) Another way ofincreasing the value ofLo would be to improve

the approaches as mentioned above in addition to providing a few

sleepers on each span with creep resistant fastenings. The creep

resistant fastenings will hold the rail and prevent the gap at fracture

from becoming excessive.


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However, provision ofcreep resistant anchors impliesinterplay offorces between the rail and grider. Hence the following

stipulations are made for bridge provided with rail free fastenings

and partly box anchored (with single span not exceeding 30.5m and

having sliding bearings at both ends).

(1) On each span 4 central sleepers will be provided with creep

resistant fastenings and remaining sleepers with rail free fastenings.(2) Bridge timbers laid on girders shall not be provided with through

notch but shall be notched to accommodate the individual rivet

heads.

(3) The girders shall be centralized with reference to the location

strips on the bearing before laying LWR/CWR.

(4) The sliding bearings shall be inspected twice a year and oiling

and greasing ofthe bearing carried out once in two years.

These provisions ofLWR manual are enclosed as Annexure-I.

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7.0 POSSIBLE SOLUTIONS :

Primarily the problem in laying LWR over the bridges is that

there is a severe limitation in the individual span length, the overall

length ofthe bridge and the disposition/type ofthe bearings as per

Para 4.5.7.1.i & 4.5.7.ii ofthe LWR manual. These can be partially

overcome by utilizing the provisions ofPara 4.5.7.iii by providing

SEJs pier to pier. This restriction in the provision ofthe SEJ can also

be overcome by continuing the LWR across the entire bridge by

utilizing the provisions ofPara 4.5.7.iv ofthe LWR manual. But

from the understanding ofthe behavior ofthe LWR this implies that

the SEJs have to be designed for greater movements i.e. wide gap

SEJs need to be used. Already on the Indian Railways wide gap


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SEJs with 190 mm gap have been approved by RDSO (Drg. No.RDSO/T-6039 & T-6262 for 52 kg. & 60 kg. Rails respectively).

The limitations in the length over which LWR can be carried

over bridges can be extended/overcome by undertaking a

rationalized analysis ofthe forces and stresses as explained in this

paper by utilizing the UIC code 774-3.This analysis will permit

increased length ofLWR that can be laid over a bridge. In fact,generally speaking, the maximum expansion length ofLWR laid on

bridges with ballasted deck (without expansion devices) can be 60 m

for steel structures and 90 m for concrete structures. Iffixed bearing

is used in the middle, the above lengths can be doubled. But this is

possible only in new constructions where the bridges and their

bearings can be suitably designed for the above forces and

constructed accordingly. Since the above limitations are for major

and important bridges, for which detailed analysis and design is

undertaken before hand, the analysis for forces due to LWR should

also be attempted by utilizing the UIC code 774-3.

On sub-urban /metro sections where the axle loads are less,

since there is a relieffrom the axle loads and also longitudinal

forces, the LWRs can be designed and provided for greater lengths

over bridges.

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To prevent the transfer offorces between the LWR and

bridge, improved design ofzero restraint longitudinal fastenings


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should be designed. In fact some trials are currently under way onthe Indian Railways with steel channel sleepers and also concrete

sleepers.

In individual cases, where practically possible, by imposing

restrictions in the braking and acceleration ofthe trains, extension of

LWR over the bridge can be attempted.

Another solution to the problem can be utilizing better quality

rails with either or both increase in the sectional area or allowable

stresses in the rails.

8.0 LIST OF REFERENCES:

1.Long welded rails - IRICEN/Pune.

2. UIC Code 774-3R. Track/bridge interaction-Recommendations

for calculations

3.UIC Code 720-R Laying and maintenance of CWR Track.

4. Manual of instructions on Long Welded Rails 1996

5. CWR on unballasted open deck bridges Vinod Kumar


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Annexure-

RELATED PROVISIONS OF THE LWR MANUAL FOR

LAYING LWR ON BRIDGES.

4.5.5 Location ofSEJ:

The exact location ofSEJ shall be fixed taking into account thelocation ofvarious obligatory points such as level crossings, girder

bridges, points and crossings, gradients, curves and insulated joints.

SEJ with straight tongue and stock shall not be located on curves

sharper than 0.5 degree (3500 m radius) as far as possible. SEJ shall

not be located on transition ofcurves.

4.5.6 Bridges with ballasted deck (without bearing):

LWR/CWR can be continued over bridges without bearings like

slabs, box culverts and arches.

4.5.7 Bridges with/without ballasted deck

i)LWR/CWR shall not be continued over bridges with overall

length as specified in para 4.5.7.1 for BG and not more than 20

metre for MG.

ii)Bridges on which LWR/CWR is not permitted/provided shall

be isolated by a minimum length of36 metre well anchored

track on either sides.

4.5.7.1 i) Bridges provided with rail-free fastenings

(single span not exceeding 30.5 metre and having sliding

bearings on both ends)


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Overall length ofthe bridge should not exceed the maximum as

provided in Table-1 with following stipulations:-

a) Rail-free fastenings shall be provided throughout the length of

the bridge between abutments.

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b) The approach track upto 50 m on both sides shall be well

anchored by providing any one ofthe following:-

i) ST sleepers with elastic f astening.

ii) PRC sleepers with elastic rail clips with fair T or

similar type creep anchors.

c) The ballast section ofapproach track upto 50 metre shall be

heaped upto the foot ofthe rail on the shoulders and kept in

well compacted and consolidated condition during the months

ofextreme summer and winter.

4.5.7.1 ii) Bridges provided with rail-free fastenings and

partly box-anchored (with single span not exceeding

30.5 metre and having sliding bearings at both ends)

Overall length ofthe bridge should not exceed the maximum as

provided in Table-1 with following stipulations:-

a) On each span, 4 central sleepers shall be box-anchored with fair

V or similar type creep anchors and the remaining sleepers shall be

provided with rail-free fastenings.


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b) The bridge timbers laid on girders shall not be provided withthrough notch but shall be notched to accommodate individual rivet

heads.

c) The track structure in the approaches shall be laid and maintained

to the

standards as stated in item 4.5.7.1 (i) (b) and (c) above.

d) The girders shall be centralised with reference to the location

strips on the

bearing, before laying LWR/CWR.

e) The sliding bearings shall be inspected during the months of

March and October each year and cleared ofall foreign materials.

Lubrication ofthe bearings shall be done once in two years.

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4.5.7.1 iii) Welded rails may be provided from pier to pier with

rail-free fastenings and with SEJ on each pier. The rail shall be box-

anchored on four sleepers at the fixed end ofthe girder ifthe girderis supported on rollers on one side and rockers on other side. In case

ofgirder supported on sliding bearings on both sides, the central

portion ofthe welded rails over each span shall be box-anchored on

four sleepers.

See Fig.4.5.7.1(iii).

4.5.7.1 iv) LWR/CWR may also be continued over a bridge with

the provision ofSEJ at the far end approach ofthe bridge using rail-free fastenings over the girder bridge (Fig. 4.5.7.1 (iv)). The length

ofthe bridge in this case, however, will be restricted by the capacity

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ofthe SEJ to absorb expansion, contraction and creep, ifany, ofthe

rails. The length ofthe bridges with the above arrangement that can


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be permitted in various rail temperature zones for LWR/CWR with

SEJs having maximum movement of120 mm and 190 mm are as

follows:-


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Cwr Over Bridges