dr. david rhodes may 2017 - 한국철도학회에 오신것을...

Dr. David RhodesMay 2017


TRACK-BRIDGE INTERACTION

The old way: “Rigid” bridge structure with short spans;“Flexible” track structure with jointed rails, wooden sleepers and ballast.



The new way: “Flexible” bridge structure with long spans;“Rigid” track structure with continuous rails, on concrete slabs.



TECHNICAL ISSUES:

1. Effect of temperature on axial forces and displacements



Bridge decks are different: They are longer on hot days thanthey are on cold days.

Continuous welded rail does not expand or contract –Whatever the temperature it stays the same length.


In cold weather the bridge deck contracts –the longer the bridge, the more the free end of the deck moves.

Bri

dg

e d

eck

dis

pla

cem

ent



“Ad

dit

ion

al”

rail

stre

ss

In cold weather the bridge deck contracts –the longer the bridge, the more the free end of the deck moves.

If there is enough “give” between the track and bridge, the track is not affected.



Note: Here (and for the rest of the presentation!):

“Additional” rail stress means rail stress which is added to the stress which would be present of the same track form were to be used in the same conditions but without the bridge being there.

We’ll also talk about “additional” track forces, displacements, etc., etc.



In real life, there is never enough “give” and so the bridge deck drags the rail with it,

Rai

ld

isp

lace

men

t



In real life, there is never enough “give” and so the bridge deck drags the rail with it,generating a stress in the rail which is proportional to the slope of the displacement.

Rai

l str

ess



For a single span bridge net forces in the rail are reacted at the abutments, whichare usually very stiff in compression, but for multi-span bridges it is also necessaryto think about the stiffness of piers and the forces at the bearings on each pier:


?



TECHNICAL ISSUES:

1. Effect of temperature on axial forces and displacements2. Effect of traction and braking on axial forces and displacements


Similar effects to those of temperature, but this time the (train drags the) rail drags the bridge deck.




TECHNICAL ISSUES:

1. Effect of temperature on axial forces and displacements2. Effect of traction and braking on axial forces and displacements3. Effect of vertical loads on axial forces and displacements



Bridge deck end rotation.



Bridge deck end rotation.Longitudinal movement plus rotation / uplift



Bridge deck end rotation (DB DS804 App 29.)



TECHNICAL ISSUES:

1. Effect of temperature on axial forces and displacements2. Effect of traction and braking on axial forces and displacements3. Effect of vertical loads on axial forces and displacements4. Characteristics of structures



The key factors determining the significance of Track-Bridge Interaction effects are ….

a. Expansion length of bridge deck(s)





b. Resistance to shear forces between the rails and the bridge deck

FO

RC

EDISPLACEMENT OF RAIL RELATIVE TO BRIDGE DECK


FO

RC

E

DISPLACEMENT OF RAIL RELATIVE TO BRIDGE DECK

Useful approximations:

u0

k

u0 is the maximum elastic displacement:For some cases (e.g. long bridges withlongitudinally stiff track) you can make a further simplification by putting u0 = 0.

k is the force per unit length of track i.e. itis the force in two rails. The units are kN/m but this k is NOT a stiffness!!







c. Stiffness of piers






c. Stiffness of piers

d. Detail design of bridge deck ends



TECHNICAL ISSUES:

1. Effect of temperature on axial forces and displacements2. Effect of traction and braking on axial forces and displacements3. Effect of vertical loads on axial forces and displacements4. Characteristics of structures5. Combinations of load cases



For example:

• Additional rail stress due to temperature with no train present: 54 MPa

• Additional rail stress due to braking forces at neutral temperature with “zero axle load”: 25 MPa

• Additional rail stress due to deck end rotation at neutral temperature with no traction or braking: 16 MPa

• Therefore total additional rail stress due to Track-Bridge Interaction effects: 95 MPa

This simple approach is accepted by most codes and standards – but it gives the wrong answer!



Think about rail force vs.displacement:

FORCE

DISPLACEMENT

F temp




FORCE

DISPLACEMENT

F brake




FORCE

DISPLACEMENT

F deck end



Think about rail force vs.displacement: (Simple linear superposition)

DISPLACEMENT

FORCE

F temp

Ftemp + F brake



Think about rail force vs.displacement: (Realistic superposition?)

FORCE

DISPLACEMENT

F temp + F brake + F deck end



Simple linear superposition of load cases generally leads to over-estimates of additional rail stress …….

…. but it also tends to under estimate forces on piers .



TECHNICAL ISSUES:


TECHNICAL SOLUTIONS:

1. Bridge design



Bridge design – basic principles

1. Accept policy on use of rail expansion devices and acceptable rail stresses

2. Short, stiff, simply supported spans are good for Track-Bridge Interactions!

3. Control expansion lengths by positioning thermal fixed points

4. Control bridge deck end effects by attention to detail design

5. Integral / semi-integral bridges need integrated structural design (i.e. analyse the bridge and track together as a total structure)



TECHNICAL ISSUES:


TECHNICAL SOLUTIONS:

1. Bridge design 2. Track design


Track design – basic principles

1. Retain standard track configuration and component selection as far as possible

2. Decide policy on use of rail expansion devices and acceptable rail stresses




Rail expansion joints in ballasted track



Rail expansion joints in ballastless track


Rail fastenings for “Reduced Longitudinal Restraint”



Rail fastenings for “Zero Longitudinal Restraint”


Use of ZLR fastenings to optimise positioning of REJ




Ballast retention at structural movement joint



TECHNICAL ISSUES:1. Effect of temperature on axial forces and displacements2. Effect of traction and braking on axial forces and displacements3. Effect of vertical loads on axial forces and displacements4. Characteristics of structures5. Combinations of load cases

TECHNICAL SOLUTIONS:1. Bridge design 2. Track design

ORGANISATIONAL ISSUES



ORGANISATIONAL ISSUES:

BRIDGE DESIGN TEAM

TRACK DESIGN TEAM

Initial design concepts

Select optimum configuration

Detail design calculations

Track-Bridge Interaction assessment

Re-evaluate configuration??

?




BRIDGE DESIGN TEAM TRACK DESIGN TEAM


Select optimum configurationDetail design calculations


Final bridge design

Agree Track-Bridge Interaction principles

Final track design




BRIDGE DESIGN TEAM

TRACK DESIGN TEAM


Select optimum configuration

Detail design calculations


Final bridge design

Set Track-Bridge Interaction limits

Consultation

Consultation

Final track design




Key considerations:

1. Some track design parameters must be established from the outset.

2. Track-Bridge Interaction effects must be assessed before the final bridge configuration is fixed.

3. Any assumptions made by bridge designer about track configuration must be agreed with track engineers.

4. Any assumptions made by bridge designers about track configuration must be recorded.






“CULTURAL” ISSUES



“CULTURAL” ISSUES: Bridge Department

TrackDepartment



“CULTURAL” ISSUES: Bridge Department

TrackDepartment

Each project designed from a “clean sheet of paper”

Maximum use of standard components and track designs

Calculate from first principles for Ultimate and Service load states

Empirical data underlie strength and durability requirements

Explicit safety and dynamic factors applied to theoretical load cases

Safety and dynamic factors implicit in design and test requirements






“CULTURAL” ISSUES

CODES and STANDARDS



CODES and STANDARDS:

Begin by understanding the technical issues –then see how codes and standards should be applied!

(Later this year (or early in 2018?) CEN will publish a Technical Report to explain the technical issues)




In Europe,

for bridge engineers, the key document is in the Eurocode EN1991-2:2003 section 6.5.4

for track engineers, the key document is UIC 774-3R




Standards set limiting values on additional axial rail stress and displacements at structure movement joints.

Limiting values on additional axial rail stress are only valid for a ‘standard’ trackform:Standard gauge ballasted trackStraight track (or large radius curve)60E1 (“UIC60”) or 60E2 railConcrete sleepers >250kg massRail steel grade R260 or better

For other track configurations, the track engineer must confirm that the same limits are acceptable, or set alternative requirements.

dr. david rhodes may 2017 - 한국철도학회에 오신것을...

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