behaviour of the track in hot weather. rail thermal forces for jointed and cwr track

Constantin Ciobanu CEng MRes FPWI MCIHT

Principal Track Engineer

West of England 28/02/2017

Behaviour of the track in hot weather Rail thermal forces for jointed and CWR track

The physics of rail thermal expansion

ΔL = α L ΔT

- ΔL is rail extension - α is the expansion coefficient of the rail steel - L is the rail length - ΔT is the rail temperature variation

N = (α E ΔT)·A = σ A

- E is the steel’s elasticity modulus - A is the area of the rail section - σ is the rail stress generated by the temperature variation, ΔT.

Track parameters influencing the rail thermal behaviour

• Installation parameters

• Rail type

• Rail temperature

• Track longitudinal resistance

• Track lateral resistance

• Joint minimum and maximum gap - for jointed track

• Joint resistance - for jointed track

• Adjustment switch gap – for CWR

Installation parameters

• installation temperature and joint installation gap for jointed track,

• stress free temperature (SFT) for CWR track.

Rail type

N = (α E ΔT)·A = σ A

• Rail temperature range: [-14°C, 53°C] (NR/L2/TRK/3011).

Rail temperature (ΔT)

N = (α E ΔT)·A = σ A

Joint maximum gap Gmax = Bf + Df + Dr – 2Db – 2Br

Long rail. Short rail

Track longitudinal resistance

Three levels of action:

• P1 – resistance due to the friction forces between the rail and the fastening.

This is usually the first to activate, at very short movements of the rail. In this case the rail can move through the fastenings and the sleepers are stationary.

• P2 – resistance due to frictional forces between the fastening system and the sleeper.

The majority of rail fastenings don’t allow any relative movement at this level and in this common case P2 is ignored and the longitudinal resistance is only analysed for movements at the other two levels.

• P3 – resistance due to frictional and passive resistance forces between the sleeper and the ballast. In this case the rails and sleepers move together relative to the ballast.

This resistance is typically in the range of 6 (tamped) to 10 (consolidated) kN/sleeper.

Track longitudinal resistance – old vs new

Old track components

BR2 baseplate with Macbeth spring spike anchors

Bullhead rail Panlock chair fastening

P1 ≈ 0

P1 > P3

Pandrol Fastclip

Pandrol ZLR – Zero Longitudinal Resistance

P1 = 0

Modern track components

Track longitudinal resistance

Track lateral resistance

Three levels of action:

• Sleeper bottom (30-50%)

• Sleeper sides (40-60%)

• Sleeper end (10-30%)

Methods to increase the track lateral resistance

• Dynamic track stabilisation (DTS)

• Increase ballast shoulder dimensions

• Lateral resistance plates

• Glue ballast

• Heavier/special sleepers

Joint resistance force

The fishplated rail expansion joint has two main functions:

• to maintain the alignment of the rail running surface.

• to reduce the rail thermal forces by allowing rail expansion or contraction.

R = 4 n N f

Jointed track response to temperature variations

Jointed track response to temperature variations

… the detailed calculation process will be presented in the PWI Journal – Vol 135 Part 2 – April 2017

The track resistance forces retain thermal forces in the rails and define two envelope branches of the joint gap loop: - 12-13 - compression force - 7-8 - tension force Significant difference compared to the free thermal expansion model. Delayed response of the track to rail temperature variations: A one day temperature loop is contained in the envelope loop. Morning : 15°C, joint gap of 9 mm. The temperature increases during the day to a maximum of 35°C ,joint gap is reduced to 4 mm. During the night the temperature decreases to 10°C , gap increases to 6.3 mm. The next morning the temperature reaches 15°C, the gap remains 6.3 mm because the resistance forces have not been reversed during the 5°C increase from the minimum 10°C reached during the night.

• At installation the thermal forces through the length of the rail are consistently null.

• This state will never return naturally throughout the service life of the track. The thermal forces will never be consistently equal through the entire length of the rail, unless joint gap resetting or any similar maintenance works are undertaken.

• From thermal perspective the track behaves as a hysteretic model where the current state of the internal thermal forces is dependent on the rail temperature history.

Continuous Welded Rail (CWR)

The track composed of long rails which develops a central immobile (fixed) zone, where no rail movement due to temperature variation occur. (UIC definition)

Lb – Stress transition length (Breathing length) ≈90 -120 m. CWR L > 200m. Shorter lengths of track can be considered CWR from maintenance perspective.

L > 37 m NR/L2/TRK/3011 (2012) and L > 30 m NR/L2/TRK/2102 (2016)

Stress transition zone – one day temperature variation

Stress transition zone

Rail temperature difference • Tunnel (covered track) to natural sunlight track • Significant changes in sunlight exposure – passage from cutting to

embankment, changes in the direction of the track • Passage over a river – condensing water will reduce the rail temperature

compared to the track over embankment • Closure weld / stressing procedure (heat influence zone, different SFT)

Stress transition zone

Track structure variation • Presence of S&C • Change in rail type • Track over the mobile bearing of a bridge

S&C – thermal forces

• 2 = 4 ? • Switch rails are free to expand • Point operating equipment allowed to switch tracks

S&C – thermal forces. 2 = 4

• Stress transfer block – closure rail transfers the stress fully to the stock rail. CR/SR tied together • Switch/Stock rail thermal interaction devices (ball & claw or similar). CRs have limited independent expansion. Creep monitor. It is not a monitoring device but a partial stress transfer device. • POE designed to fully allow the switch rail expansion . CRs expand freely relative to SRs

Ball and Claw - Switch/Stock rail thermal interaction devices

Adjustment Switches

Joints with overlapping rail ends, allowing longitudinal rail movement and so dissipating thermal forces when CWR abuts jointed track or other features not designed to withstand thermal forces.

• CWR to jointed track

• CWR to CWR (when the track structure changes)

• Over the mobile bearing of long bridges

UIC 774 R. Track Bridge Interaction

Adjustment Switches

Joints with overlapping rail ends, allowing longitudinal rail movement and so dissipating thermal forces when CWR abuts jointed track or other features not designed to withstand thermal forces.

Adjustment Switch – rail breathing: CWR to Jointed Track (18.288m rails)

theoretical calculation

Adjustment Switch – rail breathing: CWR to Jointed Track (9.144m rails)

theoretical calculation

Adjustment Switch – rail breathing: CWR to CWR

Special design might be required, especially if one CWR section is over a long bridge. (UIC 774 R. Track Bridge Interaction)

theoretical calculation

Track buckling

Track buckling

The track buckling has two main stages: • Trigger phase (A-B) – track reaches unstable equilibrium • Energy release phase (B-C) – track releases tension and assume a new

stable equilibrium state

Track buckling (UIC 720R)

The buckling triggering energy is evaluated and a temperature limit is established, dependant on required factor of safety. Definition of the Critical Rail Temperature (CRT) – a safe rail temperature increase.

CRT – Critical Rail Temperature

NR/L2/TRK/001 Module 14. Inspection and Maintenance of Permanent Way. Managing Track in Hot Weather. The rules for evaluating the CRT are based on buckling calculations.