Of course, more expansion joints might provide relief if the SFT remains 27C.
As posted previously, there are no expansion joints in CWR. Having any expansion joints makes it not continuously welded, and there would be no point in stressing it.
Of course, more expansion joints might provide relief if the SFT remains 27C.
Good point! Too much focus on buckled rails recently - I was simply thinking of ways to combat rail heat stress generally.As posted previously, there are no expansion joints in CWR. Having any expansion joints makes it not continuously welded, and there would be no point in stressing it.
I'm now very confused with the situation regarding expansion joints - I'm sure I've seen them fitted on the UK rail network, but if they aren't appropriate for CWR where are they appropriate for?
Areas where CWR join up to jointed track or to certain types of switches and crossings (that are known as ‘unstrenghtened’). This is typically an adjustment ‘switch’ (colloquially known as a ‘breather’), where the joint is diagonal over about 300mm if viewed from above. Where you see these, you will usually find short pieces of rail bolted to the centre of the sleepers across the joint; this prevents the sleepers moving relative to each other and thus allows the ‘free’ rail to expand up to the anchored rail.
The main reason for CWR is the to reduce track damage and wear. Jointed track is vulnerable to rail end breakage. The fisplate bolt holes are a weak point and when jointed track was the norm, breakage at that weak point was a major failure mode. In addition, the rails would bend as heavy axle loads pass over them causing the ballast to loosen beneath the adjacent sleepers. Thus, jointed track needed so much more maintenance that today's high speeds might not be economically viable.Thanks for the reply! This is the example I have seen and it makes sense, I'm puzzled as to why they couldn't fit this type of expansion joint at regular intervals on long stretches of CWR, the main benefits of CWR being noise and comfort these seem to be much quieter than your standard fishplate joint, and would appear to help with the issue of expansion on extreme heat.
Thanks for the reply! This is the example I have seen and it makes sense, I'm puzzled as to why they couldn't fit this type of expansion joint at regular intervals on long stretches of CWR, the main benefits of CWR being noise and comfort these seem to be much quieter than your standard fishplate joint, and would appear to help with the issue of expansion on extreme heat.
The main reason for CWR is the to reduce track damage and wear. Jointed track is vulnerable to rail end breakage. The fisplate bolt holes are a weak point and when jointed track was the norm, breakage at that weak point was a major failure mode. In addition, the rails would bend as heavy axle loads pass over them causing the ballast to loosen beneath the adjacent sleepers. Thus, jointed track needed so much more maintenance that today's high speeds might not be economically viable.
A question to the more knowledgeable folk.
A little scenario. A long length of CWR in a high temperature environment. The track is in “perfect” condition, so good sleepers, ballast etc.
So the rail is effectively supported and “clamped” into position. As the temperature rises, the rail expands. Where does that expansion go? Is it a case of no matter how good the track, at some point it’s going to buckle if it gets hot enough?
Having said all that, whilst the rail can’t expand in length, it does of course does expand width and depth, but in fractions of a millimetre!
However, rail will move longitudinally in stress transition zones - The more obvious one where the forces in the rail tend towards zero at the adjustment/breather switch.
Stress transition zones can occur in the middle of CWR sections. Tunnels are a good example as there is a varying difference in rail temperature compared with that outside in the sunshine.
Is there a tendency for the rail to "work" back and forth near breather switches as temperature fluctuates, so that the expansion range of the switch is gradually used up? Or are the fasteners guaranteed to hold the rail in a fixed position once a certain distance from the switch?However, rail will move longitudinally in stress transition zones - The more obvious one where the forces in the rail tend towards zero at the adjustment/breather switch.
Another idiot question - which in my role of 'man in the pub' I couldn't answer the other evening:
When heat does cause track to buckle
- how much (length-wise) of the track buckles?
- what happens next? Does the track go back into gauge when everything cools down, or does the buckled section need to be replaced?
- how long does sorting things out take?
I appreciate that these are all 'how long is a piece of string' type questions, but (simple) answers would be appreciated...
Thank you.To answer in ‘how long is a piece of string’ style:
1) it varies, it can be a small kink, it can be up to 100metres or more
2) no it doesn’t go back into gauge, or even remotely close to its correct position. It needs to be physically moved back.
3) that depends on the amount that has shifted. A small kink would take a couple of hours, a big buckle may take up to 24 hours or so. This also depends on the rail temperature, as it is difficult to do in the heat. Fortunately, most buckles happen between 1200-1700, and by the time resources are in place it is mid to late evening and things have cooled down. If there is an accompanying derailment, it can be longer of course.
Is there a tendency for the rail to "work" back and forth near breather switches as temperature fluctuates, so that the expansion range of the switch is gradually used up? Or are the fasteners guaranteed to hold the rail in a fixed position once a certain distance from the switch?
Not that I'm aware of.Are there any restrictions on the use of CWR on Waybeam bridges?
Thinking of short span bridges less than 10m.
The adjustment switch blades are set (at Stress Free Temperature) to allow 100mm of rail movement but couldn't tell you how much of that got used up in the latest heatwave.
I might be misinterpreting your post, but are you implying that rail will expand and contract (cyclically from night to day, sunshine to cloud et cetera) but over time the rail won't contract back as much as it's expanded and slowly eat into that 100mm spacing? Some sort of heat-induced expansion creep? Sounds plausible but I suspect if there is such a phenomenon it'll be very small. Activities, namely tamping, will have a greater effect on the 100mm and there are maintenance standards requiring routine inspection of the switches.
This chap, who is an academic and also works for a well-known UK design consultancy, has a very good permanent way blog - He covers stress transition zones in this:
https://pwayblog.com/2017/08/30/rail-stress-transition-zone/