Single Track versus Twin Track Cantilevers

[edwin_m]

I've pulled these from the Transpennine Upgrade site as it's really a wider question:

I understand the different role played by the SSA, but why the constant changes from TTC to STC.

Do they have different advantages or disadvantages? There doesn't seem to be any pattern to it, often swapping one to the other as on the sweeping bend between the White Rose Centre footbridge and the Natasha Elliot footbridge for example. I would have thought simplicity and cost would mandate the use of TTC as often as possible? Less steel, fewer piles? What am I missing?

Regarding advantages and disadvantages, a single track cantilever of course requires much less steel and is less visually intrusive, however, when placed on the inside of a curve they can block the view of signals, hence twin track cantilevers are used on the approach to signals, with the supporting post placed on the outside of the curve. This has been standard practice with electrification since the OLE renewals on the WCML and after the Ladbrooke Grove crash which brought OLE structure and signal placements in the spotlight in the early 2000s.

I think the relative benefits must be pretty marginal for double track. The Midland used TTCs almost exclusively on the double track sections between Bedford and Wellingborough but TRU appears to be using STCs except where there is a reason (such as signal sighting) to use TTCs instead. STC has more piles and masts to install, though they are smaller, but TTC needs an extra visit to site to crane in the boom before the cantilevers can be attached.

 

[GRALISTAIR]

Thanks for starting this thread. A fascinating subject which will be beneficial to all electrification enthusiasts such as myself. I will almost certainly put this thread in the pinned thread too.

Here is what I think is a fairly obvious one. TTCs being used at Blackpool North as otherwise the canopy would have had to have been cut back etc.

 

[Nottingham59]

. The Midland used TTCs almost exclusively on the double track sections between Bedford and Wellingborough

I assumed that was for ease of access on a four track railway.

 

[twpsaesneg]

STC's are cheaper, but it is quite marginal when you factor in 2 foundations vice 1 in the overall scheme of things. But marginal enough to make STC's worth using!

TTC's on MMLE and GWEP were as has rightly been said, for ease of access on the 4 track railway.

The main reason for the adoption of TTC's on 2-track railways are signal sighting, obstructions in the cess that are too difficult / costly to avoid, poor ground conditions, avoiding an obstruction (e.g. a skew bridge foundation). Very occasionally one will be put in if the reach (distance from OLE structure to wire) is too great for a normal cantilever to deal with. This is not however an exhaustive list.

The disadvantages of a TTC are that the foundations are typically far deeper and thus more expensive and more prone to refusal, the steelwork is fabricated rather than a plain section and thus more expensive and heavy. There is also a limitation on the load that can be put on a standard Circular Hollow Section steel pile connection which heavily loaded TTC's can breach, necessitating a more expensive foundation solution.

For more information see https://ocs4rail.com/

 

[GRALISTAIR]

The disadvantages of a TTC are that the foundations are typically far deeper and thus more expensive and more prone to refusal, the steelwork is fabricated rather than a plain section and thus more expensive and heavy. There is also a limitation on the load that can be put on a standard Circular Hollow Section steel pile connection which heavily loaded TTC's can breach, necessitating a more expensive foundation solution.

For more information see https://ocs4rail.com/

Pile - then auger out, then backfill with concrete.

 

[twpsaesneg]

Pile - then auger out, then backfill with concrete.

Wet trades are very expensive, augering costs around 50 - 100% more than a percussively / vibro driven CHS pile.

Also, when you hit rock due to the depth of foundation you need it becomes even more of a pain

We use Augered foundations where necessary, but they are not in favour!

 

[GRALISTAIR]

Old fashioned concrete gravity pads then such as was used on Preston - Manchester? Including this bad boy at Salford Crescent?

 

[DelW]

Pile - then auger out, then backfill with concrete.

I would guess that most of the load on the pile will be bending rather than axial. Concrete fill won't help (much) with that unless you put a cage in it as well. I'd imagine the connection detail is the limiting factor rather than the CHS itself.

Old fashioned concrete gravity pads then such as was used on Preston - Manchester? Including this bad boy at Salford Crescent?

Multiple visits by different trades - excavate / blind / side forms / steelfix / concrete / strike forms / backfill. Much more to organise and co-ordinate.

 

[HSTEd]

Has anyone ever done any studies on the use of structural aluminium sections for this sort of stuff?
The metal is more expensive but they can achieve some impressively lightweight solutions.

 

[edwin_m]

I assumed that was for ease of access on a four track railway.

I was thinking of the bits round Wymington where it's two separate double tracks. Or maybe there's no easy access to the bit in between for large plant?

 

[quantinghome]

Has anyone ever done any studies on the use of structural aluminium sections for this sort of stuff?
The metal is more expensive but they can achieve some impressively lightweight solutions.

I don't think the structural dead load is the problem. They're pretty light structures already.

 

[Nottingham59]

I was thinking of the bits round Wymington where it's two separate double tracks. Or maybe there's no easy access to the bit in between for large plant?

Looking the shadows on Google Maps, it looks like around Wymington it's pairs of single cantilevers on the fasts, which are straight. And twin track cantilevers on the slows, located on the outside of the curves.

So I would guess it's for better sight lines.

 

[GRALISTAIR]

The disadvantages of a TTC are that the foundations are typically far deeper and thus more expensive and more prone to refusal, the steelwork is fabricated rather than a plain section and thus more expensive and heavy. There is also a limitation on the load that can be put on a standard Circular Hollow Section steel pile connection which heavily loaded TTC's can breach, necessitating a more expensive foundation solution.

For more information see https://ocs4rail.com/

These to me seem to be the key points.

 

[twpsaesneg]

Old fashioned concrete gravity pads then such as was used on Preston - Manchester? Including this bad boy at Salford Crescent?

Yep, where we have shallow rock or another reason why we can't use a CHS or Auger.
Precasts are cheaper, but then need transport to site. Fine for portal structures, and development work has been done for segmented precasts for TTC. Very much an outlier though.

I don't think the structural dead load is the problem. They're pretty light structures already.

TTCs have a quite substantial cross track loading because of the weight on the nose, but wind loading on the wires is a large factor.

Has anyone ever done any studies on the use of structural aluminium sections for this sort of stuff?
The metal is more expensive but they can achieve some impressively lightweight solutions.

The bending loads the steelwork has to take just end up with a massive aluminium section. However, pretty much all of the non Furrer+Frey wiring equipment is aluminium extrusions / castings and are immensely strong but lightweight.

I'd imagine the connection detail is the limiting factor rather than the CHS itself.

Multiple visits by different trades - excavate / blind / side forms / steelfix / concrete / strike forms / backfill. Much more to organise and co-ordinate.

Completely correct.

 

[GRALISTAIR]

I know this thread is mainly about TTCs and STCs but I thought that bringing in Single Support Anchors (SSAs ) aka wicketkeepers would be useful. Here is a distant and close up shot of one. There used to be a crossover installed 3.9 miles north of Preston just before the start of the Barton Loop. It was removed a few years back and so new infrastructure had to be installed. You can tell by the newness relative to the 1973 era masts etc.

 

[HSTEd]

In photos of Japanese railways I often see gantries or twin track cantilever type structures made out of tubular sections.

I expect this is because of the high cost of labour in Japan, increasing the price of latticework structures.
In any case, has such steelwork ever been used in the UK?

EDIT:

An example is this photo from Wikipedia of the Chūō Line.

They also appear to sometimes use structures that almost resemble headspans, but using angle steel instead of wires.

 

[twpsaesneg]

In photos of Japanese railways I often see gantries or twin track cantilever type structures made out of tubular sections.

I expect this is because of the high cost of labour in Japan, increasing the price of latticework structures.
In any case, has such steelwork ever been used in the UK?

EDIT:

An example is this photo from Wikipedia of the Chūō Line.

They also appear to sometimes use structures that almost resemble headspans, but using angle steel instead of wires.

Yes, there are a few parts of the West Coast that use tubular masts, one that springs to mind is the area between Winsford and Weaver Junction.
Anglia has also got a few exquisite portal structures fabricated from tubular section - around Harlow is one location - they must have cost an absolute fortune to manufacture and to attach to but look very pretty. Well, to me anyway.

 

[etr221]

Tubular masts were common on street tramway and trolleybus systems.

The big railway user in Europe were FS (Italy) - the pali tipo M (Mannesmann) were first adopted c 1910 for the Giovi electrification out of Genova, and then became a characteristic of Italian electrified railways, both for the earlier three phase and the later 3kV dc schemes - while there were other types, only in the 1960s was its supremacy challenged with lattice tipo LS (Linea Secondaria). There were many variations of the tipo M - and in stations/yards substantial poles would have arms on two sides carrying wires for several (up I think to 4 or 5) tracks.

 

[Nicholas Lewis]

Wet trades are very expensive, augering costs around 50 - 100% more than a percussively / vibro driven CHS pile.

Also, when you hit rock due to the depth of foundation you need it becomes even more of a pain

We use Augered foundations where necessary, but they are not in favour!

Pre privatisation BR foundations were relatively slight compared to what i see now. Majority of those foundations are still standing up after 60+ years so why have post 2000 foundations become so much larger and deeper? What has changed on the design of the OLE vs BR MkIII that has necessitated this?

 

[HSTEd]

Pre privatisation BR foundations were relatively slight compared to what i see now. Majority of those foundations are still standing up after 60+ years so why have post 2000 foundations become so much larger and deeper? What has changed on the design of the OLE vs BR MkIII that has necessitated this?

Well the specification for the GWML electrification was rather over the top.

Multiple pantographs at 140mph was a downright silly specification for any conventional UK rail project.
Things have wound back a bit but we are still a long way from Mark III in durability and other terms.

 

[edwin_m]

Pre privatisation BR foundations were relatively slight compared to what i see now. Majority of those foundations are still standing up after 60+ years so why have post 2000 foundations become so much larger and deeper? What has changed on the design of the OLE vs BR MkIII that has necessitated this?

The pre-privatisation method was, IIRC, to auger a hole then place an expanded polystyrene former of the same shape as the mast, and pour concrete around it. When that had thoroughly cured, a solvent was poured in which would dissolve the polystyrene and leave a hole for the mast. This means at least two visits a month or so apart, and I suspect the solvent method wouldn't be allowed today on either fire safety or environmental grounds.

By contrast if a pile works the mast can be attached the next day. But it sometimes doesn't.

 

[twpsaesneg]

Pre privatisation BR foundations were relatively slight compared to what i see now. Majority of those foundations are still standing up after 60+ years so why have post 2000 foundations become so much larger and deeper? What has changed on the design of the OLE vs BR MkIII that has necessitated this?

There was a change in Railtrack days which forced most foundation design to be pushed into the hands of the geotechnical engineers, who were not able to use the empirical method of foundation design used by OLE designers for decades because of the design rules they had to follow by standard. This has, in the last 10 years or so, been more or less rescinded so we are back being able to allocate using the original methods.

The actual foundation types are still broadly the same, the main difference being that hand dug and clamshell bucket dug foundations are out of favour (lots of temp works design / confined spaces permits needed for the former, and less available plant to do the latter) in favour of augered ones. These do have advantages in that there is no formwork required as a tubular casing is used instead. The CHS piles are almost identical to those used from the 70's onwards.

Network Rail are more nervous about mining risk than may have been the case in British Rail days, which has driven a lot more pad-type foundations than would have been the case before (in mining areas, naturally).

The pre-privatisation method was, IIRC, to auger a hole then place an expanded polystyrene former of the same shape as the mast, and pour concrete around it. When that had thoroughly cured, a solvent was poured in which would dissolve the polystyrene and leave a hole for the mast. This means at least two visits a month or so apart, and I suspect the solvent method wouldn't be allowed today on either fire safety or environmental grounds.

By contrast if a pile works the mast can be attached the next day. But it sometimes doesn't.

Yes, the planted masts which you describe are now gone, however to be honest the move to bolted base was in British Rail days anyway. Burning out polystyrene would be seen as a Bad Thing nowadays (and rightly so really) and you also need to grout the mast into position in the core, which is another wet trades visit.

CHS Piles became very popular when we stopped doing large scale electrification works where multiple revisits were easy to do because the teams would be working on a route for months or years. Post-privatisation, with only small scale OLE works being done a one-hit solution was much better, as you say you can pile and land the steelwork in the same night which is great if you only have a handful of structures to install.

 

[Southsider]

Tubular masts were common on street tramway and trolleybus systems.

The big railway user in Europe were FS (Italy) - the pali tipo M (Mannesmann) were first adopted c 1910 for the Giovi electrification out of Genova, and then became a characteristic of Italian electrified railways, both for the earlier three phase and the later 3kV dc schemes - while there were other types, only in the 1960s was its supremacy challenged with lattice tipo LS (Linea Secondaria). There were many variations of the tipo M - and in stations/yards substantial poles would have arms on two sides carrying wires for several (up I think to 4 or 5) tracks.

Tubular masts were used on the Cathcart Circle, Neilston, Newton electrification in the 1960s. Still standing though some are at a jaunty angle!