GIRDERS BETWEN PLATFORM CANOPIES- WHY?

[JOHNMCDRAGON]

On looking at various station photographs I've often noticed light steel girders, sometimes of a lattice formation similar to the platform canopy supports , between the platform canopies at several terminal stations. e.g Glasgow Buchanan St, Aberdeen, St. Enoch. I can understand load bearing beams between solid platform walls in stations built in cuttings and so forth, but why these? Are they purely ornamental or do they serve an engineering function, or were they just a Scottish fashion in station building at the time. ?

[Jamie]

Without recourse to photos, I expect they tie the structure(s) together for improved stability.

[JOHNMCDRAGON]

Hi, Jamie and thanks for replying. I was referring to the girders over the tracks that connect platform canopies seperated by these tracks (if you see what I mean). I just could n't figure out what they did since they were not supported from the ground but by the canopies themselves. A recent photo on Railscot's website of a Black 5 hauling a failed DMU at St. Enoch shows an example.

[The Stationmaster]

I think basically to improve the overall strength of the total structure but not structurally essential as some that used to exist at Reading were taken out years ago and nothing has since fallen down.

[Rivercider]

Not Scottish, but this is what you mean?

 

Sheffield units 50018 + 56044, 18/9/81

 

cheers

[JOHNMCDRAGON]

Yes Rivercider, but usually over double tracks at termini. Why always terminal stations or bays? Are they more likely to collapse than through platforms ?

[Bomag]

Supporting a structure like a canopy is fairly simple. The difficult problem for a large span is to keep it rigid and to stop it deflecting to much in the wind, therefore the bracing, If you let a structure like that flutter around in the wind then you can easily get fatigue cracking.

[Edwin_m]

To expand on what Bomag has said:

 

Looking at Riverciter's photo the wide and heavy canopy is supported by only one row of columns on each side. Thus, if each was free-standing, any imbalance in the weight distribution or loading from wind etc will lead to very high stresses at the base of the columns (if you built a plastic model and knocked it, that is the place it would snap!). This is reduced by tying the two structures together rigidly across the tracks.

 

An alternative would be to support each structure by two rows of columns on the platform, or tie it into any platform buildings. As to why this type of construction is mostly limited to termini/bay platforms, I wonder if there would be an aerodynamic issue with putting beams over tracks where trains run at higher speeds.

[LMS2968]

Yes Rivercider, but usually over double tracks at termini. Why always terminal stations or bays? Are they more likely to collapse than through platforms ?

It wasn't just termini. Leigh station (long gone) in Lancashire had such a system. It was built on an embankment in an area subject to mining susidence. so a more even weight distribution was needed. No doubt there are others.

[Guest stuartp]

More non-termini - Stationmaster has already mentioned Reading, and there was a similar arrangement at Paisley Gilmour Street. The canopies at St Enoch, mentioned by John, are supported by a single line of fairly light columns so the bracing girders save material in allowing a lighter structure than would otherwise be needed (as Edwin has said).

 

However, Stationmaster has also mentioned that the braces were removed at Reading and it didn't fall down. Conversely no-one could accuse the arrangement at Stranraer Harbour of being an attempt to save material, there was half a foundry spanning the four roads to support two not terribly substantial canopies. It does get very windy in Loch Ryan though.

 

http://www.railbrit.co.uk/imageenlarge/imagecomplete2.php?id=36573

[John_Miles]

Bomag has the correct answer. Looking at the photographs, the columns supporting the roofs would be cantilevers unless you provide the cross beams across the track. The largest loads on a structure of this sort will be wind and snow (yes we do design for snow in the UK). As Edwin_m says, there would be a stress concentration at the base of the columns which will tend to vibrate in the wind. Again the cross members will help to resist this by forming what is known as a portal frame (I am a retired Civil Engineer so apologies for the technical terms). One final comment, it is an excessively ugly structure. Civil Engineering structures are seen by far more people than exhibits in art galleries, so we should do our best to make the aestheitically pleasing. Sadly, this doesn't happen often enough

[Edwin_m]

Might some of these, like Stationmaster's at Reading, be the remains of overall roofs that were removed to help smoke dissipate or due to maintenance problems?

[50A55B]

The prominent concrete cross 'girders' at Sheffield Victoria always struck me as being a bit of an oddity - they didn't seem contemporary with the rest of the station buildings.

[Edwin_m]

Looking at that photo again, I wonder if there were originally two bays here, and at some stage they were replaced by a single bay positioned near the centreline of the previous tracks, with both platforms and canopies being widened. The inclined sections of canopy closest to the track look newer than the rest and the columns seem to be near the centre of the horizontal part but off-centre for the structure as a whole. This lack of balance would explain the need to tie the canopies together (think arch) and the curious upward pitch is probably so the beams can link the original lower canopies over the top of a train while themselves remaining straight.

 

Must take a look next time I'm changing trains there.

[The Stationmaster]

Might some of these, like Stationmaster's at Reading, be the remains of overall roofs that were removed to help smoke dissipate or due to maintenance problems?

You will find a photo showing some of the Reading ones here http://www.rmweb.co.uk/community/index.php?/gallery/image/5086-6159-at-reading-c1963/ As can be seen they were between very ordinary looking GW platform canopies (which are still there - even as demolition proceeds to the east of them) and a vaguely similar arrangement existed between canopies over the single line bays. I suspect they probably went as a result of corrosion from years of smoke. To me they look more like bracing - but I'm not an engineer and it shuld be noted that similar size canopies can be seen elsewhere without any such bracing.

[lmsforever]

A few of the surburban stations on Glasgow South Western had these and still have have a look at pics on Scot-Rail website.

[JOHNMCDRAGON]

Thanks for replies guys and it's helped my understanding of why they're there. I never thought too much about the why and the wherefore until I came to model them then it made me ask the question of what purpose do they serve. Thanks again

JOHN

[Edwin_m]

You will find a photo showing some of the Reading ones here http://www.rmweb.co....-reading-c1963/ ... To me they look more like bracing - but I'm not an engineer and it shuld be noted that similar size canopies can be seen elsewhere without any such bracing.

 

To me they don't look as if they serve any structural purpose at all, but they don't look sufficient to support any sort of covering either. If the canopy structures were at risk of tipping then these would just bend at the thin ends where they join the canopies.

[Guest Belgian]

Bournemouth West had these between the canopies on the main arrival and departure platforms, the main departure platform sporting the station offices and thus having no columns on the platform itself (which was quite narrow- see these pictures on the SEmG site and this on e-bay). The main arrival platform was an island platform and had a single row of columns.

 

JE

[definate maybe]

Could it be that they were originally part of a larger glass canopy?

I know on Ian Futers brilliant Fort William layout that he said he left out the glass in the steel strucure over the two platforms and it looks similar to the above link to Bournemouth West

[JOHNMCDRAGON]

My curiousity started with these structures because my layout is loosely based on Glasgow Buchanan St and when looking at old photos of locos here and stations like Aberdeen you notice them and presume they extend the length of the platform, but closer perusal reveals that there are sometimes just two or three at the 'country ' end of these stations. Something not too apparent when you see the usual 3/4 front image of the train/loco at the platform but which stands out when you get a modellers eye view of a layout from above. It then intrigued me of why they were there,just a few cross members at one end , but the explanation of giving greater structural stability in wind makes sense.