RMweb Premium 47137 Posted March 16, 2015 RMweb Premium Share Posted March 16, 2015 My new layout has one track passing over another. The lower track is on a curve and gradient, and the upper track is on a tighter curve (about 20 inch radius) and a gradient. The scenic setting will be "very rural". I expect the bridge track will have continuous check rails. There will be stone abutments each side to hold the bridge deck. Maximum loading is imagined to be 30 imperial tons, 10 tons per axle. My original idea was to have two steel joists (or fabricated girders) across the span, with timber baulks laid across them and the rails spiked directly onto the timber. However I'm not sure how to arrange this at the two ends, with such a sharp angle. Perhaps, the deck should represent something made steel throughout, or even pre-stressed concrete, with the rails attached to pads on the deck. I'm happy to adjust the fictional story of the model, but I would like a design which will find broad acceptance with a civil engineer! It would be nice to be able to see through the deck onto the track below, which suggests timber or a steel lattice of some kind. So any suggestions especially pointers to obscure prototypes would be welcome. - Richard. Link to post Share on other sites More sharing options...
RMweb Premium jamie92208 Posted March 16, 2015 RMweb Premium Share Posted March 16, 2015 The type of construction would depend on the period when the branch was constructed. If it was roughly prior to the 1950's two girders like you suggest would be quite normal. The cross beams though would probably be rivetted girders perpendicular to the side girders. Due to the skew the shorter end ones of these would actually be grounded onto the abutments. Normally these cross girders would form the base for brick arches which I think were known as jack arches that crossed between the girders. the trackbed would then be ballast on top of them. However there were always what are known as ballast walls just inside the side girders. these stop ballast dropping down onto the girders and give access so that the space can be cleaned out to stop the main girders rusting away. This happenned quite recently on the G & S W main line near Kilmarnock where he ballast walls had not been maintained and a derailment was caused. Hope this is of some use. I do have some original drawings of jack arch girders and could give you the spacing if you wanted. They are usually 3 to 4 ' apart but I would have to dig the drawings out to check. Jamie Link to post Share on other sites More sharing options...
RMweb Premium 47137 Posted March 16, 2015 Author RMweb Premium Share Posted March 16, 2015 Yes 'jack arches' - and a quick Google for 'jack arch railway bridge' brought up this link with several photos: http://www.newrailwaymodellers.co.uk/Forums/viewtopic.php?f=25&t=16973&start=45 The jack arches could be made of steel instead of brick, and they can be running along the cross girders as you describe. Steel ones might be easier to model, and to put in for people who look up underneath it! Alternatively there might be more than two girders between the abutments, with the jack arches running the length of the bridge. I wonder, for such a highly skewed location, with a relatively short span, the second of these arrangements would be easier to build? Thanks for this, it's a lot easier when you have just one useful search term. However - I have dug out my one and only photograph of a bridge on the old Shropshire and Montgomeryshire (Potts) line in Shrewsbury. This is the sort of structure I'd like to emulate, but it is of course on a straight length of line. I took this photo in July 1986 - when I went back later it had all disappeared with the construction of Old Potts Way. I think it was over Rocke Street. There is another photo here, the eighteenth photo on the page. As far as I can see, there were two girders. The footway is some planks fixed on top of the sleepers. The sleepers are on top of pads of some kind. Having found the photo, the construction looks simple to model - the girders are actually on top of the walls, not set into them. They carry seven or so sleepers on top of the embankment - perhaps 17 feet of overlap. The difficulty (or lack of realism) will be in dealing with the curve on the track, but it looks to me as though the girders were most of 12 inches wide which helps. If anyone can point me towards photos of similar bridges or this one this would be very helpful. - Richard. Link to post Share on other sites More sharing options...
Gordon A Posted March 16, 2015 Share Posted March 16, 2015 Theres the bridge on the Pontypool & Blaenavon railway where the Big Pit branch crosses the main line. Gordon A Bristol Link to post Share on other sites More sharing options...
RMweb Premium 47137 Posted March 16, 2015 Author RMweb Premium Share Posted March 16, 2015 A search for images of 'steel skew bridge' is quite enlightening and includes laterally curved girders too. I have a feeling the bridge on Rocke Street was a replacement span on existing abutments. The S&M was pretty basic, but not that basic. I remember the line being served by a class 08 diesel and a few oil tankers, so the strength of the construction seems just right for my own layout. I'm happy for more suggestions before I buy the plastruct! - Richard. Link to post Share on other sites More sharing options...
RMweb Premium jamie92208 Posted March 16, 2015 RMweb Premium Share Posted March 16, 2015 As to curves that's fairly simple. Curved girders were hard and expensive to build so they just put them a little wider apart to accommodate the curvature. Greyhound Bridge on Lancaster Green Ayre is like that with 9 spans on a very highly curved main line but each span is straight. rather larger girders than yours but built on exactly the same principles. Extra longitudinal girders would be difficult as they would have to be not as deep as the outer ones and thus carry less load. jack Arches solves that problem. From the number of rivet heads visible it looks like a built up girder. The upper and lower webs would be several plates all 1/2" thick. Stating from 1 plate thickness at the outer ends and then going to 5 to 7 plates thick in the centre, this giving maximum strength where it is needed. This could easily be represented by layers of plasticard. This shows the effect on curves but on a slightly larger scale. Jamie Link to post Share on other sites More sharing options...
Gordon A Posted March 16, 2015 Share Posted March 16, 2015 Copied from P&BR web site. The leaning baulks of timber are the old supports that have shrunk with age. Gordon A Bristol Link to post Share on other sites More sharing options...
Mythocentric Posted March 16, 2015 Share Posted March 16, 2015 Hi Richard I had a similar situation to you in that I needed to carry a branch line across a double-track line on a girder bridge and used this bridge at Lancaster Station for inspiration. The photos show jack arch detail of both longitudinal and cross beam types. Note the steel jack arches on the outside only. Whilst the longitudinal beams would be easier to model on a skew the cross beam is more common unless the branch was severely weight restricted. I also photographed the abutments and girders from above and below but rather than clutter the thread up I’d be happy to send you copies in a PM if you prefer! Regards Bill Link to post Share on other sites More sharing options...
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