Torr Giffard LSWR 1951-71 Posted January 13, 2013 Share Posted January 13, 2013 Evenin' all, I'm currently designing a similar but smaller version of the Tavy viaduct for Tor Giffard. I have the option of 1 masonry arch at each end of four bowstring steel spans or two masonry arches at each end of 3 bowstring steel spans. On my layout the river won't be tidal, the water (actually plate glass) will occupy approx 4ft of the 5ft width allowed for the river. The model bridge will span between solid rock at both ends. The link below shows the Tavy viaduct http://www.google.com/imgres?imgurl=http://s0.geograph.org.uk/geophotos/02/87/97/2879726_f5d8d986.jpg&imgrefurl=http://www.geograph.org.uk/photo/2879726&h=427&w=640&sz=56&tbnid=-anjaexzhNhzaM:&tbnh=90&tbnw=135&zoom=1&usg=__T2JyoU-bNSUpvHZQVYCJAaCL8Sw=&docid=4Mbq8JUfaUz3ZM&itg=1&hl=en&sa=X&ei=SfTyUNGqJcyp0AWOr4DYAw&ved=0CD4Q9QEwBA&dur=343 The 1 masonry arch option looks more impressive as a bridge. Is anyone aware of any bridge design/prototypical rules that would be broken if the one masonry arch approach was adopted? Cheers Dave Link to post Share on other sites More sharing options...
Ron Heggs Posted January 13, 2013 Share Posted January 13, 2013 Evenin' all, I'm currently designing a similar but smaller version of the Tavy viaduct for Tor Giffard. I have the option of 1 masonry arch at each end of four bowstring steel spans or two masonry arches at each end of 3 bowstring steel spans. On my layout the river won't be tidal, the water (actually plate glass) will occupy 4ft of the 5ft width allowed for the river. The model bridge will span between solid rock at both ends. The link below shows the Tavy viaduct http://www.google.com/imgres?imgurl=http://s0.geograph.org.uk/geophotos/02/87/97/2879726_f5d8d986.jpg&imgrefurl=http://www.geograph.org.uk/photo/2879726&h=427&w=640&sz=56&tbnid=-anjaexzhNhzaM:&tbnh=90&tbnw=135&zoom=1&usg=__T2JyoU-bNSUpvHZQVYCJAaCL8Sw=&docid=4Mbq8JUfaUz3ZM&itg=1&hl=en&sa=X&ei=SfTyUNGqJcyp0AWOr4DYAw&ved=0CD4Q9QEwBA&dur=343 The 1 masonry arch option looks more impressive as a bridge. Is anyone aware of any bridge design/prototypical rules that would be broken if the one masonry arch approach was adopted? Cheers Dave Hi, Dave No rules as such apply It would appear that the use of masonry arches was available due to the availability of reasonable ground conditions at the river banks, and that the steel bowstring sections were required within the tidal section of the river As you are only going to model a part (reduced section) of the Tavy Viaduct then I would go with whatever you are comfortable modelling. Either of your options would produce an impressive structure Cheers Ron Link to post Share on other sites More sharing options...
Torr Giffard LSWR 1951-71 Posted January 13, 2013 Author Share Posted January 13, 2013 Thanks for the advice Ron. Regards Dave Link to post Share on other sites More sharing options...
Torr Giffard LSWR 1951-71 Posted January 13, 2013 Author Share Posted January 13, 2013 Ron, Do you have any knowledge as to why the bridge appears to carry ballasted track in the 1960s (pictures from each end in the Okehampton line book by the Irwell Press) and yet a recent view from the cab of a train crossing the bridge clearly shows that there was no ballast on the bridge. The reduction of weight and therefore maintenance is one possibility, ease of access to track fittings might be another. Would there be a consequent speed restriction on the structure? This is the GeoLocation picture http://www.geolocation.ws/v/W/File%3ATavy%20bridge%20-%20geograph.org.uk%20-%20706597.jpg/-/en Cheers Dave Link to post Share on other sites More sharing options...
Edwin_m Posted January 13, 2013 Share Posted January 13, 2013 Notice that the last pier of the masonry part is thicker than the others on this view. An arch imposes outward forces at both ends, and a beam bridge imposes only downward forces. Hence a pier between two identical arches has balanced forces from each side (except when a train goes over, and that is much less than the deadweight of the arches) but the pier at the end of the masonry has to withstand an unbalanced force. To be realistic you will need to use a similar thicker pier at the end of however many masonry arches you have. Its thickness does not depend on the number of arches but will depend on their size. Link to post Share on other sites More sharing options...
Torr Giffard LSWR 1951-71 Posted January 13, 2013 Author Share Posted January 13, 2013 ...thank you Edwin. I had noted the thicker stone work but it is good to hear the science and engineering behind such features. Regards Dave Link to post Share on other sites More sharing options...
Ron Heggs Posted January 14, 2013 Share Posted January 14, 2013 Ron, Do you have any knowledge as to why the bridge appears to carry ballasted track in the 1960s (pictures from each end in the Okehampton line book by the Irwell Press) and yet a recent view from the cab of a train crossing the bridge clearly shows that there was no ballast on the bridge. The reduction of weight and therefore maintenance is one possibility, ease of access to track fittings might be another. Would there be a consequent speed restriction on the structure? This is the GeoLocation picture http://www.geolocation.ws/v/W/File%3ATavy%20bridge%20-%20geograph.org.uk%20-%20706597.jpg/-/en Cheers Dave Hi, Dave The replacement rail fixing is on longitudinal steel plating fixed directly to the transverse steel beams forming part of the bridge structure obviating any need for sleepers and their ballasting support This approach to track laying is now common on existing steel structured bridges as the track elevation does not change over time, as it would do with ballasted track Ron Link to post Share on other sites More sharing options...
Torr Giffard LSWR 1951-71 Posted January 14, 2013 Author Share Posted January 14, 2013 ....thank you Ron. Looking at pictures of a similarly built bridge (Little Petherick creek on the Padstow line, in the 1960s) it is easy to see that there is no ballast on the bridge. I think that this coupled with your answer and the GeoLocation image is the way to go. Regards Dave Link to post Share on other sites More sharing options...
Ron Heggs Posted January 14, 2013 Share Posted January 14, 2013 One of the Castlefield viaduct bridges on Manchester Central which was originally ballasted, and is now used by the Metro Trams, has the rails fixed to longitudinal plating on the original steel structure - - 1961 Ballasted (steam & diesel days) - Track lifted - Metro track Link to post Share on other sites More sharing options...
Torr Giffard LSWR 1951-71 Posted January 14, 2013 Author Share Posted January 14, 2013 ...thanks for the images Ron. I'm looking at ways of improving the sound qualities of a train passing over the model bridge and ideas of a hollow metal box between the track and the bridge deck along the length of the bowstring section was one option. If the bridge was ballasted then this would be easier to conceal but I'm more inclined to go with the non-ballasted route as the method of construction which I intend to use will mean that it won't be visible from the normal viewing positions for the the layout anyway. Cheers Dave Link to post Share on other sites More sharing options...
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