There are not many ready-made or kit versions of bridges available from the model railway trade and consequently, there is often a need to 'purpose build' them for a particular situation on a layout.
Unfortunately, it can be difficult to build a bridge correctly which actually looks right because bridge construction is often not well understood by the modeller or even the manufacturers in the model railway trade, for example, some of the Wills products have buttresses which are just completely wrong and wouldn't be build thay way in real life.
Typically, modellers build a bridge by laying the bridge deck as a piece of timber and then attaching cosmetic 'girders' to the side of that timber. This is why they often do not look right. The strength of a real bridge is in the 'girders'. The girders are constructed first and then the deck is suspended between them - the other way around to the way modellers often build them.
This article covers the building of a steel bridge which carries a road over 4 railway tracks.
In building a model of anything, there is no substitute for photographs of the real thing. If we model what we see in a real picture, there is a good chance that our model will look right.
The 'Ashprington Road' layout is is based on South Brent in Devon in terms of location, operation and a few geographic factors, although it is not an actual model of South Brent.
One area where 'Ashprington Road' is similar to South Brent is the provision of a steel bridge. This is the bridge at South Brent in 2004:
There are a number of over bridges on the Western Region mainline west of Newton Abbott. The one at South Brent however, is an odd one out because it is the only one which passes over more than two railway tracks - it passes over the two main lines and the trackbed of the former Kingsbridge branch line, the unoccupied area under the right of the bridge in the above photos.
In the context of Ashprington Road however, the South Brent bridge is too short and therefore, a similar bridge needed to be identified. The one at Taffs Well (Walnut Tree Junction) was identified:
Picture courtesy Hugh Ballantyne
This bridge is a single span across 4 directly adjacent tracks and represents the maximum limit of a single span. Note how the height of the girders is related to the length of the span - a larger span requeries higher girders. Because Ashprington Road's tracks are not directly adjacent to each other, the use of the Taffs Well bridge would mean that the bridge would require extra supporting piers.
The buttresses and the main structure of the 'girder' bridge were constructed using 3mm MDF:
The buttresses were then covered with plaster:
The plaster was scribed with a mini counter-sink attached to a Dremel to scribe out the stonework. This was then painted with water colour.
The Girder Bridge
L shaped plastic card strips were attached to the spans create the girder effect. The deck of the bridge was supported with girders underneath - the whole girder bridge was built as a separately removeable component and built in the same way that prototype bridges are constructed: girders with a deck suspended between them.
The supporting piers were fabricated from strip plastic card and H section plastic girder.
On the road side of the girders, squares of thick card were fitted to simulate planels - generally, for safety reasons, the actual girdering was panelled in to prevent injury to the public.
The girders were spray painted using a Humbrol Nautical grey colour.
The Finished Article
Here is a selection of photos of the finished bridge, although weathering has yet to be applied:
Note the cross beams on the underside of the deck between the two main supporting girders. Modellers often omit items they cannot see, however, unusual camera angles always show up the bridge which has no underside detailing, the tunnel which has no 'tube' and the platforms which have no brick/stone facia because they face the opposite direction to the normal viewing direction!
The road surface has been made using sheets of fine grade 'wet & dry' paper. This has been rubbed with a further sheet of 'wet & dry' to make the road surface smoother and take away the glass 'reflectiveness' by rubbing the dust back into the road surface. The result is very effective and has also been used on Ashprington Road's platforms.