By Michael Leckenby

Caradoc of the NHLR posing on the completed ‘Banwy Bridge’. Clearly some toning down of the timber and metalwork is required.

“I’d like a deck plate girder bridge but can you make it an approximation of the W&L Banwy Bridge?” That’s how it started. The vast majority of the bridges I build are bespoke in terms of length, width and perhaps a few minor individual features. However requests for models of specific full size prototypes are thankfully much less frequent. Producing bridges commercially does not really lend itself to the construction of unique scale models from scratch.

Ex-Sierra Leone Railway No. 85 posed on the Banwy bridge. Photo: Andrew Charman.

Nevertheless, browsing through the few photos available online of the Banwy Bridge showed that my standard plate girder design could fairly easily be adapted to support the timber track deck. The deck itself can be seen in some detail on the internet and this convinced me that a reasonable representation of the W&L bridge would be possible.

Unfortunately I didn’t make a stage by stage photographic record of construction, so I am using those photos I did take to illustrate specific points.

The completed timber deck, showing the main features of the cross beams, track, walkways and parapet rails.

Constructing the Plate Girders

The main aluminium structure consists of the two plate girder assemblies joined by cross beams. The plate girders are, in effect, large ‘I’ girders made from a thin sheet for the web, framed by ‘T’ section along all four sides. I use 1.5mm thick sheet for the web. In general I make the depth 80mm, but this can be varied to suit the bridge you are building. The four pieces of ‘T’ section are mitred at the ends to provide a proper frame around the sheet. The sheet is reinforced by vertical stiffeners on both sides; these can be either channel or angle sections. I generally use 3/8” channel on the outside and angle on the inside, but angle with rounded ends seemed nearer to the prototype. In adapting my standard structure for the Banwy Bridge I also decided to use 3/4” x 3/4” x 1/16” ‘T’ section rather than 1” x 1” x 1/8” which I thought would be much too bulky for the Banwy Bridge model. The four corners of each girder are reinforced with small pieces of 1/2” X 1/2” X 1/16” angle riveted in place. All the main components of the plate girders are fastened together with 2.5mm stainless steel screws and nuts.

The underside of a standard MVL Deck Plate Girder Bridge illustrating the main girder structure of sheet aluminium framed in ‘T’ section, with angle section stiffeners and riveted corner reinforcements.

The width of the deck girder bridge is set by the length of the cross beams. These are made of 5/8” x 5/8” x 1/16” channel. They are fixed between the plate girders with their open side facing downwards. Normally I would rivet two longitudinal plates to the top cross beams to support the track, but in the case of the Banwy Bridge the track is supported by timber cross beams set on top of the plate girders.

The basic ‘Banwy Bridge’ timber deck structure. Alternate cross beams are drilled at the ends to take the parapet posts. The walkways are single strips of 3mm timber, which are later cut into three planks. The track will be dismantled for its rail, which will be pinned directly to the timber beams.

The Timber Deck

Having completed the metal girder structure using my well tried and tested construction methods I was now entering into the realms of timber working which is somewhat less familiar to me. Close examination of the available photos showed there are several parts to the timber deck structure of the Banwy Bridge. At the lowest level are the cross beams which sit on top of the girder and extend outwards the full width of the bridge. They in turn support timber walkways and two substantial longitudinal track beams. Bearing in mind that the bridge model may have to spend most of its life outside subject to everything the British weather can throw at it, it was important that the timber used should be able to withstand changes in moisture and temperature without warping and twisting. My preference was to use a hardwood which was durable enough without having to be constantly coated or soaked in preservatives. Cost was an additional consideration, particularly as I had in mind that if this model was successful I might consider producing further models of this bridge or perhaps others which incorporated timber.

A search of various timber suppliers and prices of small section hardwood showed that it would be much more cost effective to cut the sections I needed from larger blocks. Obviously there are many hardwoods which could be used. I finally made the decision to use a timber which is fairly new to the UK market, called Red Grandis. For a little over £30 I acquired more than enough to complete this bridge and perhaps several more.

Working from photographs I cut sufficient 10mm square section cross beams. These were equally spaced the length of the bridge and bolted directly through the top web of the girder.

The walkways either side of the track were each made with three equal width strips approximately 9mm x 3mm. Single strips were used for the full length of the bridge but it would probably be more realistic to use several shorter planks.

Constructing the Trackway

It was clear from photographs that construction of the track way would require some care and accuracy. Sleepers are not used to hold the rails in gauge. The separate rails, including check rails, are pinned to two substantial beams along the bridge. Never having made track using the traditional method of fastening rails using separate chairs this was going to be a challenge. Closer examination of the photos shows that the track rails and check rails are not actually held in place by chairs but by track nails. I decided to replicate this method fairly closely.

I needed a large quantity of small but strong nails with decent sized flat heads. They had to be strong enough to hammer into the hardwood without bending, and to have a head which would grip the foot of the rail. The most successful result was achieved using escutcheon pins, which were 14mm in length with a head roughly 1.5mm across. An additional advantage was their antiqued brass/copper finish.

The track beams were completed separately using the following procedure for laying the track:

  1. Draw a centre line and mark equal intervals to match the cross beam spacing. Knock nails into pre-drilled holes leaving the heads just high enough to clear the foot of the rail.
  2. Holding the rail tight against the nails, knock in a second row to secure this rail to the beam.
  3. Make the check rail by bending it slightly at each end.
  4. Fit the check rail using a third row of nails.

Detail of the track and deck to show how the track nails are spaced to secure the track rails and check rails. Note the same escutcheon nails are used to hold the walkway planks in place.

The rails need to be firmly held in their vertical position, but not too securely as this will be done after the beams have been bolted to the bridge and the gauge is checked.

Ensuring that the track beams were fastened to the bridge accurately would require extreme care. However, always looking for tips and shortcuts, I decided to thread three plastic sleeper sections from the Tenmille bullhead track (which provided the rails) onto the rails, but on the top. Rather like gauge clamps, they held the two rail assemblies at just the right gauge for me to drill through and fasten to the cross beams. Once the rails were in place I simply slid the sleepers off the ends.

At this stage the rails looked fairly straight, but there was clearly some slight waving to one side or the other depending on the position and depth of the individual track nails. Fine tuning was achieved using a thin punch or a fine screwdriver to adjust the nails and give them a final tap to securely grip the rail. The gauge and the clearance between the rails and check rails were checked continually using a piece of rolling stock.

A long view of the finished timber deck. The tops of the plate girder beneath can just be seen. The spacing of the girders is not to the scale of the Banwy Bridge, which is much narrower, but the result is very similar to the original.

The original Banwy Bridge on the W&L line, for comparison with our model.
Banwy Bridge: Welshpool & Llanfair Light Railway (Gareth James) / CC BY-SA 2.0

Parapet Rails

The parapet rails are a significant feature of the Banwy Bridge. The tall slender posts of the original appear to be very fine cast iron uprights, tapering slightly upward and incorporating three holes for the handrails. It seemed impossible that the same effect could be achieved economically. I therefore resorted to the method which would have been adopted on many a private railway. Old sections of rail were cut to size, a length of around 18mm was rounded off at the base, and a thread was cut on this to take a 6BA brass nut. Three holes were drilled through the web of the rail at equal distances up each post to take the wire rope handrail. The top end was then rounded off.

Checking with the full size bridge again told me that the parapet posts were fixed to the ends of every second cross beam. The beams had already been drilled so it was an easy task to push the threaded end through the cross beam and attach the brass nut.

An overall view of the deck structure and parapet rails.

Finally, using my standard parapet rail method I threaded 2mm diameter stainless steel wire through the parapet posts. This was held in place at the end posts with a small aluminium ferrule crimped onto the wire.

In retrospect, it would have been best to either paint or powder coat the girder structure before constructing the timber deck. The full size Banwy Bridge is distinctly dark, possibly black, so the bright aluminium finish is somewhat unrealistic in this case. Painting the structure after assembly will not be so easy. The Red Grandis hardwood is a particularly light pink colour in its bare state so a stain was applied during construction. In time it would be hoped that the timber will age to a more realistic grey.

The parapet posts are cut from used brass rail which has been left in its dark tarnished state. Threads were cut on the bottom 18mm to take 6BA brass nuts. Holes were drilled to take 2mm diameter stainless steel wire rope.

Text and photos by Michael Leckenby, except where indicated.