Building a truss girder bridge

01 January 2023
If you saw our display at the National Garden Railway Show this year, you'll have been impressed with the 3ft long bridge. Mark Thatcher tells the story of its construction.

I don’t think I can recall a single supplier with a more eclectic mix of kits like Rannoch Sidings. I reviewed their transfer wagon in Garden Rail 334 and working tipper wagon more recently, but this bridge kit takes things to a completely different level. If Rannoch produced LEGO kits then this truss bridge would be their equivalent of LEGO’s Taj Mahal kit; still one of the largest sets available, with 5923 pieces. The part count of this bridge kit does not quite make that number, but still with around 480 pieces I could see this would be a very detailed and time-consuming kit to build. Rannoch suggests 24 hours is around the norm, although as I painted as I went along, as it would be a tough ask to do otherwise, it was closer to a 30-hour build for me – but worth every minute.

What’s in the kit?

To be honest, when cracking open the box, I felt somewhat overwhelmed by the array of parts that confronted me, but as soon as I got into the groove, I could soon easily get to grips with the building process, by following the step-by-step instructions, spread across eight video segments on YouTube. As well as the larger parts which make up the main sides and deck of the bridge, there were a plethora of X girder support pieces to form an internal layer between the two bridge sides, some more X supports to make up the three optional overhead trusses, and probably around 200 individual rivet plates and overlay pieces which cleverly hide any of the visible tabs, as this kit utilises the tab and slot method to attach parts together.

This kit is suitable for 32mm or 45mm track and is scaled to accept one standard three foot length of PECO 32mm track. The material used is MDF, so clearly a good deal of waterproofing would be required if the bridge was left outside, but, frankly, I would treat this as I would one of my finer model buildings, setting it in place for a running session, and stored carefully afterwards.

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Let the games commence!

There are four main parts of this build, and I think it is helpful for me to break them down for you. They are the two main bridge sides, in a three-layer sandwich, the bridge deck, the three optional overhead trusses and the two piers which the bridge locates onto.

Starting with the sides, this is probably the most testing part of the kit for two reasons. All need to be built up on a flat surface, such as a glass table, although I did not have one to hand and ended up using my occasional table in the lounge as this was the only one which would accommodate the length of the bridge. Each side is basically a three-layer sandwich, where you join two inner pieces together to create the 3ft span, then carefully glue in the X frame risers to create the depth on the sides, before adding another identical layer to the first layer to create one finished side.

Wonky bridge

Now – this is the tricky bit! As explained in the video, it is essential to file a little off the locating tabs on these middle X sections, otherwise you will inevitably use too much force whilst seating these into place damaging them. This is a time-consuming process and quite repetitive, If you have not filed enough of the tabs down, when trying to add the upper layer and you get a sticky part, you may have to remove the top layer and start again.

This whole process just takes time, care and a deft hand, but don’t be tempted to use a lump hammer to ‘ease’ these parts into place! On the other hand, it is also essential to ensure all the tabs are located properly into the top and bottom layers of the sides, as if not, you will end up with a wonky bridge, which you don’t want. After the two side sections were completed I elected to spray paint these in red oxide primer before moving on to the next stage.

The bridge deck

The deck of the bridge is much easier to assemble and once you have mastered the sides, the build gets much easier. Similar to the sides, the deck comes in two sections which you join together. To begin with the resultant three-foot-long section is a little delicate, but as soon as you add the fifteen lateral arched cross beams to the underside of the deck, the whole structure becomes far more stable and rigid.

On the subject of the overall rigidity, initially, I had my doubts as to how solid the final bridge would be, but, in Rannoch’s usual style, one of their shorter videos demonstrated how strong it actually is. In that video they mounted a micrometer, which measures any movement, to the middle of the bridge, then started to add weights to the deck. They stopped at 19.75kg (as they had ran out of weights), but even with this load, the bridge only showed a 0.75mm downward deviation and returned to normal after the weights were removed.

One side of the deck even includes a pedestrian walkway & handrail and this was to be my Achilles heel later on in the build however. I elected to paint the deck in satin black as a contrast to the red oxide sides. It was then time to adjoin the two sides to the deck and to marvel at how big the resulting structure would be!

A useful jig

It was a very simple job to fit the deck as the slots in sides of the bridge accommodate tabs in it, but it is also mission-critical to get the sides perpendicular to the deck. Fortunately, Rannoch have thought of this too, providing two square sections of MDF with two tabs at the bottom and two hooks at the top of each jig. The hooks hook over the inner top section of each side of the bridge, and the tabs locate into two slots in the deck. So after getting the angle roughly right, you just drop in one jig onto the deck, around a quarter of the distance from each end, and leave it in position whilst the glue dries. This is another good example of the 'no stone unturned' approach used by this manufacturer.

A bit like Bingo!

As there are a lot of visible tabs when the layers of the sides and deck are mounted together, it is now time to hide them all. Again, another brilliantly executed stage of the kit. On each of the sides there is an engraved number which is visible at the bottom of each upright girder. These are matched to the corresponding number on a separate upright overlay. So just find the two matching numbers and away you go. A bit like Bingo!

This however can be a time-consuming process as there are a total of 13 of these to add to each face of each side, by my maths that is 52 upright overlays to add! TIP: Sort these strips into four piles (1-13, 14-26 and so on) or if you have space sort them all out in order, which will take the pain out of locating each one in a massive pile of bits.

The overlay action does not stop there however as each upright added in the stage above also has another number scribed on the base of it, which matches the rivet plates which will be added next. The numbers grow here too as there are 24 rivet plates to add to each side. So that’s another 96 parts to add making a total of 148 overlays for the bridge sides. I think you can now begin to see the full magnitude of the level of components and detail provided in this kit. If you think of this kit a bit like a 3D version of a painting by numbers set, you won’t be far wrong!

A double Six

This was by far the most exciting of the sub-assemblies in this kit for me. When you add the top line of rivet plates to the inside of the bridge sides, you have an alternative. If you don’t wish to add these parts (with small square holes in) then, in order to increase the loading gauge of the bridge, they can be dispensed with and replaced with overlays without the holes. Although, as these trusses add so much character, and further rigidity, to the whole ensemble, it would be criminal not to use them.

With these top trusses added, the loading gauge of the bridge will be around 185mm, enough for most Roundhouse or Accucraft models, but of course, it becomes unlimited without these fitted. However I thought I would roll the dice with these and fitted the appropriate rivet plates with the slots in them, which the tabs on the trusses drop into. And guess what? They turned out to be a perfect tolerance fit due to the very accurate nature of this kit! I may have rolled a double six here but really should not be surprised. So now, if I have a loco with a particularly tall chimney, I can just pop these trusses out to accommodate it.

There is a third configuration which I particularly like too. You can remove the two outermost trusses, leaving the central one in place, which I think looks most attractive and has the advantage that you can more easily see your locos and rolling stock as they trundle across the bridge.

Pier Pressure

Perhaps I have been judged by my peers in the past, but now you may judge me by my own piers! Again I am running out of superlatives with this kit, but these piers have been ingeniously designed with two tabs on each pier, which locate into corresponding slots in underside of the bridge deck, making it very easy to detach the components from each other. And if you are so inclined, these bridges can be combined and you simply discard the top layer of each pier (which also has a similar locating tab) and just use the lower sections to join multiple bridge sections together. I have to say these piers are great standalone kits in themselves.

My mistake here, and my only mistake during this build was failing to realise the piers are handed, as there are two small lengths of extra walkway for you to add, to extend this from the bridge across onto the piers. It was a schoolboy error as I had one set of locating pips for these on the wrong side of one of the piers. It was an easy work-around though. Thank goodness of Perfect Putty from Deluxe Materials is all that I will say on this matter.

Finally, two nameplates are provided to disguise the last visible central join on the sides of the model, but, I dispensed with these and printed my own 3D nameplates to make this bridge more relevant to my current location.

Attention to detail

I should mention that if you paint lazily with wasteful rattle cans like I do, I went through one spray can of black and another two of red oxide primer, 25g (half a large bottle of Superglue) and plenty of exterior-grade wood glue too. That equated to adding around £25 to the build costs. It is something to be aware of, but unavoidable with this behemoth of a kit.

I did start this article comparing it to a fine LEGO kit, and later on, to a painting by numbers kit. I am hesitant to use the phrase ‘exhibition model’ as that is a very subjective term. Yes, it takes time to build, but more time = more fun. Yes, it is repetitive in places, there is no getting away from that. Let’s say it will take you 25 hours to build. At £125.00, that is a fiver an hour. I can just about buy a pint of wallop for that. But building this I don’t need to, with the added benefit that I am not cooking my liver either.

Don’t expect to be able to knock this kit out over a weekend. I think this would make an ideal winter’s project, as when the nights draw in once again, you can repair to your own modelling space and really enjoy seeing this wonderful kit coming together. Take your time with it and make the building experience last. You won’t regret it.