Arched Truss Bridge for Two-track Operation

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Arched Truss Bridge, picture 1


  • Lightweight construction
  • Capable of bearing two heavy trains at the same time, spanning the whole bridge length and moving at high speeds

Since I am a member of a LEGO User Group (SwissLUG), I participate in our club meetings. From time to time we organize an exhibition with our creations where usually a common train layout is the main attraction. For such a train layout, I have been asked by my colleagues to build a two-track train bridge that can span the length of twelve straight LEGO track segments, which add up to about 1.5 m.

Arched Truss Bridge, picture 2

I decided to span the whole length without intermediate supports and thus ended up building a truss bridge. Of the two options available, I chose the arched truss design, while the other solution would have been a straight truss. Thanks to the good work of my colleague TJ Avery of Houston, Texas who made Microsoft® Excel spreadsheets to calculate the shape of such an arched truss, the overall design was laid out quite fast. Thank you, TJ!
Since the trains that would cross the bridge were eight-wide and even more, there was the need for a sleek design of the truss structure. The first idea that comes to the mind when trying to build a truss, is to use Technic beams. Because at most of the knots of the arch, four beams come together, the width would be at least five studs - four Technic beams and a five stud long axle connecting them. This was too much and - in my opinion - didn't look too good. Therefore I tried another solution with the truss members built of 1 x X plates with their studs to the sides. This allowed for an overall width of only seven plates or about three studs for a main truss member. It could have been built even sleeker, but at the cost of rigidity. At each knot where four members come together, there are two plates for the two main chords, another two for the vertical member and one plate for the diagonal member.

Arched Truss Bridge, picture 3

The two truss arches are connected at four points by crossmembers. Each of which consist of a straight truss itself. Thus the main supporting framework was complete.
The next step was to build the track system that would be hung from the arches subsequently, connecting their anchor points and thus close the flow of the forces. Put in other words: if the anchor points of the arches on one side of the bridge would not be connected with the ones on the other side, the weight of the track system and the trains hanging on the arches would make them bend and consequently forcing the anchor points to move outwards. While the hangers of the track system are made of 1 x X plates, like the arches themselves, the framework for the two railroad tracks is made of Technic beams. The tracks lie between the arches while on both outer sides there is each a pedestrian walkway.

Basically the bridge was now finished. All that was needed were some pillars on both sides to lift it up to the right level to match with the rest of the train layout. Instead of simple supports, I decided to go for bridge portals on both sides, reflecting late 19th century architecture. The fictional date of construction of the bridge shall be 1897. For each track there is a separate opening in the portal as well as for each pedestrian walkway. The bridge portals feature a balcony like structure at the top with a Swiss flag on a flagpole and the year of construction displayed using old 1 x 1 number bricks. Furthermore there are two cross-shaped reliefs representing the Swiss crest on both Portals. Finally details like railings for the walkways, mainly made of Technic pins, pedestrians, a workman, a swimmer and even a skeleton hanging from the truss framework complete the model.

Unfortunately I am not able to show pictures of the brigde as part of the train layout. For a SwissLUG exhibition, which was part of a toy fair, we set up our layout the evening before the opening day. During the night there was a fire at a booth in the same hall, covering all the exhibits under a layer of ashes and cinder. Luckily, the LEGO models themselves where not destroyed, but most of them had to be taken apart and the bricks had to be cleaned. So my bridge found a premature end before it could prove its strength and impress the beholder with its lightweight and sleek construction.

Arched Truss Bridge, picture 4Arched Truss Bridge, picture 5Arched Truss Bridge, picture 6
Arched Truss Bridge, picture 7Arched Truss Bridge, picture 8

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