I built this 5 foot long arched truss bridge in 1995. I did not model the bridge after any specific structure, but I did draw inspiration from the Sydney Harbor Bridge in Australia and the Hell Gate Bridge in New York.

The model was composed primarily of Technic beams (using all that I had) and axles for the joints. The roadway was made from beams and large plates, and simple bricks were used for the abutments.

Total Length: Approximately 5 feet (1.5 m)

Ultimate Capacity: 35 pounds (at failure)

Structure Type: Arched Through Truss Bridge

I sized the truss structure in a series of trial and error attempts by adjusting the lengths of different members. I slowly assembled part of the arch (one half and one side of it, or 1/4 of the arch) by this trial and error method and stopped when I was happy with the shape. I used the "1/4 arch" as a prototype to then copy and complete the whole arch.

Once the "1/4 arch" was nearly completed, I took careful inventory of my Technic beams and planned out the entire bridge so that it used nearly ALL of the beams I had. I also arranged each color to be in a particular section (symmetrically, of course).

It would have been nice to do it all in one color, but I was going for maximum size and I wanted to use all my Technic beams. My feeling was that by placing each color in its own section, the bridge looked much better than by using colors randomly (i.e. the "built by a 3-year-old look").

It's rather difficult to make a 3D truss from Lego bricks. It can be done, but it can take a lot of parts. A simple, easy, and "brick effective" (minimum number of bricks- maximum size) solution was to use cables (string) in the plane perpendicular to the main truss.

I connected the main trusses (the two main parallel trusses making the arch shape) with simple 2x plates, then I tied string tightly in an X pattern to connect the two. The result is a strong, stiff structure that is properly braced.

I used double vertical members to "sandwich" the diagonal and main members. The double vertical members helped to even out the eccentricity of the joint (the members met in parallel planes, but not in the same exact plane).

Most of the members were made of two or three Technic beams joined lengthwise by long 1x plates. Many of the long members incorporated 1x2 plates with the one offset stud on top. This allowed me to make members with 1/2 brick width increments. Surprizingly, a member made in this fashion is relatively strong, especially because it's a TRUSS (no bending or twisting in the members- just axial loads).

NOTE: Images created with MLCAD. See www.ldraw.org for info on the program.

LDRAW files (DAT) of these models:

* Typical Bridge Joint: bridge_joint.dat

* Member in 1/2 Brick Increment: member.dat

The roadway was made from large plates and Technic beams. Its construction was simple and looking at the pictures explains it all.

It was supported in small spans by the truss. At the ends of the bridge, the roadway was directly connected to the members of the truss (with axles or friction pegs). In the middle span, cables (string) suspended from the arch connected to the roadway to support it.

The ultimate capacity of 35 pounds was measured by placing weight on top of the arch and not on the roadway (the actual roadway wasn't capable of that load). At 35 lbs, one of the long 1/2 brick increment members snapped. A simple 1x plate connection (as mentioned above in the detail of the members) came apart and started a chain reaction, causing more members to come apart. The arch collapsed to the floor, but not a single Lego piece was broken.

That was the end of the bridge and it was disassembled.

The abutments were made from simple bricks. As you can see from the right abutment, I ran out of bricks and just capped it off with plates.

I was just a poor college student at the time and buying more bricks for a project just wasn't an option. Oh well, the arched truss was the main attraction, the abutments were a minor thing.

And I didn't bother with approaches either. I ran out of room and Lego bricks. People were impressed, nevertheless.

I consider this my first "Professional" model. I built it after having taken a few engineering courses at college and learning the basics of structural engineering.

Also, I put a lot of time into the planning of this model and had a strong "build-it-right" attitude. In other words, a large project such as this has many small steps of partial completion along the way. If each step produces something that works well, rather than- "ehhh, it sorta works", then the final model is well made and functions correctly. In other-other words, take your time and do it right!

I also developed a good technic for making a TRUSS which has allowed me to build some rather large and impressive structures that are relatively strong.

This is a classic example of my building style. I went for the largest model I could build, being limited by the size of my collection. It may not have been aesthetically pleasing but it was functional.