First Truss Bridges

Two Truss Bridges

Intro
In 1995, I took my first entry-level engineering courses at Texas A&M University and learned how trusses function. (I've written an article about trusses that describes how they work and how to build them with LEGO parts.)

During my long holiday break that year, I tried out the concept and built a large truss bridge. It was an arched through-truss type structure and turned out to be surprisingly strong. In the following years, I built a second truss bridge in a slightly different style. The details and photos of both bridges are below.

BRIDGE 1: Arched Through-Truss

Built: c. 1995
Total Length: Approximately 5 feet (1.5 m)
Ultimate Capacity: 35 pounds (at failure)

I started simply by pinning Technic beams together to form triangles. It was that basic. After pinning together a series of triangles (therefore making a truss), I had the beginnings of a nice bridge.

I free-handed the shape of the main arch, or rather, formed the shape by trial and error. I pinned together members and then took a step back to look at the shape. If it didn't look right, then I adjusted the member lengths.

Each member of the truss was made of Technic beams connected end-to-end by 1xn plates. I also fine-tuned the arch shape by making members in 1/2-stud increments where needed. There's a diagram to the left that shows this.

I started the arch at the mid-point and worked my way back to the abutment. I first made a quarter of the total arch to establish the shape and final design of the bridge. Once that quarter was completed, I simply made three more copies to complete the arch.

As I built, I also kept talley of my total inventory of Technic beams. In the end, with carefully planning, I used nearly all that I had. To make the longest bridge I could, I used all colors. (I was going for something big, and I wasn't concerned about looks :-)

There's also a diagram that shows the joint detail (see left). Each joint, or intersection, was a connection of several Technic beams. They were all pinned together with a long axle.

As I assembed the four quarters of the truss arch, I realized that sturdy cross bracing would be absolutely necessary (otherwise, the bridge would sag side-to-side and possibly flop over). Since I had used most of my beams and plates on the arch, I decided to use string X-bracing as an economic alternative. And it worked pretty well :-) (keep in mind that this was in the day before 1/2-stud wide liftarms existed, a piece that I often use now to make cross-bracing in my truss structures)

The abutments were towers of basic bricks. I built one completely, and then ran out of bricks while building the second one. Oh well, the truss arch was the main feature :-)

After a time, I decided to load test the bridge. At 35 pounds (applied to the mid-point), one of the truss members came apart, and the whole thing crashed to the ground. I was very pleased to see this much capacity, and it proved to me the strength of this type of construction of LEGO parts.

After experiencing the success of this bridge, I started to incorporate trusses into many of my models. So what do I mean by success? Well, here are a few high points:

The overall structure was relatively light, i.e. high strength to weight ratio (which is characterisitc of a truss)
Aside from figuring out the geometry by trial and error, the bridge was relatively easy to build (now, I use a spreadsheet to plan the geometry before I ever pick up a brick)
Pinning together multiple Technic beams actually produces an eccentric joint (i.e. all the members are not perfectly in-line), but this construction did work well and produced a strong bridge.
The string cross bracing worked well for the bridge, however it did not for later trusses that I built with different applications (e.g. crane booms). I eventually switched to rigid cross bracing made of 1xn plates and connected to the main chords of the truss with 1/2-stud wide liftarms.

BRIDGE 2: Straight / Arched Truss

Built: c. 1997
Total Length: Approximately 8 feet (2.4 m), the free span was probably about 4 feet
Ultimate Capacity: unknown

About a year and a half after I built the previous bridge, I made a new bridge based on the same building concepts. This new one was slightly different in type, having a straight surface on top (to apply a roadway or train rails) and arching down to two supports.

The design of this bridge also differed in the make-up of the vertical truss members. In the previous bridge (the through-truss arch), I used doubled vertical members so that the loading on the truss joints wasn't too eccentric (i.e. out of plane, or not in-line). In this new bridge, I reduced the verticals to just single members. This made the joints even more eccentric than before, but it still worked and the bridge functioned well.

As in the previous bridge, I laid out the truss pattern for 1/4 of the bridge by trial and error. Since the bridge is symmetric, I simply copied the pattern three more times to complete the entire truss.

The bridge sits on two sets of piers or columns. Each column is made of basic LEGO bricks. Since they are relatively thin compared to their height (and therefore subject to toppling over easily), I put the columns into pre-compression.

Pre-compressing the columns is much like pre-stressed concrete structures today. The concept is that if you squeeze together a brittle material (and a stack of bricks acts as a brittle material, i.e. it can take compression but not much tension), then it will be able to resist more tension and bending loads. When you bend something, it will see compression on one side and tension on the other.

In other words, with the columns of this bridge stacked and squeezed together, they would not crumble if the bridge was hit from the side or rocked back and forth.

The pre-compression or pre-stressing was accomplished by means of dual cables inside each column. These cables were tensioned by means of a ratcheting winch. I simply cranked down on the winch until the cables were taut.

This was more of a bridge "study" than a final model. I was really experimenting with this building concept (truss) and testing its limits. It was valuable to learn that I could get away with single vertical truss members (and therefore having more eccentric joints) as this saves material and makes the overall structure lighter. By saving material, I could build even bigger trusses!

Thomas J. Avery 2008