A “truss” is what you see when you look at a truss bridge from one of its sides. A truss is typically made up of a lot of triangles, but some uncommon truss designs don’t have any. The purpose of a truss is to help a bridge support a load (car, train, person) from any point along the span of the bridge. Without a truss, you simply have a beam bridge.
Overview of Terms
Let’s define a couple terms to help you understand how to study truss design.
Shown here in red is the Truss Frame. The frame is the outermost parts of the truss.
The frame is made up of several parts: Top chord, bottom chord, and two end posts. This diagram shows the frame in an expanded view so you can easily see each part. Practically, you might use different sizes or shapes of wood for each of these parts due to the force being put on each part is different.
Now we will add the truss members, which are shown in black in this diagram. The truss members are simply an arrangement of triangles (most of the time) that transfer the force/s put on the bridge to the ground. The way these triangles are arranged or shaped is the essence of truss design. You will see examples of the most common designs further on this page.
These terms will be helpful to keep in mind as we talk more about truss design. Now let’s take a quick look at the history of truss design, particularly in the United States.
Brief History of Truss Design
While trusses have been used for both roofs and bridges for many centuries, there was an explosion of truss advancement in the 19th century in America. The need for bridges to span longer distances in this era, as well as to hold increasingly heavy loads, brought about many creative solutions in the form of new truss designs.
Three names stand out as true pioneers in these early truss bridges: Timothy Palmer (1751-1821), Louis Wernwag (1770-1843), and Theodore Burr (1771-1822). These men, along with other bridge builders who followed them, designed and built many bridges, especially in New England. Theodore Burr came up with a design that was used in many iconic covered bridges, and some are still standing today. These men came up with practical solutions for bridge building, and did not know or have access to the theory behind their designs.
Interestingly, building bridges in the 18th and early 19th century was more about quality of construction. Skilled carpenters were needed, and most of the engineering was practical and not theoretical. Wood was the primary material available in these early years, but iron and then steel came along and changed everything.
With iron and steel, and the expansion of railroads that carried heavier and heavier loads, new bridge designs were needed. The Howe and Pratt trusses in particular were designed to incorporate iron rods in the truss. These two designs, which you can see from the original patent images, do not look exactly like the truss designs that we associate with those names today. This is a bit of a mystery to me, but you can see semblances of the original designs in the modern depictions. Both the Pratt and Howe patents were very much concerned about methodology of construction more so than the actual design.
Bridge history is fascinating, and there is so much more to learn. This short section is meant to whet your appetite, but now we turn to the application of truss design to model bridge building.
Common trusses used in model bridge building
Each of the following truss designs are very common in both real and model bridges because of their sound engineering and ease of construction. As I mentioned earlier, the key for us model builders is how these designs transfer forces throughout the bridge and eventually to the bridge supports. Each of these designs does that in a different way.
Take some time to read up on each of these designs before deciding on one to use for your bridge. Perhaps you will end up not using any of these designs but creating something on your own based on the principles of force transfer.
I have chosen to highlight these four examples of different trusses to get you started with some very solid examples that you can easily use on your bridge. There are other, more complex, designs that aren’t shown here. You can do a web search for truss design and see many more examples. I’m a fan of keeping things simple, but it is possible that your unique bridge project would benefit from one of the more exotic designs.
If you are interested in learning more about trusses and truss design, check out Truss Fun, Second Edition from Amazon. This is a comprehensive study on the engineering principles behind the design of bridges. It is easy to understand and to follow, and is a great fit for students who are just learning, but advanced enough to be a great resource to those with more experience. For more great resources, see this list of other great bridge books.