Lateral what, you ask? Exactly. Lateral bracing is perhaps the most important aspect of your bridge. It is vital to the success of your bridge to understand what it is.
Lateral bracing is the term we use to refer to any pieces on a bridge that help keep the bridge from twisting. It also helps keep the top chords of the bridge from bending or deforming in or out. In the figure below, the lateral bracing is marked red:
Why is lateral bracing so important?
The shorter a piece of wood, the more compression it can hold before failure. Lateral bracing serves to break the top chord into smaller sections, giving it more strength. Simply put, lateral bracing is the “truss” that goes on top of your bridge. It isn’t quite like a truss, however. The differences are that Lateral Bracing doesn’t have to withstand the same amount of force as the truss members, under normal circumstances.
Why only on top?
The bottom chords of a bridge are being pulled in tension, and they are not going to bend or twist in the same way as the top chords will. Remember that the top chords are being pushed together. When you push something it tends to bend or deform. The bottom of a model bridge doesn’t need lateral bracing because they aren’t twisting or bending. The top chords will want to bend and twist, and they need the extra support to stay straight.
Why an X?
An X shape for lateral bracing is beneficial because the x’s make the top chords into a series of triangle shaped sections. Triangles are the go-to shape when building structures, especially model bridges. The triangle is really good at resisting being deformed, which is the whole point of lateral bracing. You could make a zig-zag pattern across the top instead of a series of x’s if you are trying to save some weight or materials.
There is a place and time when you can use straight pieces for lateral bracing instead of X’s. That idea is illustrated by the Fernbank Bridge. Notice that this bridge uses an L-beam for the top chord. And a large L-beam at that. The key lies in that fact; the straight pieces across the top had a very large surface area joining them to the top chord. This automatically made them stiff, and able to resist deforming. This is imitating what a triangle does, and worked very well for the Fernbank Bridge. This idea will not work unless you have the large surface to glue the lateral bracing to the top chords, so be careful if you decide to try it out.
How much lateral bracing?
The amount of lateral bracing you need to use is based on the shape of your top chords. The top chords are most often rectangle shaped, with the skinny side facing up and down and the wide side facing horizontally. A rectangle shaped piece of wood is easy to bend in one direction, but not in the other. If your top chords are rectangle shaped, remember this and use enough lateral bracing to keep the top chords from bending in or out.
Other shapes for top chords are square, T, L, or I beams. The square is probably the least efficient, but is the easiest to manufacture. If you want to use an T, L, or I beam on your model bridge, chances are that you will have to make it yourself by gluing two or more pieces of wood together. These types of beams will need less lateral bracing. However, they are tricky to make and you need to be precise and accurate when you make them. Otherwise they won’t work well.
Generally, most bridges don’t use enough lateral bracing.
Lateral bracing is supporting less of a load than the truss members, so the pieces used for lateral bracing can be smaller. See this Science Olympiad bridge for another example of small but very effective lateral bracing.