Lateral What? Exactly. Perhaps the most important aspect of your bridge and you aren’t sure what it is.
Lateral bracing is the term I use to refer to any pieces on a bridge that help keep the top chord from bending horizontally. In the figure before, lateral bracing is red:
Why is lateral bracing so important?
As you may have read on this website that the shorter a piece of wood, the more compression it can hold. 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.
Why only on top?
The bottom member of a bridge is in tension, and is not going to bend or twist. Wood has the same tensile strength no matter how long it is. The top chord will want to bend and twist, and needs support to keep it straight.
Why an X?
The reason many builders choose X’s is because they make triangles, which resist deforming. Of course, it doesn’t have to be X’s. You could use half X’s in a zigzag pattern, just take away half of the pieces in the figure. Or you could use straight pieces, circles, or any combination. However, straight pieces would just make a rectangle, which won’t really help.
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 kind of made a triangle, so to speak.
How much lateral bracing?
The amount of lateral bracing you need to use is based on the ratio of dimensions of your top chord. You want to use only just enough and not overdue it in order to get the best efficiency. However, there is a danger of not using enough. As a general rule of thumb, I try and use the same of amount of bracing joints as I have truss joints to the top chord. You can see this idea in the example pictures.
There is another thing to consider. Let’s say your top chord is 5 units wide and 5 units tall (a square) That beam is going to be equally hard to bend in any direction. However, if your top chord is 2 units wide and 8 units tall, even though it has the same total mass as before, you will need to use more lateral bracing.
There comes a point when it is no longer useful to add lateral bracing. If your top chord is only 1 unit wide, and 9 units tall, your top beam is going to bend and twist like a slippery snake. There isn’t a lot you can do about it except increase the width.