I want to put x-shaped supports on my bridge, but I don’t know how best to join them. I’ve heard of something, I think it’s called a half-lap joint, that I think might work best. You cut half of the wood off the part of the strip that will be the actual joint, and then connect the two pieces at those joint parts. There isn’t any part of the wood sticking out, and they are very connected, I think, as far as how the weight/force is distributed, which I figured would be good. But is there a better way to make an x?

Perhaps a more simply way to describe this is cutting a notch out of both pieces of the X at the point of intersection. The reason to do this is to avoid one pieces laying on top of the other, which makes it slightly harder to glue and adds additional pre-load force on that piece.

However, I think cutting notches is more trouble than it is worth. All the bridges I have built (see my Science Olympiad Bridge ), using X’s for lateral bracing, have had one piece laying on top of the other. This does mean you have to use clamps or tape to hold the pieces down while gluing. But I consider this less of an inconvenience than trying to cut two notches out of the lateral bracing. In fact, most of the time my lateral bracing is so thin that I couldn’t cut a notch out of it even if I wanted to.

The notches must be cut exactly to the right size. If they are even a little too large, then you weaken your lateral bracing pieces. This may not be a problem, however, because most people use pieces of wood that much bigger than they need to for lateral bracing. Still, I do not see cutting notches as the way to go.

Another way around this issue is to not use X shaped lateral bracing. Instead, you could use a zig-zag pattern. This will look like a Warren truss on top of your bridge. A stronger pattern is the Warren with vertical supports for lateral bracing.

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.