Hub Hobby Shop

Balsa Bridge

Balsa Bridge

Balsa Bridge


 by Garrett Boon



The Forces

Compression: Compression is a pushing (compressing) force. You can take the middle a straw and squeeze it between two fingers, the straw flattens. However, if you hold each end of a straw and push together it is harder to make the straw compress, or flatten. The shorter a piece of wood is, the more compression it can hold. The longer a piece of wood is, the less compression it can hold.

Tension: Tension is pulling. It would be hard to break a straw if you held both ends and pulled apart. Wood also has the ability to resist tension. Tension may be applied parallel to the grain of the wood, but should be avoided perpendicular to the grain. Wood is very strong in tension parallel to the grain, but extremely weak in tension perpendicular to the grain.





Lap Joint: The lap joint is one of the strongest, and you should use it whenever you can. It strengthens compression members because it adds stiffness. The lap joint has one drawback, however. The joint is only as strong as the face of the wood. That means that only one part of your piece of wood is in contact with each other. Most Balsa wood is not


Notched Joint: The notched joint gives more strength than the end joint, but less than the lap joint. It is more difficult to build, so it is not very common.

Gussets: If you have to use an end joint, it is a good idea to add a gusset to make it stronger. The gusset creats a lap joint, which is strong in both tension and compression. Usually I try to make each part of the gusset the same length. If there is more glued to one piece, the one with less surface area for the glue will pull away first anyway.

Lateral Bracing: 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.

Why is lateral bracing so important? As you may have read, 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 Fernbankbank Bridge (shown below). 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.

Bending Wood










Garrett Boon has spent over five years participating in Science Olympiad building and technology events. He has won over ten awards in the Bridge Building & Tower Building events, including three 1st places. In the spring of 2004, he took first place at the Georgia State Finals in Bridge Building, breaking a 4-year efficiency record with his bridge. More bridge building tips can be found on his website, Model Bridge Design.