Why balsa bridges fail

When a bridge breaks, it breaks in one place first, and the rest follows. Knowing the usual first failures tells you where to put your effort.

1. A compression member buckles

The stick that is being pushed bows out sideways and snaps. This is the classic failure of a balsa truss, and coaches list it first.

How to prevent it: shorten the unsupported length, add bracing, use a stiffer or larger stick, or both. See truss basics.

2. A joint pulls out

The glue joint lets go, or the stick tears away at the joint. Coaches' rules of thumb say that end-grain joints are the worst offenders, and that overlapping side faces (lap joints) are preferred.

How to prevent it: make joints with a tight fit, enough overlap and a thin glueline, and don't butt a stick end into the side of another with only a dab of glue. See gluing balsa joints.

3. The bridge twists or falls sideways

A tall, narrow truss can tip over sideways well before any stick breaks, because nothing holds the top of the truss in line.

How to prevent it: brace the two trusses against each other with cross-members, and keep the load centered.

4. Local crushing under the load or the supports

The load point or support presses into the soft balsa and crushes it. Spread the load over more wood with a short pad or small plate where the rules allow it.

What we don't know yet

Real data on which failure is most common is thin. We searched Science Olympiad, ASCE and ASEE material and found no teardown statistics of where bridges break, only coaching experience and individual examples. The order above reflects that experience. If your team keeps records of how its bridges fail, they are more useful than anything here.

Learn from every break

1. Photograph the bridge before and after testing.
2. Note where it broke first, not where it ended up.
3. Keep the pieces, since the broken ends show whether the wood or the glue gave way.
4. Change one thing in the next version.

See testing your bridge safely for how to run the test.