Another special feature of trusses is that the members don't bend. They get pulled apart (in tension) and pushed together (in compression), but they don't bend like the plank does when you stand on it. The members stay straight from end to end until they break. This doesn't mean the bridge will stay straight, though. As heavier loads are put on the bridge, it will still sag in the middle. This is because the individual members of the truss are getting longer (if they are in tension) and shorter (if they are in compression).
A Belt Isn't the Only Thing That Buckles
Many materials, in theory, have the same strength when being squeezed together (in compression) as they do when pulled apart (in tension). The problem is that if you press the two ends of a thin member (like a ruler) together, it doesn't simply stay straight and get shorter, but instead it bends out to the side. This is called buckling, which is the way that most tall, skinny things break when compressed end-to-end.
How Can My Truss Fail?
There are three ways your truss can fail. If a member buckles enough, it will bend and break in the direction in which the craft sticks have a low moment of inertia. This may be prevented if the loading frame supports partially buckled members. Another type of failure is that a craft stick pulls apart in the middle in tension. The third type of failure possible is joint break-out. This is when the craft stick breaks right where the bolt is connected.