If the two points where the ends of the straw had been attached move apart, the straw was under tension. If the two points move toward each other, the straw was under compression. It is interesting to compare where the tension and compression elements are in bridges versus cantilevers. Here, the line of straws along the top are in tension and the line of straws along the bottom are in compression. Many engineering principles emerge from these activities.
Pin joints are common in real structures.
For instance, rivets are used with steel beams and nails are used with wood planks in the same way that pins are used with straws in this activity. Students should be encouraged to test the strength, stability, and durability of their structures. In this way, they gain insights into the behavior of structures that are subjected to outside forces, and they can see how to improve their structures so as to strengthen weak points. You can also leave the structures intact overnight, to discover which ones are still standing in the morning. An additional interesting extension is a search for unnecessary straws.