In these fews pages it talks about colums, beams and arches. These are all part of a structural system that helps a building stands up.  The important thing is how they are supporting each other, meaning which way they are placed. Because the load might be different the way they are placed will affect the structure. When placing them we must pay attention to force, tension, and compression. They are different structural placements that put more force, tension and compression in different areas. If that happens we will need to know if the material will be able to support it or not. For example if two ends are being compressed towards the middle, the material should be able to hold and not snap/buckle in the middle. Another important structural element are joints, how to connect the materials. Just like we must consider how to place columns and beams to stand and support for a long time, its the same for joints. Need strong joints to withold the weight of the structure.