Science 7 – Unit D: Structures and Forces Topic 5: How Structures Fail Levers Create Large Forces Lever: device that can change the amount of force needed to move an object. When enough applied force is put on one of ends of the lever, it can be used to lift very heavy loads. A lever is an arm rotating on a pivot, called the fulcrum. Lever Action When an external or internal force acts on a one end or side of a frame structure, deformation can result. The joints between the components are weakened and, if nothing is fixed, the structure will fail. Resisting Structural Failure Basically any force, from bending to twisting to shearing can cause deformation. The right materials and designs must be chosen to deal with the forces that the structure will have to face. For example, the cables in a hanging sign should be metal, because metals have a high tensile strength. Any component dealing with compression (eg. parking lot columns) should be made of concrete. Why do Materials Fail? Materials fail because the forces acting on them are strong enough to rearrange or break apart the connections between the particles of the material. If you bend a paper clip back and forth enough times, you will notice that it becomes easier to do this: the connections between the particles are weaker. It’s only a matter of time before a crack or break will form. When this weakening occurs, engineers call it metal fatigue. It is an issue in everything from buildings to cars to airplanes.