Bridge Failure  Fracture
[1, 2]

Fatigue failure occurs when a material fails at loads lower than the determined yield strength and is usually caused by stress from cyclical loading over a period of time. The first sign of fatigue is a crack in an area of concentrated stress, followed by crack propagation, and, quickly thereafter, fracture failure [a]. The progression of cracking to failure is shown in Figure 2. Cyclical loading, like opening doors in a building or traffic on a bridge, is the primary cause of fatigue.

While fatigue occurs in buildings and bridges, bridge fatigue tends to be more catastrophic than building fatigue. The repetitive flow of traffic interrupted by heavy trucks are the primary causes of fatigue in bridges. Because fatigue failure occurs relatively quickly, it is crucial to inspect areas of concentrated stress in bridges to avoid fatigue. In general for buildings, fatigue occurs in small parts and will not cause failure of an entire structure; only the moving parts would need to be replaced. Both concrete and steel, the main building materials for large structures, are susceptible to fatigue. In some areas around the world, “fluctuating” wind loading is constant enough to cause fatigue. [b] Fatigue caused by wind can be very damaging to a lateral bracing system, especially one without redundancy, and cause building failure. Other causes of structural fatigue include earthquake loading and machine vibration. [c]