Fatigue cracks are caused by repeated stresses on metal structures, and can lead to catastrophic failure. They have predictable phases and can be discovered by inspectors. Inadequate steel plates caused the 2007 Minneapolis bridge collapse. Routine inspections can prevent fatigue failure, but it can occur in any steel structure.
Fatigue cracks are cracks caused by repeated stresses on a structure over a period of time. Many times, fatigue cracks can be superficial, but other times they can cause catastrophic failure leading to disastrous results. Fatigue cracks are a serious engineering concern that can only be discovered by inspectors, who are able to tell how bad the cracks are.
Perhaps the best-known example of how disastrous fatigue cracks can be comes from an accident that occurred in Minneapolis on August 1, 2007. During the afternoon rush hour that day, a bridge on Interstate 35 over the Mississippi River collapsed , causing 13 people to fall. their death below. It was a national tragedy that prompted many states to immediately step up their bridge inspection policies.
It was later determined by the National Transportation Safety Board that the bridge collapsed due to the inadequate size of the large steel plates used to connect the bridge pieces together. The inadequate size caused fatigue cracks which eventually led to full-blown fractures. Once that happened, the rest of the plates gave way in a chain reaction event. This is known as catastrophic fatigue failure.
There are a number of characteristics unique to fatigue cracks. Primarily, they occur in metal structures. Secondly, they also have predictable phases. These include the initial crack, progressive growth and final fracture leading to fatigue failure.
While fatigue cracks can be a big problem, they can also be very predictable during these stages. Only an engineer can tell if fatigue cracks are a serious problem for a structure. In some cases, redundant parts may be present to prevent fatigue failure.
While the thought of fatigue cracks can be terrifying, in many cases they can also be predicted and prevented. Routine inspections often locate and make note of fatigue cracks. In some cases, repairs can be made quickly and easily by replacing critical parts of the structure. The good news is that fatigue cracks occur on the external, visible sections of the steel the vast majority of the time.
While much of the attention regarding fatigue cracks is focused on bridges, it should be noted that they can affect any steel structure and lead to catastrophic results anywhere. Steel rails in railways can cause trains to derail if they fail. Even steel in buildings, if sufficiently stressed, can fail. This is especially dangerous in multi-story buildings. However, fatigue failures in buildings are not as common as failures in other applications.
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