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Loading History and Fatigue Design

By Bill Mohr on Thursday, June 24th, 2010

Welcome to my blog on structural integrity. I thought it would be a good idea to discuss a general concept as the first item for this blog post.

This post considers the importance of fatigue design for different types of structures. Many designers never need to deal with metal fatigue. Others deal with it routinely, since otherwise their structures will fail well before their desired useful life.

A big reason why shows up in the difference between peak loads and routine cycles of load. It is obvious that a device, say one used in a single event emergency, built only for a single peak load will not need a fatigue analysis. As the cycles get bigger relative to the peak load, the need for fatigue assessment increases non-linearly.

Take a tower loaded by storm winds. It must be designed for some single biggest wind speed and also allow for all of the history of storms during its lifetime. The ratio of the single biggest wind speed to the range of speeds experienced during “normal” storms can differ in different parts of the world. Think of places that have many calm days and few storms, but may be subjected to a very large hurricane. This tower must be designed to resist the hurricane and will likely have more than enough resistance to the few smaller storms. The fatigue design effort can be minimal.

Now consider moving the location to one with the same normal weather, but no hurricanes. Strangely enough, this one may need a more extensive fatigue estimate. The thinner, lighter members used where the peak load is small, may be at more risk during the storms. Similarly, a place with many large storms and the same peak conditions as the hurricane would also need more work on the fatigue assessment.

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