Lumber Fatigue And Decay
Question:
What is fatigue in lumber and how does long-term fatigue affect lumber? Does long-term fatigue cause cracks in lumber? What is the difference between long-term fatigue and decay? How do you determine whether lumber has been subject to long-term fatigue or decay?
Answer:
Fatigue is directly related to the duration of the load. Fatigue properties are generally of little concern in many applications of wood but can become important in applications where there are many repetitions of stress. Long-term fatigue can cause damage to lumber if it is not considered in the truss design when repetitions of design stress or near-design stress are expected to be more than 100,000 cycles during the normal life of the structure. The Forest Products Laboratory can provide data on the fatigue properties of wood.
Decay is a hazard for wood completely different from fatigue. It results from the action of certain types of fungi, which use the wood substance as a food source. Wood-destroying fungi require favorable conditions of moisture, temperature and access to air as well as access to wood. Lack of any of these will inhibit the growth of fungi. Usually, trusses are designed with wood members with a moisture content of 19% or lower. Keeping wood below 20% moisture content is the most effective way of preventing decay.
Any of the following lumber associations might be able to give you more case-specific information these topics:
American Forest & Paper Association
Southern Forest Products Association
Western Wood Products Association