Designing for creep in wood structures

The National Forest Products Association Design Specification for Wood Construction suggests a creep factor of 1.5 for glued laminated timber and seasoned sawn lumber, and 2.0 for unseasoned sawn lumber. That is, deflection due to long-term or permanent loads is assumed to be 1.5 to 2.0 times the immediate elastic deflection caused by the load. A stochastic creep model previously developed for predicting the creep response of structural lumber was used to evaluate these design “rules-of-thumb” regarding creep. The effect of duration of the applied load and moisture content on the creep factors was evaluated. The factor of 1.5 for seasoned sawn lumber was found to be nonconservative for loads of duration greater than approximately 2 to 3 weeks. The factor of 2.0 for unseasoned lumber was found to be sufficient for durations of load of 2 months up to 50 years.

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