Creep of Douglas Fir Beams Due to Cyclic Humidity Fluctuations

This report describes tests of the hypothesis that creep in Douglas fir beams of commercial size is influenced by cyclic humidity fluctuations.Twenty 3.5- x 3.5-inch (89- x 89-mm) beams were creep-tested for a period of approximately 1,200 hours (7 weeks). Deflection data were recorded for loaded beams while the relative humidity of their environment was cycled at 70 F. The environmental EMC was varied from 7 to 20% for periods of 24 and 168 hours, respectively. Three regression models were fitted to the relative creep data. The effect of stress levels and humidity cycles were compared.The study showed that creep for cycled specimens greatly exceeded creep for uncycled specimens