The effect of a cyclic thermal loading on the load-duration behavior of structural lumber in bending is presented. Select Structural and No. 2 grade Douglas-fir nominal 2 by 4 beams were tested under a constant bending load to determine time-to-failure. Two cyclic temperature environments were used in the investigation: 73 F to 100 F and 73 F to 130 F on a 24-hour cycle with a constant 50% relative humidity. An exponential damage accumulation model with a temperature factor was used to predict the observed times-to-failure. The damage model originally was fitted and calibrated using load-duration data from equivalent lumber samples subjected to constant temperature environments. The model predicted quite well the observed times-to-failure in the cyclic temperature environments. This is quantified using a standard errors analysis between the model predictions and the observed cyclic temperature data. These errors are comparable to those observed with the constant temperature data which were used to determine the model constants
