Effect of Loading Rate on Bending Strength of Douglas-Fir 2 By 4’s

Select Structural (SS) and No. 2 Douglas-fir lumber specimens were tested at three different rates of loading to determine the rate-of-loading effect on bending strength of lumber and, through a damage accumulation model, the duration-of-load effect. Results show that average strength increases nearly linearly with the logarithm of loading rate. Results also show that rate of loading affects the total distribution of bending strength, contrary to claims in published reports. The rate-of-loading effect and, consequently, the duration-of-load effect appears to be greater for SS than for No. 2 lumber graded on the basis of size and location of knots and slope of grain. These results will be complemented by results from constant load tests currently underway.

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