Duration of Load Tests for Wood in Tension Perpendicular to Grain

A total of 180 Douglas-fir glulam blocks 5 inch by 5 inch by 24 inch, were tested in tension perpendicular to grain at six different rates of loading. It was found that the reduction in strength with time was greater than predicted by the Madison “duration of lead” concept. The tension perpendicular to grain strength measured with these large blocks was less than half of what is measured with the standard ASTM specimen. It would appear that the allowable stress for tension perpendicular to grain used in Canada (65 psi) is too high. Deformations perpendicular to grain were measured, and when the wood was loaded at the slowest rate, it would deform eight times as much as it would during the quick test. The ability of wood to stretch perpendicular to grain may explain the difference in the time behavior observed between clear material and commercial material. Small beams with an artificial stress concentration showed a different time behavior from that of small clear beams.

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