Effect of Lumber Width and Tension Lamination Quality on the Bending Strength of Four-Ply Laminated Beams

Eighty 4-lamination southern pine or Douglas-fir beams were evaluated in static bending tests; all test beams were near-minimum quality. Objectives of this study were to determine: 1) the relative strengths of small glulam beams with different qualities of tension laminations, 2) which quality of tension lamination provides the desired design strength level, and 3) if there is a significant strength difference between glulam beams made from different widths of lumber. Three qualities of tension laminations (L1/No. 1D,302-20, and 302-24) and two lumber sizes (2 by 6 and 2 by 10) were included in the experimental design. There was no significant difference between bending strengths of beams by size; there was by tension lamination qualities. Beams with 302-20 tension laminations were about 15 percent lower in strength and beams with L1 or No. 1D tension laminations about 25 to 35 percent lower in strength than those with 302-24 tension laminations. Because the near-minimum quality test samples were used, these four-lamination beam test results are expected to provide a conservative estimate of the bending strength of shallow glulam beams. However, until new information becomes available, these results will be useful to industry committees developing specifications for shallow glulam beams and recommending future research in glued-laminated timber.

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