Tension Proof Loading of Lam Stock for Laminated Beams

In two experiments, high-stress tension laminations for 17 Douglas-fir beams were proof loaded in tension before the beams were laminated. The beams were 10-12 inches deep and 16 feet long. The purpose was to demonstrate the efficacy of tension proof loading in providing assurance of beam strength for allowable design stresses of 2600f or higher. A tensile proof load of 2.0 times f was used in the first experiment with allowable design f values of 3000, 2400, and 2000 psi. Although the proof load resulted in breakage of an excess of the laminating material, all of the beams from the surviving material were sufficiently strong to have been safely assigned working stresses of 3300f. From the data of this experiment a more practical tension proof-load factor was calculated, namely, 1.4 times design f. In the second experiment, the proof-load factor of 1.4 times f was evaluated using f values for the beams of 2600, 2400, and 2200 psi. Test results indicated that all of these beams could have been assigned allowable design stresses of 2600f or higher. These experiments also demonstrated the occurrence of a reinforcing effect on the tensile strength of lumber due to laminating. Significantly higher bending stresses and strains were sustained by the laminated beams than the laminations would have been able to withstand in uniform tension as isolated members.

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