Proof Loading of Tension Laminations for Large Glued-Laminated Beams

The purpose of this study was to establish tension proof-loading criteria which would permit fabrication of large glued-laminated beams with assured minimum allowable values. The beams were fabricated from low grades of lumber, which imposed maximum reliance for strength on tension proof loading. The two outermost tensile laminations, representing 12-1/2 percent of beam depth, were proof loaded in tension parallel to grain prior to laminating the beams, while L3 visual grade was permitted in the remainder of the beams. Twenty-four experimental beams 24 inches deep by 5-1/8 inches by 40 feet long were loaded to destruction in static bending. Two beam modulus of elasticity (E) combinations were developed, namely 2.0 and 1.8 x 106 psi. The anticipated beam design stresses, f, for these E levels were 2,600 and 2,200 psi, respectively. A general E gradient was developed within the beams, from high E on the surfaces to low E in the core. Two proof-load levels were employed for each of these beam combinations, namely 1.4 and 1.2 times the allowable bending stress anticipated for the beams. All beams in the 2.0E2600f-l.4 proof-load category qualified for 2600 psi allowable design stress. Beams in the 2.0E-2600f-1.2 proof-load category qualified for 2400 psi and all beams in the 1.8E-2200f level qualified for 2200 psi. L3 visual grade could be permitted throughout the compression zone and core of beams provided that high-E material is located in the outermost laminations and E balance is maintained in the beams.

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