Solid Wood Products 2699 



Effect of laminae thickness and width on giue-line durability. — Selbo 

 (1948a) glue-laminated white oak beams from laminae 4, 8, 14, and 24 inches 

 wide, that were Vs-, !/4-, Vs-, ¥4-, 1, and l-Vs inches thick. After bonding with a 

 phenol-formaldehyde adhesive designed for curing at intermediate temperatures 

 and conditioning for 2 weeks, the beams were end-coated and exposed unpro- 

 tected for 3 years to outdoor conditions, or to 3 years of continuous salt-water 

 soaking, or alternate soaking in a 4-percent salt solution and drying. He found 

 that glue bonds of good durability could be obtained with all laminae within the 

 size range tested. The thickness and width of the laminae, within these limits, 

 apparently had no significant effect on the strength and durability of the glue 

 bonds. 



Effect of laminae thickness on beam strength. — Beams glue-laminated 

 from thin laminae are usually significantly stronger than those made of thick 

 laminae if they are cut from knotty wood or wood with areas of cross grain. With 

 thin laminae, knots and localized areas of cross grain are more randomly distrib- 

 uted within the beam and therefore weaken it less than if concentrated in thick 

 laminae or one-piece beams (Neam and Norton 1952; Koch 1967b, p. 10; 

 Schaffer et al. 1972; Braun and Moody 1977). With clear straight-grained white 

 oak, however, Finnom and Rapovi (1959) found that laminae thickness, in the 

 range from Va- to y4-inch, did not affect modulus of rupture or modulus of 

 elasticity of straight or curved laminated beams. Preston (1950) found that 1/40- 

 and 1/60-inch-thick clear yellow-poplar laminae were compressed (densified) 

 during pressing and therefore had 16 and 26 percent higher tensile strength than 

 1/10-inch laminae; also Preston found that modulus of elasticity in bending was 

 increased by decreasing laminae thickness. 



Safe bending radius for white oak laminae. — (See section 19-3). 



Effect of water soaking on strength of laminated beams. — Laminated 

 white oak beams 1.75 inches wide, 3.75 inches deep, and 45 inches long 

 comprised of six Vs-inch-thick essentially clear boards were fabricated by Freas 

 and Werren (1959) and tested in bending when at 11 to 12 percent moisture 

 content; bending properties of these dry beams were compared with those of 

 matched beams salt-water soaked for 3 months to a moisture content of about 40 

 percent and tested wet. They found that the decrease in modulus of elasticity 

 after salt-water soaking averaged about 26 percent; modulus of rupture de- 

 creased about 41 percent. 



Effect of preservatives on shear strength. — Selbo (1959) found that block- 

 shear strengths of northern red oak laminated with resorcinol and phenol-resor- 

 cinol adhesives from preservative-treated northern red oak boards were 4 to 16 

 percent less than in untreated controls. Two oil-borne and three waterbome 

 preservatives were used in the test, which lasted 3 years, during which time 

 specimens were conditioned at 80°F and 65 percent relative humidity. In all 

 specimens, shear strength declined slightly during the first three months, but 

 thereafter remained generally constant. 



Strength of laminated oak beams. — Modulus of rupture and modulus of 

 elasticity of laminated beams depends on wood species, grade, moisture con- 

 tent, arrangement of laminae, and beam size. Nemeth (1974) described an 



