2692 Chapter 22 



official acceptance in the market should find Galligan's (1977) explanation 

 useful. Koch (1981) found that estimated strength ratios of yellow-poplar 2 by 

 4's were not good predictors of their modulus of rupture. 



STRUCTURAL PLYWOOD 



Consumption of softwood plywood in the United States increased from 2.7 

 billion square feet (Vs-inch basis) in 1950 to about 20 billion square feet in 1978. 

 During the same timespan, consumption of hardwood plywood more than dou- 

 bled to about 4 billion square feet (fig. 29-14). 



Of the 182 softwood plywood plants operating in the United States in 1978, 60 

 were in the southern pine region — all 60 built since 1963 (Whitman 1979). Such 

 extraordinary expansion suggests that demand for structural panels may eventu- 

 ally outrun the available supply of softwood veneer logs. Construction of plants 

 to make composite structural panels of southern pine veneer over flake cores (see 

 discussion related to table 24-2 and figs. 24-50 through 24-52) would greatly 

 increase the square footage of panels that can be manufactured from available 

 pine veneer. 



Another possibility for expanding structural panel production is use of hard- 

 wood veneers. Veneer for this purpose need not be of appearance grade and 

 could be rotary peeled in large volumes from 4-foot bolts of hardwood that grow 

 on southern pine sites (fig. 18-252). Ideally such veneers should be strong and 

 stiff, but of moderate weight; additionally, they should glue readily. The Hard- 

 wood Plywood Manufacturers Association (1971) has provided a structural 

 design guide for hardwood plywood, and the American Plywood Association 

 (1974) has published a product standard. O'Hallaran (1979) described procedure 

 used in developing performance-based specifications for structural panels; by 

 laboratory and field work, these procedures establish criteria for structural 

 adequacy, dimensional stability, and durability during one year of weathering. 



The oaks and hickories, which comprise more than half the volume of hard- 

 woods growing on pine sites, are strong and stiff — but they are dense and 

 difficult to glue into exterior-grade panels (Stem 1947b; Craft 1970, 1971, 

 1975). Lighter pine-site hardwoods such as sweetgum, yellow-poplar, red ma- 

 ple, sweetbay, and black tupelo peel and glue readily, but have less strength and 

 stiffness than southern pine of comparable thickness. See table 24-31 for modu- 

 lus of elasticity and tensile strength of 1/6-inch-thick, rotary-peeled sweetgum, 

 white oak, black tupelo, and yellow-poplar veneers harvested in three southeast- 

 em areas. 



The gluing problem can be partly resolved by using sweetgum or yellow- 

 poplar (or other of the readily glued low-density species) for inner plys and red 

 oak or hickory for faces (Craft 1975). (For further discussion of glueing technol- 

 ogy see: Sellers, T. and J.R. McSween. 1983. Glueing Stmctural Plywood from 

 Medium-density Southern Hardwoods. Plywood Research Foundation, Tacoma 

 WA. 15p.) 



Plywood manufacturers prefer to use 8-foot lathes for both face and core 

 veneers — the core veneers are reduced to 4-foot length with splitter knives. To 



