THE GLTTIFG OF WOOD 53 
usually greater than necessary, and the strength across the grain is 
often less than desired. When pieces are glued together with the 
direction of their grain at right angles, each piece helps the other 
resist stresses across the grain, and their strengths in the two direc- 
tions, are, in effect, averaged. The result is a more homogeneous 
product than solid wood. 
Similarly, since the tendency to shrink and swell with moisture 
changes is very slight along the grain, the crossing of the plies 
counteracts the normal shrinkage and swelling of the wood across 
the grain, and, therefore, the glued cross-banded product under vary- 
ing moisture conditions is nearly constant in width and length. 
Furthermore, it is not necessary that the cross plies, such as the 
core in a 3-ply or the crossbands in a 5-ply veneered panel (fig. 3) 
be thick or that they occupy a very large part of the total thickness 
of the cross-banded product. For example, in a 5-ply, thick-core 
panel one twenty-fourth + one-twentieth + thirteen-sixteenths + one- 
twentieth + one twenty-fourth of an inch, the % -inch crossbands 
restrain the core and face plies from shrinking or swelling appre- 
ciably with moisture changes. The thickness of a cross-banded panel, 
however, can change as it would if it were made from solid wood 
since there is no restraining force in the direction of thickness. 
To realize fully the advantages of cross-banded construction the 
panels must be properly designed and glued. The tendency of 
panels to warp and twist may be even greater in improperly con- 
structed plywood and veneered panels than in the average solid wood 
of the same thickness. Therefore, in cross-banded construction where 
the movement of the wood is prevented when it loses or absorbs mois- 
ture, large stresses are set up. The adjoining plies try to shrink or 
swell in directions which are at right angles to each other, but each 
ply restrains the ply or plies next to it. The contending forces, 
therefore, tend to break the glue joint or to distort the panel. The 
development of these stresses can not be prevented if the moisture 
content of the wood changes, but their effect can be largely controlled 
by proper design and well-glued joints. 
In cross-banded products that are properly designed, the forces 
exerted by the plies on one side of the core under changing moisture 
conditions balance in magnitude and in direction the forces exerted 
by the plies on the other side of the core. This balance is partly 
accomplished by the use of an odd number of plies so arranged that 
for any ply on one side of the core there is a corresponding parallel 
ply on the other side at the same distance from the core. (Fig. 3.) 
If only two plies are glued together with the grain at right angles 
to each other, each ply tends to distort the other when moisture 
changes occur, and cupping of the panel results. On the other hand, 
two or more adjacent plies with the grain parallel have much the 
same effect as a single ply with a thickness equal to their combined 
thickness. For example, a 3-ply panel, with the grain of two adja- 
cent plies parallel but at right angles to the third ply will cup the 
same as a 2-ply panel when both are subjected to moisture changes. 
If, however, a fourth ply parallel and similar in properties to the 
one single ply is glued as a face to the parallel plies face the construc- 
tion is balanced and in effect three ply. 
