general result of this work, extending 

 over many years, has been to place wood 

 on a technical footing with other 

 modern engineering materials. Hun- 

 dreds of thousands of tests have 

 defined and differentiated the various 

 lands of strength of more than 160 

 wood species, including all the more 

 important woods produced for the 

 Nation's markets. Both the methods 

 and the results of this research have 

 been adopted as standard by engineer- 

 ing authorities in the United States and 

 abroad. The data provide a funda- 

 mental basis for design, for selection of 

 species for particular uses, and for fmd- 



M-28743-F 



Modern metal connectors act as dowels or 



keys in the construction of strong 



timber joints. 



ing servicable substitutes for the scarcer 

 and higher-priced woods. Many minor 

 species remain to be investigated, and 

 problems of the strength of second- 

 growth material are assuming larger 

 importance with the changing character 

 of the American timber stand. . 



Structural investigations 



On the foundation of reliable strength 

 values for the principal species, Labora- 

 tory research is able to deal effectively 

 with problems of wood structures and 

 structural parts, in which the form, size, 

 and condition of members must be 

 taken into account. Tests of full-sized 

 timbers have demonstrated in quantita- 

 tive fashion the influence of common 

 defects, such as knots and checks, on 

 strength, with the result that structural 

 designers have been supplied with more 

 efficient working-stress values, building 

 codes are being modernized, and timbers 

 are bought and sold on the basis of 

 strength grades, by which they can be 

 rationally and economically selected for 

 their intended loadings. The value of 

 the Laboratory's contributions to struc- 

 tural practice is further illustrated by 

 the development of a special formula for 

 wood-column design, which has re- 

 placed the less accurate ones formerly 

 in wide use, and by the discovery of a 

 new engineering principle applying to 

 beams under shear loading, by which 

 large savings of material are made 

 through the improved design of railway- 

 and highway- bridge stringers and other 

 large members. 



Joints and fastenings ( 



Since the joints of a structure are 

 usually its critical points, the strength 

 of the fastenings used in wood is fully 

 equal in engineering importance to the 



16 



