MECHANICAL PROPERTIES OF WOODS GROWN IN UNITED STATES, 3 



SCOPE AND METHOD OF EXPERIMENTS. 



ORIGIN OF DATA. 



The data in this bulletin are based upon about 130,000 tests, 

 probably the greatest number ever made in one series upon any ma- 

 terial. For this reason, and for others explained later, the data are 

 the most thorough and accurate that are available on the mechanical 

 properties of American woods. The tests were begun about six years 

 ago at the Forest Products Laboratory, which is maintained by the 

 United States Forest Service with the cooperation of the University 

 of Wisconsin. One hundred and twenty-six species of wood have 

 been tested, and it is planned to continue the series until all species 

 which are important, or which give promise of becoming so, have 

 been included. 



SMALL CLEAR SPECIMENS USED. 



Small clear specimens are used in the tests in order that considera- 

 tion of the influence of defects may be ehminated from calculations 

 to determine the relation between strength and density, moisture, 

 locality of growth, soil conditions, etc. These various relations are 

 referred to in the present bulletin, however, only when it is necessary 

 in order to render the data thoroughly understandable. The speci- 

 mens are 2 by 2 inches in cross section,, Bending specimens are 30 

 inches long; others shorter, depending on the kind of test. 



SELECTION OF MATERIAL. 



The material for any given species and locality is cut from typical 

 trees, usually five in number. These are selected by representatives 

 of the Forest Service, careful descriptions being made of each tree and 

 of the conditions under which it has grown. As a rule the test 

 specimens are taken from the top 4 feet of the 16-foot butt log. The 

 number of test specimens from each tree varies from 40 to 120, 

 depending on the size of the tree. Eventually each important species 

 will be represented by tests from at least five typical trees from each 

 of several localities distributed throughout its range of growth. 



OTHER DATA INCLUDED. 



Data derived from tests previously made by the Forest Service and 

 under practically the same conditions as the present series are in- 

 cluded in Tables 1 and 2. The tests were made at Purdue Univer- 

 sity and at the Universities of Colorado, California, and Washington 

 in cooperation with those institutions. 



TESTS ON LARGE TIMBERS. 



A large number of tests have also been made by the Forest Service 

 on full-sized timbers, such as bridge stringers, factory-building tim- 

 bers, and car sills. These tests have demonstrated the influence of 

 defects such as knots, shakes, and checks on strength, and they serve 



