The Forest Products Laboratory 



"better" grades will be iiniformh^ stronger than the poorer ones. This 

 allows higher fiber stresses and consequently more efficient use. since 

 allowance does not have to be made for the occasional weak piece which 

 slips in under grading rules patterned after lumber grading rules. 

 The laboratory has studied and analyzed the effect of defects and 

 physical properties upon the strength of structural material, and has 

 drawn up grading rules for the two princij^al structural timber spe- 

 cies. These rules have been adopted by the associations concerned and 

 are now in everyday use. Tables of allowable fiber stresses under 

 various conditions of service to be used with these rules, have also been 

 drawn up. So far as known, this is the first successful attempt on a 

 commercial scale to grade structural timber on a basis of strength. 



Efficient design of any article requiring strength demands not 

 only that sufficient strength be present, but also that this strength be 

 secured at a minimum expenditure of material. To balance the con- 

 struction by the elimination of surplus material is frequently more 

 difficult than simply to strengthen an admittedly weak part by the 

 addition of more material. Through a series of tests upon white oak 

 barrels, in which the barrels were subjected to various kinds of tests, 

 such as internal pressure tests and drop tests, the laboratory deter- 

 mined the proper relation among stave thickness, head thickness and 

 hoop spacing to yield the best service with the smallest amount of 

 material. New types of barrels, based upon the results of these tests, 

 have been approved by the Interstate Commerce Commission. Just 

 as in the design of barrels, so also with boxes, crates, and other types 

 of shipping containers, one of the most important design problems is 

 to secure a "balanced construction" affording greatest strength at 

 least cost. The proper selection of the species or kind of wood for 

 various uses is also very important. 



The laboratory has perfected a box testing drum giving consistent 

 results which indicate the value of any type of construction for the 

 shipment of goods and has developed various standard types of box 

 constructions which have been adopted by the associations of manu- 

 facturers and of users of boxes. It has also investigated and tested 

 the relative value of many woods for box making and divided them 

 into four groups on this basis, the woods in each group to be used inter- 

 changeably. This work represents the greatest single step forward 



