43 



across grain, especially the latter, owing to the tendency of an excessive crushing 

 stress across grain to indent the timber, thereby destroying the fiber and increasing 

 the liability to speedy decay, especially when exposed to the weather and the con- 

 tinual working produced by moving loads. 



The aim of your committee has been to examine the conflicting test data at hand, 

 attributing the proper degree of importance to the various results and recommenda- 

 tions, and then to establish a set of units that can be accepted as fair average values, 

 as far as known to-day, for the ordinary quality of each species of timber and corre- 

 sponding to the usual conditions and sizes of timbers encountered in practice. The 

 difficulties of executing such a task successfully can not be overrated, owing to the 

 meagerness and frequently the indefiniteness of the available test data, and especially 

 the great range of physical properties in different sticks of the same general species, 

 not only due to the locality where it is grown, but also to the condition of the timber 

 as regards the percentage of moisture, degree of seasoning, physical characteristics, 

 grain, texture, proportion of hard and soft fibers, presence of knots, etc., all of which 

 affect the question of strength. 



Your committee recommends, upon the basis of the test data at hand at the present 

 time, the average units for the ultimate breaking stresses of the principal timbers 

 used in bridge and trestle constructions shown in the accompanying table. 



In addition to the units given in the table, attention should be called to the latest 

 formulae for long timber columns, mentioned more particularly in the appendix to 

 this report, which formula? are based upon the results of the more recent full-size 

 timber column tests, and hence should be considered more valuable than the older 

 formula? derived from a limited number of small-size tests. These new formula? are 

 Professor Burr's, Appendix I; Professor Ely's, Appendix J; Professor Stanwood's, 

 Appendix K, and A. L. Johnson's, Appendix V; while C. Shaler Smith's formula? 

 will be better understood after examining the explanatory notes contained in Appen- 

 dix L. 



Attention should also be called to the necessity of examining the resistance of a 

 beam to longitudinal shearing along the neutral axis, as beams under transverse 

 loading frequently fail by longitudinal shearing in the place of transverse rupture. 



In addition to the ultimate breaking unit stress the designer of a timber structure 

 has to establish the safe allowable unit stress for the species of timber to be used. 

 This will vary for each particular class of structures and individual conditions. 

 The selection of the proper "factor of safety" is largely a question of personal 

 judgment and experience, and offers the best opportunity for the display of analyt- 

 ical and practical ability on the part of the designer. Tt is difficult to give specific 

 rules. The following are some of the controlling questions to be considered: 



The class of structure, whether temporary or permanent, and the nature of the 

 loading, whether dead or live. If live, then whether the application of the load is 

 accompanied by severe dynamic shocks and pounding of the structure. Whether 

 the assumed loading for calculations is the absolute maximum, rarely to be applied 

 in practice, or a possibility that may frequently take place. Prolonged heavy, steady 

 loading and also alternate tensile and compressive stresses in the same place will call 

 for lower averages. Information as to whether the assumed breaking stresses are 

 based on full-size or small-size tests or only on interpolated values, averaged from 

 tests of similar species of timber, is valuable in order to attribute the proper degree 

 of importance to recommended average values. The class of timber to be used and 

 its condition and quality. Finally, the particular kind of strain the stick is to be 

 subjected to and its position in the structure with regard to its importance and the 

 possible damage that might be caused by its failure. 



In order to present something definite on this subject, your committee presents 

 the accompanying table, showing the average safe allowable working unit stresses 

 for the principal bridge and trestle timbers, prepared to meet the average conditions 

 existing in railroad timber structures, the units being based upon the ultimate 



