42 



University, St. Louis, afford us to-day, in connection with the work of the above- 

 mentioned experimenters, our most reliable data from a practical standpoint. 



The test data at hand and the summary of criticisms of leading authorities seem to 

 indicate the general correctness of the following conclusions: 



(1) Of all structural materials used for bridges and trestles timber is the most 

 variable as to the properties and strength of the different pieces classed as belong- 

 ing to the same species; hence it is impossible to establish close and reliable limits 

 for each species. 



(2) The various names applied to one and the same species in different parts of 

 the country lead to great confusion in classifying or applying results of tests. 



(3) Variations in strength are generally directly proportional to the density or 

 weight of timber. 



(4) As a rule, a reduction of moisture is accompanied by an increase in strength; 

 in other words, seasoned lumber is stronger than green lumber. 



(5) Structures should be, in general, designed for the strength of green or moder- 

 ately seasoned lumber of average quality and not for a high grade of well-seasoned 

 material. 



(6) Age and use do not destroy the strength of timber unless decay or season 

 checking takes place. 



(7) Timber, unlike materials of a more homogeneous nature, as iron and steel, has 

 no well-defined limit of elasticity. As a rule, it can be strained very near to the 

 breaking point without serious injury, which accounts for the continuous use of many 

 timber structures with the material strained far beyond the usually accepted safe 

 limits. On the other hand, sudden and frequently inexplicable failures of individual 

 sticks at very low limits are liable to occur. 



(8) Knots, even when sound and tight, are one of the most objectionable features 

 of timber, both for beams and struts. The full-size tests of every experimenter have 

 demonstrated not only that beams break at knots, but that invariably timber struts 

 will fail at a knot or owing to the proximity of a knot, by reducing the effective area 

 of the stick and causing curly and cross-grained fibers, thus exploding the old prac- 

 tical view that sound and tight knots are not detrimental to timber in compression. 



(9) Excepting in top logs of a tree or very small and young timber, the heart wood 

 is, as a rule, not as strong as the material farther away from the heart. This becomes 

 more generally apparent, in practice, in large sticks with considerable heart wood 

 cut from old trees in which the heart has begun to decay or been wind shaken. 

 Beams cut from such material frequently season cheek along middle of beam and fail 

 by longitudinal shearing. 



(10) Top logs are not as strong as butt logs, provided the latter have sound timber. 



(11) The results of compression tests are more uniform and vary less for one species 

 of timber than any other kind of test; hence, if only one kind of test can be made, 

 it would seem that a compressive test will furnish the most reliable comparative 

 results. 



(12) Long timber columns generally fail by lateral deflection or "buckling" when 

 the length exceeds the least cross-sectional dimension of the stick by 20; in other 

 words, when the column is longer than 20 diameters. In practice the unit stress for 

 all columns over 15 diameters should be reduced in accordance with the various rules 

 and formulae established for long columns. 



(13) Uneven end bearings and eccentric loading of columns produce more serious 

 disturbances than are usually assumed. 



(14) The tests of full-size long compound columns, composed of several sticks 

 bolted and fastened together at intervals, show essentially the same ultimate unit 

 resistance for the compound column as each component stick would have if con- 

 sidered as a column by itself. 



(15) More attention should be given in practice to the proper proportioning of 

 bearing areas; in other words, the compressive bearing resistance of timber with and 



