Economical Designing of Timber Trestle Bridges

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1902 - 57 pages
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Page 43 - factor of safety ' is largely a question of personal judgment and experience, and offers the best opportunity for the display of analytical and practical ability on the part of the designer. It 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...
Page 42 - Beams cut from such material frequently season check 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...
Page 42 - 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 belonging 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...
Page 42 - ... 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...
Page 43 - ... 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 formulae are based upon the results of the more recent full-size timber column tests, and hence should be considered more valuable than the older formulae derived from a limited number of small-size tests....
Page 43 - ... 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 piece, 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...
Page 44 - ... the experimental work on the strength of American timbers being conducted by the Forestry Division of the United States Department of Agriculture, and to suggest that the American Association of Railway Superintendents of Bridges and Buildings indorse this view by official action and lend its aid in every way possible to encourage the vigorous continuance of this series of Government tests, which bids fair to become the most reliable and useful work on the subject of strength of American timbers...
Page 8 - Table 1 gives the different species now employed in various parts of these structures, and a mean estimate of the length of life of each. These separate estimates, however, were very erratic, in many cases being little better than a guess, so that the mean given in the table is by no means reliable. This lack of information is scarcely less remarkable than it is unfortunate. Although...
Page 41 - ... tests, owing to the greater freedom of small selected test pieces from blemishes and imperfections and their being, as a rule, comparatively drier and better seasoned than full sized sticks.
Page 42 - ... 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 considered as a column by itself. (15) More attention should be given in practice to the proper proportioning of bearing...

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