Bulletin (United States. Division of Forestry)U.S. Government Printing Office, 1896 |
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12 inch A. L. Johnson Aver average Bald cypress beam in inches bearing area bearing value bolt braces bridge and trestle caps and sills cent Compression corbels cost of stringers cost per linear cross breaking crushing strength deflection Douglas fir end bearing extreme fiber F. E. Kidder factor of safety feet Forestry Division formula FULL-SIZE TESTS grain H. T. Bovey Height of beam Limits linear foot Live oak load in middle Longleaf pine modulus of elasticity modulus of rupture Norway pine Oregon paper piece pine Pinus Pitch pine posts pounds per square present practice railroad companies red fir Redwood safe loads safe unit stresses seasoned shearing Shortleaf pine SMALL-SIZE TESTS square inch strength of bridge strength of timber Superintendents of Bridges Table Thurston timber structures tion Transverse rupture trestle timbers Ultimate breaking stress uniformly loaded Washington yel Watertown Arsenal White oak White pine Yellow pine
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Էջ 11 - Since the strength of timber varies very greatly with the moisture contents (see Bulletin 8 of the Forestry Division), the economical designing of such structures will necessitate their being separated into groups according to the maximum moisture contents in use.
Էջ 44 - Transverse rupture, modulus of elasticity 2 Shearing with and across grain 4 In conclusion, your committee desires to emphasize the importance and great value to the railroad companies of the country of 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...
Էջ 43 - 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 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 continual working produced by moving loads. The aim of your committee has been to examine the...
Էջ 43 - 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...
Էջ 44 - 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 ever undertaken. With additional and reliable information on this subject far-reaching economies in the designing of timber structures can be introduced, resulting not only...
Էջ 7 - Timber structures, on the other hand, have been designed according to the general principle that the Lord takes care of His own, as the great number of fatalities resulting from failures of these structures will attest. Experience is not only a dear teacher, but requires endless time to tell what he knows. That this time has not yet expired is evidenced by the anachronisms of design existing in the present practice. It is a well-known principle that a chain is no stronger than its weakest link, and...
Էջ 42 - 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...
Էջ 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 the application of the load is accompanied by severe dynamic shocks and...
Էջ 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...
Էջ 12 - Class B (moisture contents, 15 per cent. ) — Structures under roof but without side shelter, freely exposed to outside air, but protected from rain, such as roof trusses of open shops and sheds, covered bridges over streams, etc. "Class C (moisture contents, 12 per cent.)— Structures in buildings unheated, but more or less protected from outside air, such as roof trusses of barns, enclosed shops and sheds, etc.