. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 144 LECTURE point b. To do this Ave may either introduce a strut, c b, or a tie, a d, as in Figs. 18 and 19. If we use a strut or wooden "brace, we shall have the arrangement shown in Fig. Fig. 33. In this arrangement the heam cannot assume the shape shown in Fig. 32, without its diagonals becoming shorter; and since the braces are in the most favorable position for resisting—that is, with the strain acting in the direction of the
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 144 LECTURE point b. To do this Ave may either introduce a strut, c b, or a tie, a d, as in Figs. 18 and 19. If we use a strut or wooden "brace, we shall have the arrangement shown in Fig. Fig. 33. In this arrangement the heam cannot assume the shape shown in Fig. 32, without its diagonals becoming shorter; and since the braces are in the most favorable position for resisting—that is, with the strain acting in the direction of their length—a small amount of mate- rial will do a great deal of work. If it is desirable to use an iron tie instead of a wooden brace, we shall have the form shown in Fig. 34. Fig. 34. For any beam or truss, which is only intended to bear a constant and quiet weight, this bracing is sufficient, but if the load is variable and passing, as in the case of a railroad bridge, something more is needed. In a structure of considerable length, the effect of the load at any point between the centre and the end will be to cause a rise of the cor- responding point on the other side of the centre ; and since the braces are not calculated to prevent such a rise, oscillations will take place which may soon destroy the structure. Such a rise at any point can only take place by a change in the shape of the rectangle ; and if, therefore, we introduce another brace in the direction of the other diagonal, we shall prevent change of figure in either direction. X XI X X X X In Fig. 35 we have such an arrangement. Such braces are called counter braces, and since the strain upon them is a secondary one, and always small, they maybe made much lighter than the main braces. A little consideration will show that ties may be substituted for struts in a variety of ways, and vice versa. For instance, in Fig. 33, the addition of ties running in the same diagonal as the struts will counter brace the truss,
Size: 2800px × 893px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorsmithsonianinstitutio, bookcentury1800, bookdecade1840
