. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. D Fig. 23. the bridge, we can put it above by a simple change, as in Fig. 21, where the point C is firmly fixed by the braces A C and C B, and there- fore the centre point of the beam, A D, may be suspended from C by the tie rod C D, thus changing the long span, A B, into two short ones, A I) and D B. Again, if we find that A C and C B are so long as to be too flexi- ble, we may support their center points by additional braces, D E and D F,
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. D Fig. 23. the bridge, we can put it above by a simple change, as in Fig. 21, where the point C is firmly fixed by the braces A C and C B, and there- fore the centre point of the beam, A D, may be suspended from C by the tie rod C D, thus changing the long span, A B, into two short ones, A I) and D B. Again, if we find that A C and C B are so long as to be too flexi- ble, we may support their center points by additional braces, D E and D F, Fig. 22; thus firmly fixing the points E and F, and should A D and D B be too weak they can be supported from the fixed points E and F by tie rods E G and F H. So we arrive by this simple process at a form which is comparatively complex. If it is desirable to make use of a material like wrought iron for stiffening, since it is peculiarly adapted to bear tensile strain, we may make use of it in a most economical manner. In Fig. 23 we have a beam, A B, trussed, as it is termed, by the iron rod, ADB, which passes under a post or strut, C D ; now it will be im- possible, when all the parts are tight, for the point C to sink without the lines A D and B D becoming longer. Since the strain upon the tie in this case is a direct tensile strain each fibre will be made to bear its share of the load, and it will be a very economical mode of using our material. We may modify this in such a way as to show that the strain upon such a tie is precisely the same as on the lower edge of a beam. Let us suppose, in Fig. 24, that the strut £ f* is made so short as to disappear, and permit ^ D the rod to touch the beam throughout its Fig. 24. whole length, it will still act as the tie in Fig. 23, but with diminished effect, owing to its being nearer to the neutral axis, and the moment of the resistance of its fibres being therefore less. This mode of strengthening a beam is some- times resorted to
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