But if it passes on the level of the lower chord (Fig. 176a.) the lower chord may be simply suspended upon the posts; and not be depended upon for resisting tension. The lower chord in this case may also be entirely dispensed with; for cross-ties, or joists, may be secured to the posts and longitudinal joints be placed upon them. If the lower chord is used and is made continuous so as to resist tension, it vir tually changes it into a Whipple truss in which the long sus pension-rods are unnecessary members. Still, in this case, the truss-especially the panel rods, are not so proportioned as to make it safe to omit the long suspension-rods. 638. Fink Truss. The outline or skeleton of a Fink truss is shown in Fig. 177. Fig. 177-Fink Truss. A B the upper chord, g the lower chord, a g, bh, etc., are posts, A C, C B long suspension-rods. Ah, D, etc., secondary suspension-rods. This truss consists of a primary system of king posts, A C B, Fig. 177; two secondary systems, A h D and Dk B; four tertiary systems, A g b, bi D, Dje, and e l B, and so on. The posts, suspension-rods and chords may be similar in detail to the systems previously described. The noted Louisville bridge, across the Ohio River at Louisville, is made upon this plan. DIMENSIONS OF THE LOUISVILLE BRIDGE. It is 5,294 feet long, divided into the following spans from centre to centre of piers: ..... ..... Kentucky abutment.. 32.5 feet. 2 spans of 227.0. 1 span of 370.0.. 6 spans of 245.5. 1 span of 400.. 3 spans of 180. 1 span of 149.6.. Total length. 5,294.0" 639. Post's Truss. The main peculiarity of this truss is in its form. The upper ends of the posts are carried towards the centre of the bridge, an amount equal to half a bay, and as all the bays are equal the posts in each half of the truss are all parallel to each other (Fig. 178). Fig. 178-Side view of panels of a Post Truss. A A are struts. B B, main ties. C C, counter ties. E E, bottom chords. II, top chords. D, ends of floor-beams. G, lower horizontal diagonal ties. G', upper horizontal diagonal ties. CENTRE . CENTRE Fig. 179-Plan of the roadway. G G are brace-rods. E E, bottom chord. D are floor beams. H Fig. 180-Plan of the top of the bridge. I, top chord. H, cross-tie or strut. G', upper horizontal tie. Fig. 181-Shows details at a joint of the lower chord. F is a cast-iron block for receiving the ends of the horizontal tie-rods. K is an iron bolt which passes through the ends of the links which form the lower chord. The other letters refer to the same parts as in the preceding figures, DESCRIPTION OF POST'S IRON BRIDGE. A A (Figs. 178, 179, 180 and 181)-Are the struts, composed of two rolled-iron channel bars, with plates riveted on their flanges, forming a hollow column having a rectangular cross-section. The struts are swelled in the centre by spring ing the channel-bars and having the plates sheared to the required shape. The bearings of the struts upon the pins (K) are of either cast or wrought iron, and are enclosed between the side-plates, and abut against the channel-bars, and are riveted to both. The pin holes are bored through shoes and plates. BB-Are the main ties, or main suspension braces, and are made of flat bar-iron with die-forged heads at the ends, bored out to fit the pins. C C-Are the counter ties, made of round iron, with forged eyes at the ends to receive the pins, and having turnbuckles at a convenient distance from the bottom end, for purposes of adjustment. D D-Are the floor-beams, suspended in pairs from the chord pins at each panel point, by means of eye-bolts or by stirrups passing over the chord pins and under a bolt through the webs of the beams. E E-Are the bottom chord bars or links, made of flat bariron, with die-forged heads, and bored holes for the chord pins. The sizes of the bars in the respective panels are determined by the strains, the first and second panels having two bars, the third and fourth having four bars each, the fifth and sixth having six bars each, etc., to the centre of the span. F-Is a bottom lateral brace angle block of cast iron, fastened to the ends of the floor-beams, which form the bottom lateral strut. G G-Are the lateral brace-rods, of round iron, having screws and nuts at their ends, for adjustment. H H-Are top lateral struts, made of rolled-iron beams, or channel bars in pairs. These struts have a cast-iron shoe at their ends, and are bolted to the top plate of the top chord, by bolts passing through shoes, top plate of chord, and through the joint box in the top chord. The top lateral brace rods pass through the cast-iron shoes, with nuts on the outside. I I-Are the top chords. When made of wrought iron they are composed of channel bars with covering plate riveted to the flanges on the top, and bars riveted at intervals across the bottom flanges, either diagonally or straight across to keep the channel bars in line. Additional sectional area is obtained by riveting plates on the inside of the channel bars. The top chords are made in panel lengths, with their ends squared by machinery to insure true bearings-and when of cast iron have a rectangular cross-section, with the inside cored out to obtain the necessary sectional area to provide for the compression strain. The connection of the struts and main and counter braces is made by means of a pin passing through a cast-iron box which encloses the mall, the length of the pin being just equal to the width of the box. The top-chord sections have a recess which fits over the box, and when the connection is made in the box the pieces of top chord are laid on, and cover the whole. The joint is then secured by the bolts which pass through the top lateral strut, top chord and joint box. This bridge is composed partly of wood and partly of iron, as shown in Figs. 181a, 1816, and 181c. A, A-Top chord, packed and framed as shown in Figs. 181a and 1816. B B-Struts, framed with square end at the top entering and abutting against joint box E (Fig. 1816) and fitted at bottom ends into strut shoe K (Fig. 181c). CC-Main suspension ties, of square, round or flat iron, with eye at lower end and screw at upper end, passing through joint box E, secured by nuts. D D-Counter braces, of square or round iron, made similar to main ties. E E-Cast-iron joint boxes enclosed in top chord, and receiving the struts, main ties and counters. This box has a flange around the bottom to support the weight of the top chord, which lies upon and is bolted to it. |