On Some Common Errors in Iron Bridge Design

32 eyebars in suspension bridges years earlier was disregarded. After some years of discussion the Railway authorities yielded to the writers representations, and condemned this viaduct as a double line bridge. It now carries a single line only, though containing abundant material if rightly disposed to make an amply strong double line structure. In the bottom chord of this viaduct eyebars are also used, having the extraordinary peculiarity of the centre of the eye being an inch above the centre line of the bar. As, however, this chord is of excessive sectional area no real danger arises from this singular departure from correct practice. Structures having defective eyebars in them can be improved only by reconstruction and replacement of the weak parts. If this is impossible, the load carried should be reduced, by narrowing the deck if a public road is carried or removing a line of rails, placing the remaining line centrally as was done at the Moorabool Viaduct. Fig. 37 represents an eye from the Hawkesbury Bridge, and may be regarded as an example of tlie best and latest practice. The joints of compression parts are not often, a source of weakness. If they are solidly butted together, the only need for rivetting is to prevent lateral displacement. The usual practice, however, is to assume that the ends are not in contact, and insert sufficient rivets to carry the whole stress as in tension joints. In this case there is no need to have the single leading rivet, and the arrangement shown in Fig. 33 is admissible. Should the rivets of a compression joint be insufficient to take the stress they will distort slightly and permit the ends of the plates to come into contact, when further yielding will be effectually prevented. The practice has been sometimes adopted, and appears reasonable, of putting the girdei’s together with but a few loosely fitting “service bolts” in the rivet holes of the compression joints, then, loading them with a weight sufficient to bring the ends of the parts into perfect contact, and then rivetting up. If this method be adopted the number of rivets in compression joints may be made very small. 18. Local weakness at intersection of web members.—In the olclei’ form of lattice girders the web diagonals near the centre of the span are usually made of angle bars, which give a suitable