On Some Common Errors in Iron Bridge Design

49 path cantilever to a point about three-fourths of the height of the parapet. They may be macle of angle, T, channel, or any other stiff section of metal and securely rivetted or bolted top and bottom. In some recent and otherwise excellent bridges near Melbourne, small triangular plates of thin metal rivetted along the bottom and the vertical side have been used. These are not to be commended, as the maximum compression comes on the thin, edge of the plate that is least able to endure it, and the majority of the rivets are placed where they endure but little stress and are of proportionately little service. The same amount of material and labour put in a substantial sloping strut would give far greater strength. A parapet may be high enough and strong enough, and yet a source of danger by having openings in it large enough for children to pass through. An examination of a considerable number of such parapets leads the writer to recommend that if formed of paralled horizontal bars, these bars should not be more than 9 inches apart vertically, or if as is often, the case formed of lattice work showing square openings, such openings should never be more than 1 foot square. It is recommended that all parapets accessible to the public be altered when necessary to comply with the above requirements. In concluding this necessarily very inadequate treatment of a vast and complicated subject of extreme public importance, the writer desires to apologise for its brevity and other defects, and to express his hope that it may be the means of rendering a most important section of engineering work in. some small measure more consistent, economical, and secure against accidents than it has been hitherto. 4a