On Some Common Errors in Iron Bridge Design

10 largely reduced in length, thus economising flooring, hand-railing and area of ground occupied. This, it may be added, has been done in several more recent structures of a similar kind. A third case of unfavourable support is where the girder seat is not placed centrally to the cylinder or column. This causes the stress to be greater on one side of the column, than the other, and involves a tendency for the column, to lean over if in soft ground. An instance of this is to be seen in an important bridge illustrated in “Engineering,” vol. 43, p. 117. Such errors as these are usually irremediable in. existing structures. It is desirable, however, to guard against their repetition in future designs. 4. Imperfect expansion apparatus.—Variations of temperature affect all metal structures, and unless properly provided for may cause extra stresses of serious amount in the metal work and dislocation of the brickwork or masonry of the supports, such dislocation unfortunately being only too apparent in. not a few existing structures. The extreme temperature range in Melbourne from the Observatory thermometer records is about 150° Fahrenheit, but it is not probable that the change of temperature of considerable masses of metal work such as used in bridges will be more than 120°. As iron and steel expand not quite one part in 800 between freezing and boiling, a range of 180°, the expansion for 120° will be one part in 1200 or 1 inch in 100 feet. This simple and easily remembered rule is safe for the Victorian, climate, but would not suffice for places such as New York, where the temperature range is greater in both directions than in Melbourne. A metal structure then should be anchored or fixed at some definite point and be allowed free movement everywhere else. The point of anchorage may advantageously be near the centre, so as to divide up the motion. If this is not convenient the anchorage should be at the firmest or most solid support. If for example one end of a bridge be on solid rock and the other on a tall and somewhat flexible support, the former should be made the anchorage. At the Victoria Street Bridge, near Melbourne, the opposite course was adopted and the whole bridge was thrown somewhat out of position by movement of the support and involved the necessity of its being detached and moved back to its original position—a troublesome operation.