the counterpoise decreases as the platform rises. The system is manœuvred by an endless chain passed over the gorge of a wheel hung upon the horizontal shaft. For light platforms a counterpoise may be dispensed with, and the bridge may be manoeuvred by connecting the chain attached to the movable end of the platform to a horizontal shaft, which is turned by the usual tooth-work combinations. When the locality does not admit of manoeuvring the bridge by a chain connected with some point above the framework, the platform (Fig. 202) is continued back, from two-thirds to three-fifths its length, from the face of the abutment, to form a counterpoise for the platform of the bridge. The horizontal axis of the bridge is placed near the face of the abutment, and a well of a suitable shape to receive the posterior portion of the platform that forms the counterpoise is formed behind the abutment. The mechanism for working the bridge may consist of a chain and capstan below the platform-counterpoise, or of a suitable combination of tooth-work. In bridges of a single platform, the movable extremity, when the bridge is lowered, rests on the opposite abutinent, and no intermediate support will be required for the structure if the framework be of sufficient strength; but when a double bridge, consisting of two platforms, is used, the platforms (Fig. 200) should be supported near their movable ends, when the bridge is down, by struts movable around the joint by which they are connected with the face of the abutments. These struts are so connected with the bridge that they are detached from it and drawn up when it is raised, and fall back into their places, abutting against blocks near the movable end of the platform, when the bridge is down. By these arrangements the chains for working the bridge are relieved from a portion of the strain when the bridge is down, and it is also rendered more firm. When the counterpoise is formed by the rear part of the platform, additional security may be given to the bridge when down by attaching two chains beneath the platform, and securing them to anchoring-points at the bottom of the well. In some cases a heavy bar, fitted to staples beneath connected with the timbers of the platform, is used for the same pur pose. In double bridges the two platforms when lowered should abut against each other, giving a slight elevation to the centre of the bridge. This not only gives greater stiffness, but is favorable to detaching the platforms when the bridge is to be raised. For draw, and every kind of movable bridge, temporary barriers should be erected on each side at the entrance upon the bridge, to prevent accidents by persons attempting to cross the bridge before it is properly secured when lowered. 681. Turning-bridges. These bridges revolve horizontally upon a vertical shaft or gudgeon below the platform, which is usually thrown far enough back from the face of the abutment to place the side of the bridge, when brought round, just within this face. The weights of the parts of the bridge around the shaft should balance each other. To support and manoeuvre the bridge (Fig. 203) a circular ring of iron, or roller-way, of less diameter than the breadth of the bridge, and concentric with the vertical shaft, is firmly imbedded in masonry. Fixed rollers, in the shape of truncated cones, are attached at equal distances apart to the framework of the platform beneath, and rest upon the roller-way. The bridge is worked by a suitably arranged tooth-work, or by a chain and capstan. In some cases cast-iron balls, resting on a grooved roller-way, and fitting into one of corresponding shape fixed beneath the platform, have been used for manoeuvring the bridge. The ends of the bridge are cut in the shape of circular arcs to fit recesses of a corresponding form in the abutments, so arranged as not to impede the play of the bridge. In double-turning bridges the two ends of the platforms which come together should be of a curved shape. The plat forms should be sustained from beneath by struts, like those used for draw-bridges, which can be detached and drawn into recesses when the passage is interrupted; or else they may be arranged with a ball-and-socket joint at their lower extremity, so as to be brought round with the bridge. For the purpose of giving additional strength and security to the bridge, iron stays are, in some cases, attached on each side of the platform near the extremities, and connected with vertical posts placed in a line with the vertical shaft. Turning-bridges may be made either of timber or of cast iron; the latter material is the more suitable, as admitting of more accuracy of workmanship, and not being liable to the derangements caused by the shrinking or warping of framework of timber. 682. Swing Bridge at Providence, R. I. The details of this bridge are worthy of special study. An account of it is published in the London Engineering for March 21st, 1873. Fig. 204 is an elevation of the bridge, and the righthand half of Fig. 205 is a plan of the truss work under the roadway, and the left-hand half the plan of the roadway and truss work. Fig. 206 is a section of the turn-table for supporting the bridge. An essential part is the four compound radial arms, G &, F F, Fig. 206, the lower parts of which are of cast-iron compression members, and the upper parts of two wrought-iron rods each. The whole structure rests upon a nest of conical rollers, I I (Fig. 206), upon which it turns as it moves about. There are several small wheels b, b, b, which are under the turn-table, and serve only to steady it in case it tends to tip in any direction. The strains on the several members were computed under three hypotheses, viz.: 1st. The strains due to the weight of the truss only when the draw was open. These strains were assumed to be the same as when it was closed and unloaded, for no part of the weight of the bridge was supposed to be Figs. 204 and 205-The elevation and plan of the swing bridge over Point Street, at Providence, Rhode Island. Total span 250 feet. Depth at the centre 25 feet, and at the ends 9 feet. The part A O of the upper chord is subjected to tension only, and is composed of tension bars only; but the part C D may be subjected to both tension and compression, and is made to resist both strains. The three end panels only require counter-ties. E is a pin for securing the bridge in place when it is closed. a, a, a, are tie rods after the plan of a Whipple Truss. The part M N is subjected to compression only, and the part N O to both tension and compression, c, c, c, are main tie rods; d, d, counter-tie rods, P, the foundation for the turn table, b, b, the seat for steady wheels. 387 Fig. 206-Is the turn-table. b, b, b, are rollers for guiding the turn-table to keep it from overturning. They serve to steady it. GG is a cast-iron arm, which is supported at its outer end by the wrought-iron radial arms FF (four in number). JJ is the wheel |