The use of steam as an operating power for swing bridges has almost disappeared, being replaced by electric motors, which are more economical and do away with many difficulties encountered in the use of steam engines and their boilers, though gasoline engines are in general use where electricity is not readily obtainable.
The metal arch bridge is a type of structure developed almost wholly in recent years, and there are various modifications of this type, some being built as spandrel arches, in which the web members connect a horizontal to chord with an arched lower chord, having the floor system supported on the top chord or framed in between vertical posts at the top chord level, while others are built as braced ribs, having both chords arched and connected together with the web members and the floor supported on vertical posts resting on the braced rib at panel points. Each of these types of arch spans is further varied by being built by some engineers with two pin bearings at the two supports, and by others with the addi tion of a third pin bearing at the crown. It is contended by the advocates of two hinges that the omission of the centre hinge adds con siderably to the rigidity of the span, but it has not yet been conclusively demonstrated that the third hinge adds appreciably to the vibration, type of bridge; the Brooklyn Bridge having stone towers, the Williamsburg Bridge steel towers, and the most modern, Manhattan Bridge, rocking steel towers. The Williamsburg Bridge is the longest suspension bridge in the world, having a span of 1,600 feet. Fig. 2 shows the design for the proposed bridge over and this third hinge disposes of the great un certainty of temperature stresses and allows the exact calculation of the true stresses and sections by static methods, so that it hn con siderable advantage and is more in use than the two-hinged type. Many long spans have been built of this type, the longest span being the Pennsylvania Railroad bridge over Hell Gate, New York, which is a two-hinged span drel arch carrying four tracks, with a span of 977 feet 6 inches as shown on Fig. 1. Sus pension bridges have not been used for railroad spans to any great extent, as with wire cables and a stiffening truss along the floor line it is difficult and expensive to give them the neces sary rigidity for such heavy moving loads. However, for very long spans where the dead load exceeds the live load they can be advan tageously used, and in the most modern designs the cables have been designed of eye-bars in place of wire, which enables the stiffening trusses to be built along the cable and thus saves a very large portion of the weight. The depths of such stiffening trusses can be so de signed that neither of the chords will carry compression, so that the only compression mem bers will be the comparatively light web the North River at New York, with a span of 2,880 feet to carry eight tracks and two road ways; this design shows the stiffening trusses along the cables.
The cantilever bridge has come into very general use in recent years, not because of the economy in metal required, as this type weighs more .than a simple truss or an arch of equal
span, but because of the great advantage that it can be erected without putting falsework in the main channel. Since the government pro hibits falsework unless there are several spans with sufficient channel depth for navigation this type is practically mandatory for long spans, as, if any other type is adopted, the span must be erected- with additional material, causing it to act temporarily as a cantilever, and this additional erection material overbalances the saving of any type adopted. For this rea son the cantilever has become the standard type for spans where falsework cannot be used, though• for very long spans the sus pension type is probably more economical. It is impossible to set a limit as to when the suspension type is lighter, but for ordinary railroad loads the cantilever is probably more economical up to spans of about 2,000 feet, and members. Recent designs for suspension bridges also use rocking or flexible steel towers with the cables made fast to their tops in place of rigid towers with cables supported on roll ing shoes. The three suspension bridges over the East River between New York and Brook lyn are excellent types of the evolution of this beyond that the suspension type would probably prove the better. The cantilever over the Saint Lawrence River at Quebec, having a span of 1,800 feet, is the longest span of any kind in the world.
Cantilever bridges are comparatively new, but even in the few years in which they have been in use they have been greatly improved in their general arrangement, details and erec tion devices. The capacity of the bridge shops to manufacture eye-bars as large as 16 inches by 2 inches and 83 feet long, centre to centre of eyes, with pin holes 13 inches in diameter, and heads as large as 36 inches in diameter, as well as to build and ship single compression members weighing 100 tons, together with the ability of the erecting gangs to put these huge pieces in place with moving travelers, has given the designing engineer practically unlimited op portunities, and has brought about the con struction of several very large spans of this type in the last two years, notably the 1,800 foot span at Quebec, the 1,182-foot span at Blackwell's Island, N. Y., and the 812-foot span over the Monongahela River at Pitts burgh, Pa. The first two bridges are described elsewhere, and the Pittsburgh cantilever is shown in Fig. 3.
Stone arch bridges are an excellent type for short spans, where the owners can afford the first cost, as the repairs are practically nothing, but this type has not changed in recent years, though in the modified form of the reinforced concrete arch it is being introduced quite generally for ornamental bridges in parks and cities where the zsthetic requirements are more important than economy, and it is an excellent type for this use, but cannot be eco