In such cases, as in medium spans, the most common solution is erection by the cantilever method, generally counterbalancing the cantilever arms from previously-completed portions of the permanent structure, building both cantilevers of the same spans simultaneously, and connecting their extremities by an independent suspended span designed to resist cantilever stresses developed by erection and to function subsequently as a simple span.

Cantilever spans have been erected up to a length of 1,800 feet with results which indicate that the same method can be extended to lengths several hundred feet greater. Serious consideration has been given to the practicability of constructing cantilever spans up to 3,000 feet in length, which length approaches the present limit set by the strength of materials; and such a detailed design would, of course, involve the development of complete cantilever-erection plant. In this case it is probable that the assisted cantilever method or some modification of it, such as has already been adopted in some large cantilever spans, would be employed, whereby temporary supports would be placed under the cantilevers between the permanent piers, thus greatly reducing the erection stresses.

The length and weight of the members of the suspended trusses in a cantilever span necessitate their erection by a heavy traveler, and, if accomplished by the cantilever method, produce a great increase of the stresses developed by the erection of the cantilever arms proper. In order to avoid these stresses, and for other reasons, the suspended span has in some instances been erected complete at a low level, towed to position under the ends of the finished cantilever arms, hoisted up, and connected.

In another case of a long-span cantilever, the ends of the cantilever arms were connected by a light, temporary suspension bridge on which the permanent suspended span was erected.

The difference in local conditions, type, and other features may make any one of these methods the most economic one for the given case, so that for an extremely large structure, all of them will probably have to be analyzed for the final comparative determination.

Suspension Bridges

Short-span and temporary suspension bridges may be designed with main cables, each composed of one or more twisted wire ropes extending continuously from anchorage to anchorage, that can be delivered complete at the site, and, with ordinary appliances, pulled across the river from tower to tower and from tower to anchorage, secured in place, and adjusted, after which the erection from them of the remainder of the superstructure is comparatively simple and easy. Generally this is the most economic method.

For all long-span suspension bridges, the universal method of erection has been the preliminary construction of falsework suspension bridges,