Engineering Wonders of the World
Volume I

272 ENGINEERING WONDERS OF THE WORLD. over the water at either end. For lightness and for ease of construction it is not made solid, but is built up of triangulated truss work. Fig. 23.—BUILDING OUT THE ARMS OF THE BLACK- well’s ISLAND BRIDGE FROM THE ISLAND PIERS. The travellers have passed through the towers. (A triangle is a perfectly rigid assemblage, so differing from a square, which can be flattened down into a rhomboid shape.) On either shore there is another such built-up beam, poised on a pier near the water’s edge, and at its land end both supported by and anchored to a heavy “ anchor pier.” This beam pro- jects outward over the water, and meets the overhanging end of the middle or island section. In the Queensboro Bridge these “ cantilever arms ” project out far enough to meet. In all other cantilever bridges, however, the arms are not made long enough to Design. meet, and the open space between them is covered by an ordinary truss bridge, called the “ suspended span,” which rests on the ends of the cantilever arms. The omission of the customary suspended span is a very peculiar characteristic of the Queens- boro structure. This is of special interest, as the reader is not likely to see a second instance of this omission. Most engineers do not like the arrangement here adopted, though no one questions that it can be made just as strong as the usual form. Bridgemen think it easy to erect a cantilever bridge, because in putting up the overhanging work they do not have to bother with false- work—that is, temporary supports built up below. Once the anchor arms or shore arms are completed, the projecting parts are at- tached piece by piece from above, forming at all times a firm, stable structure, until they meet at mid-channel. Even arches are some- times erected by this method, a notable in- stance being afforded by J. B. Eads’s bridge across the Mississippi at St. Louis. This method of construction is, in fact, the reason why the cantilever type of bridge ng so often adopted when deep channels, wherein it would be difficult and costly to build false- work, have to be crossed. The appliance which makes the cantilever method of erection so easy is the “ traveller.” This is a great upright framework mounted on rollers, carrying derricks and other hoisting tackle, and fitted with a long overhanging nose at the front, from which of the bridge are hung while being pinned or riveted together so as to carry their own weight. It is only necessary to add that, before cantilever erection begins, the shore arms The “Traveller.” the new pieces Fig. 24.—ASSEMBLING EYE-BARS ON THE BLACK- well’s ISLAND BRIDGE. The pins used to hold the eye-bars together are huge steel bars 10 feet long and 16 inches in diameter.