Engineering Wonders of the World
Volume I

THE GREAT ZAMBESI BRIDGE. 97 more protracted, as the solid rock had a thick coating of debris which must be removed. About 110 feet below rail- level, on the ledges prepared, the masons laid a thick bed of concrete for the pedestals and bearings, reinforced top and bottom by iron rails. This was allowed to set for several weeks before any weight was placed upon it. Meanwhile began the con- struction of the bridge proper. As scaffolding or other direct support from below was out of the question, it was necessary to build the main span out from both banks, on the bracket or cantilever prin- ciple, until the two parts of the arch should meet and become self-sus- taining. The engineers gave their attention first to the two shore spans, resting at their land ends on abutments built into the rock. The main girders of the trusses were partly supported on temporary timber baulks and trestles, suffi- ciently strong to bear the additional weight of the jib cranes, which, when the shore spans were completed, lowered the materials for the skewbacks and end posts of the arch. As soon as these were up the shore spans were lowered on to them, and the temporary sup- ports removed. The next thing was to provide an anchorage at each end to sustain the main span during its cantilever stage. The plan adopted—a novel one—was as follows :— Some distance back from the edge of the cliff two shafts were sunk to a depth of 30 feet in line with the top of the end posts of the arch. At the bottom they were connected by a short tunnel. A number of wire ropes, specially provided for the purpose, were then (1,408) THE EARLY STAGES OF A CANTILEVER. (Photo, Cleveland Bridge Company.) attached at one end to an end post, carried down one shaft, through the tunnel, up the other shaft, and affixed to the other end post. Each rope Anchoring had separate attachments and the adjusting apparatus, so that it Cantilevers, might be made to bear exactly its fair proportion of the total strain. This gave the cantilever a large amount of Mother Earth—or rather rock—to pull on ; but to make safety doubly sure 400 tons of rails were piled on the ground between the two shafts. When once the anchorages were in, the work proceeded rapidly. The cranes, running for- wards as the cantilevers grew, lowered the parts of the steelwork to the assemblers, who quickly pinned them at the junctions. At their heels came the riveters with their forges and mechanical closing tools. To give confidence to the workmen, a huge net was slung under the points where building was in progress. Fortunately it had to catch nothing heavier than bolts and tools, and 7