Read the text and pay attention to the differences in the various techniques of superstructure erection.

The erection technique of a superstructure depends on the span type, its dimensions and material. The span length for motorway bridges usually varies between 6 and 42 m. In the case of railway bridges the spans may be up to 34.2 m long. For short spans the choice of bridge types is very wide even for heavy loadings. For medium spans, the choice is still wide enough, but for long spans, the choice is restricted to continuous lattice girder, cantilever lattice and steel lattice bridges. When the spans are in excess of 457.2 m, the stiffened suspension bridge is the only suitable form. The following erection techniques have been developed for building different bridge structures – simple setting of discontinuous beams; erection by protrusion; launch and balanced cantilever erection.

The simplest erection technique is used for single beam spans (fig. 14.1a). Standard reinforced concrete beams are produced by casting yards, shipped to the site, and set in place by cranes (fig. 14.1a). For short spans, steel beams are usually formed as a single unit. At the site, they are placed parallel to each other, with temporary forms between them so that a concrete deck can be cast on top. The beams usually have metal pieces welded on their top flanges, around which the concrete is poured. These pieces provide a connection between beams and slabs, thus producing a composite structure.

When a bridge is designed as a continuous one the builders put simple beams on two supports, which join them into a continuous beam. For longer spans, steel beams are made in the form of plate girders. A plate girder is an I-beam consisting of separate top and bottom flanges welded or bolted to a vertical web. While beams for short spans are usually of a constant depth, beams for longer spans are often deeper at the supports and shallower at mid-span. They are called haunched beams. Haunching stiffens the beam at the supports, thereby reducing bending at mid-span. In general, shorter spans are built with beams, hollow boxes (box girders), trusses and arches, while longer spans use cantilever, cable-stay, and suspension forms.

The next, span erection method is erection by protrusion. This method is used to construct very long bridge spans by employing reinforced concrete continuous girders, which are cast on the bank and then set to the place by pushing (fig. 14.1b). While being pushed, the girder works as a cantilever resisting the dead weight. As reinforced concrete has rather high density builders use a launching nose to reduce bending in the girder (fig. 14.1b). This technique has its disadvantages, which clearly limit its use: the concrete has to be prestressed in advance, and it needs time to reach a certain density.

Framed suspended bridges, framed flyovers and trestle bridges may be constructed by two methods. The first one uses the sliding formwork for building up the span with concrete (fig. 14.1c). It results in a monolithic structure. The next method represents a balanced cantilever erection of framed structures, from ready-made reinforced concrete units produced by the casting yards (fig. 14.1d).

This method is used for prestressed concrete bridges using a concrete cantilever in short segments. The previous segment, thus avoiding the need for falsework, while it is being cast, supports each new segment. The units are stuck together with an adhesive or cement. This construction method leaves no possibility for later correction, and the elements must be fixed exactly in the right position. For steel cantilever bridges, the steel frame is built out from the towers toward the centre and the abutments. When a shorter central span is required, it is usually floated out and raised into place. The deck is added last.

The most complicated construction technology is employed for arch bridges. The traditional method demands temporary piers and centering to reproduce the shape of the arch superstructure (fig. 14.2a). The precast reinforced concrete blocks are placed on an arch centering. The monolithic concrete arch is erected in a curved framework. The construction of arches afloat is a much more advanced technique (fig. 14.2b). The arches and semi-arches are assembled on the bank and transported to a barge, which carries these structures to the place.

One of the oldest engineering forms is the suspension bridge, which is constructed by mounting the roadway directly on the continuous steel cables. Cables are generally made of thousands of high-strength steel wires spun together, at the construction site. Spinning is done by rope pulleys that carry each wire across the top of the towers to the opposite anchorage and back. A traveling wheel carries the continuous cable strand from the anchorage on one side to the anchorage on the other side, where a crew receives the wheel, anchor the strand, and return the wheel, laying a fresh strand. The wires are bundled and from these successive parallel strands, a cable is built up and covered to prevent corrosion. When the cables are complete, suspenders are hung, and finally the deck is constructed – usually by floating deck sections out on ships, hoisting them with cranes, and securing them to the suspenders.

Construction of cable-stayed bridges usually follows the cantilever method. After the tower is built, one cable and a section of the deck are constructed in each direction. Each section of the deck is prestressed before continuing. The process is repeated until the deck sections meet in the middle, where they are connected. The ends are anchored at the abutments.

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